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There Is a Black Hole at the Centre of Our Galaxy · Night Science Stories

• 0:00
  Speaker 1: Good evening, and welcome to the Sleepy Scientist. I'm very
• 0:03
  Speaker 1: glad you've found your way here tonight. I hope the
• 0:07
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• 0:52
  Speaker 1: here for a while, thank you for coming back. It
• 0:55
  Speaker 1: genuinely means a great deal to have you spending a
• 0:58
  Speaker 1: little part of your evening here. Tonight we're travelling somewhere
• 1:01
  Speaker 1: that sounds rather less relaxing than it actually is. Right
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  Speaker 1: at the centre of the Milky Way, roughly 26,000 light
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  Speaker 1: years from where you are now, something enormous is hiding
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  Speaker 1: in the dark. Thankfully it is very far away, very quiet,
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  Speaker 1: and has absolutely no plans to ruin your evening. But
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  Speaker 1: it does raise a rather wonderful question. If a black
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  Speaker 1: hole gives off no light of its own, and the
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  Speaker 1: centre of our galaxy is hidden behind enormous clouds of dust,
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  Speaker 1: how did we ever discover that it was there? So,
• 1:36
  Speaker 1: get comfortable, let your eyes close whenever they feel ready,
• 1:39
  Speaker 1: and we'll make our way slowly towards the darkest point
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  Speaker 1: in the Milky Way. There is a black hole at
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  Speaker 1: the centre of our galaxy, four million times the mass
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  Speaker 1: of the Sun, and somehow we managed to find it.
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  Speaker 1: about 26,000 light-years from where you are now in the
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  Speaker 1: direction of the constellation Sagittarius, there is something we cannot see.
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  Speaker 1: It produces no visible surface light. It cannot be photographed
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  Speaker 1: in the ordinary sense of seeing a solid object illuminated
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  Speaker 1: against the darkness. And yet, packed into an extraordinarily small
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  Speaker 1: region of space, it contains roughly four million times the
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  Speaker 1: mass of our Sun. This is Sagittarius A star, the
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  Speaker 1: supermassive black hole at the centre of the Milky Way. 26,000
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  Speaker 1: light years is an awkward distance to imagine. Light itself,
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  Speaker 1: travelling at almost 300,000 kilometres every second, takes around 26,000
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  Speaker 1: years to cross the gap between the centre of our
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  Speaker 1: galaxy and Earth. So whenever astronomers study that distant region,
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  Speaker 1: they are not seeing it as it is tonight. They
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  Speaker 1: are receiving information that began travelling towards us thousands of
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  Speaker 1: years before the first cities appeared on Earth, and that
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  Speaker 1: distant black hole is enormous in mass. Our sun contains
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  Speaker 1: more than 99% of all the mass in the solar system.
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  Speaker 1: Imagine 4 million suns, and you begin to approach the
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  Speaker 1: mass associated with Sagittarius A star. Yet all of that
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  Speaker 1: mass is concentrated into a region astonishingly small compared with
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  Speaker 1: the galaxy surrounding it. A black hole does not have
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  Speaker 1: a solid surface in the ordinary sense. There is no
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  Speaker 1: dark sphere of material that you could approach and touch. Instead,
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  Speaker 1: the defining boundary is the event horizon, a region beyond
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  Speaker 1: which nothing, including light, can return to the outside universe.
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  Speaker 1: For a black hole with a mass like Sagittarius, a star,
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  Speaker 1: that horizon would span only a few tens of millions
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  Speaker 1: of kilometres across. On galactic scales, that is almost absurdly compact.
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  Speaker 1: Four million suns have effectively been concentrated into a space
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  Speaker 1: smaller than the scale of our planetary system. But perhaps
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  Speaker 1: the strangest thing about Sagittarius A-star is not simply that
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  Speaker 1: it exists, it is that it belongs to us. When
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  Speaker 1: we see photographs of spiral galaxies, it is easy to
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  Speaker 1: imagine the Milky Way in much the same way. a
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  Speaker 1: beautiful disk hanging against black space, with luminous spiral arms
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  Speaker 1: curling around a bright central region. But we never actually
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  Speaker 1: see our own galaxy from that perspective. We are inside it.
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  Speaker 1: Earth is embedded within the Milky Way's disk, travelling around
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  Speaker 1: the Sun while the Sun itself moves through the galaxy.
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  Speaker 1: The stars scattered across the night sky are not decorations
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  Speaker 1: surrounding a stationary Earth. They are other members of this
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  Speaker 1: enormous moving system. The whole scene is in motion. The
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  Speaker 1: sun lies roughly 26,000 light-years from the galactic centre and
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  Speaker 1: travels around it at a speed of hundreds of thousands
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  Speaker 1: of kilometres per hour. Depending on exactly how that motion
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  Speaker 1: is measured, a useful figure is around 220 kilometres every second.
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  Speaker 1: At that speed, you might expect the journey to feel
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  Speaker 1: rather dramatic. It does not. There is no sensation of
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  Speaker 1: rushing through the Milky Way. No wind blows past the planets.
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  Speaker 1: Nothing in the night sky appears to be streaming backwards
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  Speaker 1: as we race along. Earth, the Sun, the nearby stars
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  Speaker 1: and a vast amount of surrounding material are all participating
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  Speaker 1: in the same broad galactic motion. So from our everyday perspective,
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  Speaker 1: the journey is almost perfectly hidden. One complete orbit of
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  Speaker 1: the Sun around the centre of the Milky Way takes
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  Speaker 1: roughly 230 million years. The last time the solar system
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  Speaker 1: occupied approximately this part of its galactic journey, dinosaurs had
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  Speaker 1: not yet appeared. And somewhere far ahead along that immense
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  Speaker 1: circular path, after continents have rearranged and entire branches of
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  Speaker 1: life have appeared and vanished, the solar system will eventually
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  Speaker 1: return to roughly the same region again. At the centre
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  Speaker 1: of all this motion lies the region containing Sagittarius A-star.
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  Speaker 1: There is, however, an important detail here. It is tempting
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  Speaker 1: to imagine the Milky Way as an enormous version of
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  Speaker 1: the solar system, with Sagittarius A star replacing the Sun
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  Speaker 1: and hundreds of billions of stars circling it, because the
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  Speaker 1: black hole's gravity holds everything together. That picture is not
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  Speaker 1: quite right. Sagittarius A star dominates gravity very close to
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  Speaker 1: the galactic centre, particularly among stars passing through its immediate neighbourhood.
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  Speaker 1: But out here, where the solar system lives. The black
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  Speaker 1: hole accounts for only a tiny fraction of the gravitational
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  Speaker 1: influence controlling our motion. The Milky Way contains hundreds of
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  Speaker 1: billions of stars, enormous clouds of gas and dust, and
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  Speaker 1: a much larger halo of dark matter extending far beyond
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  Speaker 1: the bright disk that we can see. The combined gravity
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  Speaker 1: of all that material shapes the sun's orbit. If Sagittarius
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  Speaker 1: A-star could somehow be replaced by another object containing exactly
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  Speaker 1: the same four million solar masses, our distant orbit would
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  Speaker 1: barely notice the difference. Gravity responds to mass and its distribution.
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  Speaker 1: It does not become magically stronger simply because that mass
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  Speaker 1: happens to belong to a black hole. So Sagittarius A-star
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  Speaker 1: is not a cosmic drain slowly pulling the entire galaxy
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  Speaker 1: towards destruction. It is something much more interesting. It is
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  Speaker 1: an extraordinarily compact concentration of mass, sitting at the dynamical
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  Speaker 1: heart of a much larger gravitational system, and for most
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  Speaker 1: of human history, nobody knew it was there. That is understandable.
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  Speaker 1: We live thousands of light-years away, buried inside the same
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  Speaker 1: galaxy we are trying to understand. We cannot simply fly
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  Speaker 1: above the Milky Way, turn around and photograph its centre.
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  Speaker 1: Worse still, enormous clouds of interstellar dust lie between us
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  Speaker 1: and the central regions, hiding much of what is there
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  Speaker 1: from ordinary visible light telescopes, and the object itself is dark.
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  Speaker 1: This created a wonderful astronomical problem. How do you discover
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  Speaker 1: something that emits no visible light, hidden behind thousands of
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  Speaker 1: light-years of dusty galaxy, when you cannot even step outside
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  Speaker 1: that galaxy to look back at it? For much of
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  Speaker 1: the 20th century, astronomers had to approach the problem indirectly.
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  Speaker 1: They could not simply point towards Sagittarius and see a
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  Speaker 1: black hole waiting conveniently in the centre of the picture. Instead,
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  Speaker 1: they had to learn to notice what the invisible object
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  Speaker 1: did to everything around it. Gravity became the clue. Radiation
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  Speaker 1: from the surrounding region became another, and eventually the movements
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  Speaker 1: of stars passing extraordinarily close to that empty point would
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  Speaker 1: reveal just how much mass was hiding there. Sagittarius a
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  Speaker 1: star may be invisible, but four million suns cannot sit
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  Speaker 1: quietly in one place without leaving evidence. The centre of
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  Speaker 1: our galaxy has been telling us what is there all along.
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  Speaker 1: We simply had to learn how to listen, finding the
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  Speaker 1: centre of the Milky Way sounds at first as though
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  Speaker 1: it should be fairly straightforward. After all, we know that
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  Speaker 1: our galaxy is a vast flattened collection of stars. We
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  Speaker 1: know it has a central region. and if we can
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  Speaker 1: look across millions or even billions of light-years to study
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  Speaker 1: other galaxies, it seems reasonable to imagine that looking towards
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  Speaker 1: the middle of our own should be one of astronomy's
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  Speaker 1: easier jobs. But living inside the Milky Way makes the
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  Speaker 1: problem surprisingly difficult. If we could somehow rise tens of
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  Speaker 1: thousands of light-years above the galactic disk, the overall structure
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  Speaker 1: would be much easier to understand. We would see a broad,
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  Speaker 1: flattened system of stars wrapped around a bright central region,
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  Speaker 1: with spiral structure extending through the disk. Instead, we are
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  Speaker 1: buried inside that disk. The solar system sits far from
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  Speaker 1: the centre surrounded by stars, gas and dust in almost
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  Speaker 1: every direction. Looking towards the galactic centre therefore means looking
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  Speaker 1: along the plane of the Milky Way itself through an
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  Speaker 1: enormous depth of material. It is rather like trying to
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  Speaker 1: identify the exact centre of a forest while standing among
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  Speaker 1: the trees. Except in this case, the forest is tens
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  Speaker 1: of thousands of light-years deep. The direction we need to
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  Speaker 1: look is towards the constellation Sagittarius. From a dark location,
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  Speaker 1: this part of the sky can appear extraordinarily rich. The
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  Speaker 1: Milky Way becomes brighter and broader there because we are
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  Speaker 1: looking towards denser regions of the galaxy. But brightness does
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  Speaker 1: not necessarily mean clarity. The line of sight towards the
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  Speaker 1: centre passes through countless foreground stars. Some are relatively close
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  Speaker 1: to the solar system. Others lie much farther away. Beyond
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  Speaker 1: them are immense clouds of gas and tiny solid particles.
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  Speaker 1: known collectively as interstellar dust. These dust particles are extremely small,
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  Speaker 1: often comparable in size to the wavelengths of visible light,
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  Speaker 1: but there are enormous numbers of them spread throughout the galaxy.
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  Speaker 1: Individual grains would hardly seem capable of hiding anything. Collect
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  Speaker 1: enough of them across thousands of light years, however, and
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  Speaker 1: the result is remarkably effective. Interstellar dust absorbs and scatters
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  Speaker 1: visible light. Shorter wavelengths are generally scattered and absorbed more
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  Speaker 1: strongly than longer wavelengths, which is why dust can both
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  Speaker 1: dim distant objects and alter their apparent colour. When astronomers
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  Speaker 1: look towards the centre of the Milky Way using ordinary
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  Speaker 1: visible light telescopes, they therefore encounter dark lanes cutting through
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  Speaker 1: the crowded star fields. Those dark regions are not empty.
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  Speaker 1: They are places where dust is blocking the light coming
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  Speaker 1: from objects behind it. This effect is known as interstellar extinction,
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  Speaker 1: and towards the galactic centre it is severe enough that
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  Speaker 1: visible light from the central regions is dramatically reduced before
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  Speaker 1: it ever reaches us. A larger visible light telescope can
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  Speaker 1: collect more of the light that survives the journey, but
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  Speaker 1: there is a limit to how useful that becomes. If
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  Speaker 1: a particular wavelength is being absorbed before it reaches Earth,
• 12:36
  Speaker 1: simply building a bigger mirror does not magically restore the
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  Speaker 1: missing information. A telescope cannot collect photons that never arrive.
• 12:44
  Speaker 1: Astronomers needed a different way of seeing. The solution came
• 12:48
  Speaker 1: from looking beyond the narrow range of electromagnetic radiation that
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  Speaker 1: human eyes happen to detect. Visible light is only one
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  Speaker 1: part of the electromagnetic spectrum. Infrared radiation has longer wavelengths
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  Speaker 1: than visible light, and those longer wavelengths are generally able
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  Speaker 1: to travel through interstellar dust more effectively. Radio waves, with
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  Speaker 1: much longer wavelengths again, can penetrate dusty regions even more successfully.
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  Speaker 1: The centre of the Milky Way did not suddenly become
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  Speaker 1: less crowded or less dusty when astronomers began using infrared
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  Speaker 1: and radio telescopes. Instead, we became better at looking through
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  Speaker 1: the material that had been hiding it. Radio astronomy provided
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  Speaker 1: some of the earliest powerful views into this concealed region,
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  Speaker 1: because radio waves could reach us through the galactic dust
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  Speaker 1: that blocked visible light. Infrared astronomy later became particularly important
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  Speaker 1: for studying the stars close to the very centre. The
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  Speaker 1: difference can be striking. A region that appears broken and
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  Speaker 1: obscured in visible light can become densely populated with stars
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  Speaker 1: when viewed at infrared wavelengths. This revealed that the centre
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  Speaker 1: of the galaxy is not simply a dark patch hidden
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  Speaker 1: behind dust. it contains an extraordinarily crowded stellar environment. Surrounding
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  Speaker 1: Sagittarius A star is what astronomers call the nuclear star cluster,
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  Speaker 1: one of the densest concentrations of stars in the Milky Way.
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  Speaker 1: Within this relatively small central region, stars are packed far
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  Speaker 1: more closely together than they are in the neighbourhood of
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  Speaker 1: the Sun. If our solar system were somehow moved into
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  Speaker 1: such an environment, the night sky would look profoundly different.
• 14:31
  Speaker 1: Bright nearby stars would be vastly more numerous, and the
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  Speaker 1: idea of an isolated Sun sitting comfortably several light-years from
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  Speaker 1: its nearest stellar neighbour would no longer apply. But resolving
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  Speaker 1: those individual stars from Earth was extremely difficult. The problem
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  Speaker 1: was not merely dust. The galactic centre is crowded. From
• 14:51
  Speaker 1: our perspective, stars separated by enormous distances in three-dimensional space
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  Speaker 1: can appear almost on top of one another in the sky.
• 15:00
  Speaker 1: Their light overlaps, and the atmosphere above Earth adds further
• 15:03
  Speaker 1: distortion to images made from the ground. So astronomers faced
• 15:07
  Speaker 1: several problems at once. They needed wavelengths capable of penetrating dust.
• 15:12
  Speaker 1: They needed detectors sensitive enough to record faint infrared radiation.
• 15:17
  Speaker 1: They needed telescopes with excellent resolution. And they needed methods
• 15:21
  Speaker 1: capable of separating individual stars inside one of the busiest
• 15:25
  Speaker 1: regions of the sky. This is why progress in studying
• 15:29
  Speaker 1: the galactic centre depended so heavily on technology. Infrared detectors improved.
• 15:36
  Speaker 1: Large ground-based telescopes became more capable. Techniques for correcting the
• 15:41
  Speaker 1: blurring caused by Earth's atmosphere became increasingly sophisticated. Gradually the
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  Speaker 1: central regions sharpened. Astronomers could begin to distinguish individual stars
• 15:52
  Speaker 1: close to Sagittarius, a star rather than seeing only an
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  Speaker 1: unresolved glow. This was an extraordinary change in perspective. For centuries,
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  Speaker 1: the centre of our own galaxy had effectively been hidden
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  Speaker 1: behind a natural curtain. Meanwhile, astronomers could study entire external
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  Speaker 1: galaxies floating in much clearer lines of sight far beyond
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  Speaker 1: the Milky Way. In some ways, humanity learned the shapes
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  Speaker 1: and structures of distant galaxies before it properly understood the
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  Speaker 1: centre of the one it actually lives inside. There is
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  Speaker 1: a slightly pleasing absurdity to that. We could look millions
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  Speaker 1: of light-years away and recognise spiral galaxies as enormous organised systems,
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  Speaker 1: while the middle of our own remained concealed behind dust
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  Speaker 1: just 26,000 light-years from home. But that apparent disadvantage eventually
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  Speaker 1: became scientifically useful. Once infrared observations began revealing individual stars
• 16:52
  Speaker 1: near Sagittarius A-star, Astronomers no longer needed to see the
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  Speaker 1: central object itself. They could watch the stars around it.
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  Speaker 1: And those stars were moving. Some were moving extraordinarily quickly.
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  Speaker 1: Their paths across the sky would eventually provide a way
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  Speaker 1: around the final obstacle. The centre of the Milky Way
• 17:12
  Speaker 1: might be hidden from ordinary sight. The object sitting there
• 17:15
  Speaker 1: might produce no visible light of its own, but gravity
• 17:19
  Speaker 1: does not need light in order to leave evidence. Long
• 17:22
  Speaker 1: before astronomers could follow individual stars sweeping around the centre
• 17:26
  Speaker 1: of the Milky Way, there was another kind of clue
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  Speaker 1: waiting in the darkness. It did not arrive as visible light.
• 17:33
  Speaker 1: It arrived as radio waves. In February of 1974, radio
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  Speaker 1: astronomers Bruce Ballick and Robert Brown were observing the centre
• 17:42
  Speaker 1: of the Milky Way using the facilities of the National
• 17:45
  Speaker 1: Radio Astronomy Observatory. Their attention was directed towards the complicated
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  Speaker 1: region in Sagittarius that astronomers already knew was producing unusually
• 17:55
  Speaker 1: strong radio emission. What they found was something much smaller.
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  Speaker 1: Buried within the broader glow was an exceptionally compact radio
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  Speaker 1: source lying almost exactly where the dynamical centre of the
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  Speaker 1: galaxy was expected to be. It was a tiny point
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  Speaker 1: inside an already confusing part of the sky. and it
• 18:16
  Speaker 1: would eventually become known as Sagittarius A-star. The name can
• 18:20
  Speaker 1: sound slightly misleading at first, because Sagittarius A-star is not
• 18:25
  Speaker 1: a star at all. The wider region, called Sagittarius A,
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  Speaker 1: had already been identified as a powerful and complicated source
• 18:32
  Speaker 1: of radio emission near the galactic centre. It includes several
• 18:36
  Speaker 1: distinct structures, produced by hot gas, supernova remnants, magnetic fields
• 18:42
  Speaker 1: and other energetic processes. The compact object discovered by Bruce
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  Speaker 1: Ballack and Robert Brown was different. It was extremely small
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  Speaker 1: on the sky, bright at radio wavelengths, and positioned at
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  Speaker 1: the heart of this larger radio-emitting region. The word star
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  Speaker 1: was added to the name to distinguish this particularly compact
• 19:02
  Speaker 1: component from the surrounding Sagittarius A complex. Robert Brown later
• 19:08
  Speaker 1: explained that the symbol had been inspired by the notation
• 19:11
  Speaker 1: chemists sometimes used for an excited state, because the discovery
• 19:15
  Speaker 1: itself felt exciting. For astronomers trying to understand the centre
• 19:19
  Speaker 1: of the Milky Way, radio astronomy had an enormous advantage.
• 19:24
  Speaker 1: Radio waves are part of the same electromagnetic spectrum as
• 19:28
  Speaker 1: visible light, but their wavelengths are much longer. That difference
• 19:32
  Speaker 1: matters when radiation passes through interstellar dust. The microscopic grains
• 19:38
  Speaker 1: that so effectively scatter and absorb visible light are far
• 19:42
  Speaker 1: less obstructive to many radio wavelengths, a region that appears
• 19:46
  Speaker 1: almost completely hidden in an ordinary optical image. can therefore
• 19:51
  Speaker 1: become accessible when observed with radio telescopes. The dust did
• 19:56
  Speaker 1: not disappear. Astronomers simply learned how to listen through it.
• 20:00
  Speaker 1: By the 1970s the galactic centre was already known to
• 20:03
  Speaker 1: be a strange and energetic environment. Gas there moved rapidly.
• 20:08
  Speaker 1: Radio emission came from several overlapping structures. Something was clearly
• 20:12
  Speaker 1: happening in that crowded central region. But Sagittarius A star
• 20:17
  Speaker 1: added a new level of intrigue. Here was a compact
• 20:20
  Speaker 1: radio source sitting at almost precisely the right location to
• 20:24
  Speaker 1: mark the centre of the galaxy. That was important. It
• 20:28
  Speaker 1: was not, however, immediate proof of a black hole. A
• 20:31
  Speaker 1: black hole does not naturally announce itself by placing a
• 20:34
  Speaker 1: convenient label in the sky, saying 4 million solar masses here.
• 20:38
  Speaker 1: Radio emission can be produced by many astronomical processes. Hot
• 20:43
  Speaker 1: gas can radiate. Charged particles moving through magnetic fields can
• 20:47
  Speaker 1: produce radio waves. Supernova remnants and other energetic environments can
• 20:52
  Speaker 1: be powerful radio sources. So the discovery of Sagittarius A
• 20:57
  Speaker 1: star established that there was something unusual and very compact
• 21:00
  Speaker 1: at the galactic centre. It did not yet establish exactly
• 21:04
  Speaker 1: what that something was. The real problem was mass. Even
• 21:08
  Speaker 1: before astronomers could track complete stellar orbits around the centre,
• 21:12
  Speaker 1: observations of material moving nearby were beginning to suggest that
• 21:16
  Speaker 1: an enormous amount of mass was concentrated there. Gas provided
• 21:21
  Speaker 1: some of the early evidence. Just as planets move faster
• 21:24
  Speaker 1: when they orbit close to a massive star, clouds of
• 21:27
  Speaker 1: gas can reveal the gravitational field through which they are travelling.
• 21:31
  Speaker 1: Spectroscopic measurements allow astronomers to determine how quickly gas is
• 21:35
  Speaker 1: moving towards or away from Earth. by examining shifts in
• 21:39
  Speaker 1: the wavelengths of the radiation it emits. Near the galactic centre,
• 21:43
  Speaker 1: some of those velocities were very large. Infrared observations became
• 21:47
  Speaker 1: especially important. Physicist Charles Townes, better known for fundamental work
• 21:53
  Speaker 1: connected with the MESTA and laser, was also deeply involved
• 21:57
  Speaker 1: in astronomical observations of the galactic centre. Charles Townes and
• 22:02
  Speaker 1: his colleagues studied infrared emission from gas in the central region.
• 22:06
  Speaker 1: and found evidence of rapid orbital motion around the centre.
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  Speaker 1: The faster material moves in an orbit, and the smaller
• 22:14
  Speaker 1: the region through which it moves, the more mass must
• 22:17
  Speaker 1: generally be present to provide the necessary gravitational attraction. These
• 22:22
  Speaker 1: measurements increasingly suggested that millions of times the mass of
• 22:25
  Speaker 1: the Sun were concentrated near the centre of the Milky Way.
• 22:29
  Speaker 1: That created an uncomfortable question. Where was all that mass?
• 22:34
  Speaker 1: If there were millions of ordinary stars crowded into the centre,
• 22:38
  Speaker 1: astronomers might expect to see their combined radiation. Perhaps instead,
• 22:43
  Speaker 1: the region contained vast numbers of objects that were difficult
• 22:46
  Speaker 1: to detect. One possibility was an extraordinarily dense collection of
• 22:51
  Speaker 1: faint stars. Another was a cluster dominated by stellar remnants
• 22:55
  Speaker 1: such as white dwarfs, neutron stars or smaller black holes.
• 23:01
  Speaker 1: These alternatives were not unreasonable simply because they sound complicated.
• 23:05
  Speaker 1: Science has to allow explanations to survive until observations rule
• 23:09
  Speaker 1: them out. A supermassive black hole was certainly an attractive possibility,
• 23:15
  Speaker 1: particularly as black holes had become increasingly serious objects of
• 23:19
  Speaker 1: astrophysical study during the 20th century. But an attractive possibility
• 23:24
  Speaker 1: is not the same thing as demonstrated reality. The challenge
• 23:28
  Speaker 1: was to show not only that a large amount of
• 23:31
  Speaker 1: mass existed near Sagittarius A star, but that this mass
• 23:35
  Speaker 1: was packed into such a tiny region that other explanations
• 23:38
  Speaker 1: became physically implausible. That distinction would become crucial. A million
• 23:44
  Speaker 1: suns spread over a large volume can simply be a
• 23:47
  Speaker 1: star cluster. Several million suns confined into an extraordinarily small
• 23:52
  Speaker 1: volume become something much more difficult to explain. As observations improved,
• 23:58
  Speaker 1: The suspected mass near the galactic centre kept becoming more concentrated.
• 24:04
  Speaker 1: The radio source remained extremely compact. Gas continued to move
• 24:08
  Speaker 1: rapidly around the central region. An infrared astronomy began revealing
• 24:12
  Speaker 1: stars travelling at remarkable speeds nearby. Slowly the question was changing.
• 24:19
  Speaker 1: Astronomers were no longer simply asking whether the centre of
• 24:22
  Speaker 1: the Milky Way was unusual. That part was becoming difficult
• 24:25
  Speaker 1: to dispute. The more interesting problem was determining what kind
• 24:29
  Speaker 1: of object could hide millions of solar masses in such
• 24:31
  Speaker 1: a small space, while producing almost none of the visible
• 24:35
  Speaker 1: light expected from millions of ordinary stars. Sagittarius A star
• 24:40
  Speaker 1: had become a marker. It pointed towards the place where
• 24:43
  Speaker 1: the mystery was deepest. But the strongest evidence would not
• 24:47
  Speaker 1: ultimately come from the radio source itself. It would come
• 24:50
  Speaker 1: from objects surrounding it. If enough mass was really concentrated
• 24:55
  Speaker 1: at that tiny point in the sky, nearby stars would
• 24:58
  Speaker 1: have no choice but to respond to its gravity. And
• 25:02
  Speaker 1: unlike the dark object they were circling, those stars could
• 25:05
  Speaker 1: be seen. The centre of the Milky Way was hidden,
• 25:10
  Speaker 1: but it was not unreachable. The difficulty was that astronomers
• 25:14
  Speaker 1: had spent centuries learning to observe the universe mainly with
• 25:17
  Speaker 1: visible light. and visible light was precisely what the dust
• 25:21
  Speaker 1: between us and the galactic centre was very good at blocking.
• 25:26
  Speaker 1: Infrared radiation offered a way around that problem. Near-infrared light
• 25:30
  Speaker 1: sits just beyond the red end of the visible spectrum.
• 25:34
  Speaker 1: Human eyes cannot detect it, but suitably designed instruments can,
• 25:39
  Speaker 1: and because its wavelengths are longer than those of visible light,
• 25:43
  Speaker 1: near-infrared radiation passes through interstellar dust far more easily. That
• 25:48
  Speaker 1: does not mean the dust becomes transparent in an absolute sense.
• 25:52
  Speaker 1: Some infrared radiation is still absorbed. But compared with visible light,
• 25:57
  Speaker 1: the improvement is enormous. Suddenly regions of the Milky Way
• 26:01
  Speaker 1: that had looked like dark, confused patches began revealing dense
• 26:05
  Speaker 1: fields of stars. This was particularly important near Sagittarius A-star.
• 26:11
  Speaker 1: The central region of the galaxy contains an extraordinary concentration
• 26:14
  Speaker 1: of stars packed together much more tightly than those in
• 26:17
  Speaker 1: the neighbourhood of the Sun. From Earth they appear extremely
• 26:21
  Speaker 1: close together on the sky, separated by angles so small
• 26:25
  Speaker 1: that distinguishing one from another becomes a technical challenge of
• 26:28
  Speaker 1: its own. And that brought astronomers to the next problem.
• 26:33
  Speaker 1: Even if infrared light could reach us through the dust,
• 26:36
  Speaker 1: Earth's atmosphere still stood in the way. When starlight enters
• 26:41
  Speaker 1: the atmosphere, it passes through constantly moving pockets of air
• 26:45
  Speaker 1: with slightly different temperatures and densities. These pockets bend the
• 26:50
  Speaker 1: incoming light by tiny changing amounts. That is why stars
• 26:54
  Speaker 1: appear to twinkle. It is charming when you're standing outside
• 26:58
  Speaker 1: on a clear night. It is rather less charming when
• 27:01
  Speaker 1: you're trying to measure the position of a star with
• 27:04
  Speaker 1: extreme precision. Astronomers call the blurring and distortion produced by
• 27:09
  Speaker 1: the atmosphere astronomical seeing. For ordinary observations, this may simply
• 27:14
  Speaker 1: make a point of light appear slightly fuzzy. At the
• 27:17
  Speaker 1: centre of the Milky Way, where many stars are packed
• 27:20
  Speaker 1: into a tiny area of sky, that fuzziness can cause
• 27:23
  Speaker 1: neighbouring stars to blur together. What astronomers needed was exceptional
• 27:29
  Speaker 1: angular resolutions. Angular resolution is the ability of a telescope
• 27:33
  Speaker 1: to distinguish two objects that appear very close together on
• 27:36
  Speaker 1: the sky. The smaller the angular separation a telescope can resolve,
• 27:41
  Speaker 1: the finer the detail it can reveal. Near Sagittarius, a star,
• 27:47
  Speaker 1: the separations involved can be tiny fractions of an arcsecond.
• 27:51
  Speaker 1: An arcsecond itself is already a very small angle. There
• 27:54
  Speaker 1: are 3,600 arcseconds in one degree. Trying to separate stars
• 27:59
  Speaker 1: at the galactic centre, therefore required instruments capable of detecting
• 28:04
  Speaker 1: shifts far smaller than anything the unaided human eye could
• 28:07
  Speaker 1: ever notice. Two major observatories became particularly important in this effort.
• 28:13
  Speaker 1: In Chile, the European Southern Observatory operated powerful telescopes that
• 28:18
  Speaker 1: would eventually include the Very Large Telescope at Paranal Observatory.
• 28:22
  Speaker 1: High above the Pacific Ocean in Hawaii, the W.M. Keck
• 28:26
  Speaker 1: Observatory used two enormous telescopes on the summit of Mauna Kea.
• 28:31
  Speaker 1: Both locations offer dark skies, high altitude and extremely capable instruments,
• 28:38
  Speaker 1: but even the finest ground-based telescope still has to look
• 28:40
  Speaker 1: through Earth's atmosphere. One early solution was a technique known
• 28:45
  Speaker 1: as speckle imaging. Instead of taking one long exposure and
• 28:49
  Speaker 1: allowing atmospheric turbulence to smear the image, astronomers took many
• 28:54
  Speaker 1: extremely short exposures. During each tiny fraction of a second,
• 28:59
  Speaker 1: The atmosphere had less time to distort the light. Each
• 29:02
  Speaker 1: image might look noisy or fragmented on its own, but
• 29:06
  Speaker 1: by combining large numbers of them mathematically, astronomers could recover
• 29:10
  Speaker 1: much finer detail than an ordinary long exposure would provide.
• 29:14
  Speaker 1: This technique became crucial for studying the innermost stars around
• 29:18
  Speaker 1: Sagittarius A star. Two independent research programmes would become especially important.
• 29:25
  Speaker 1: One was led by Reinhard Genzel. working with researchers associated
• 29:29
  Speaker 1: with the Max Planck Institute for Extraterrestrial Physics and the
• 29:33
  Speaker 1: European Southern Observatory. The other was led by Andrea Ghez
• 29:38
  Speaker 1: at the University of California, Los Angeles. using the W.M.
• 29:41
  Speaker 1: Keck Observatory. Both groups faced essentially the same challenge. They
• 29:46
  Speaker 1: needed to identify individual stars in the crowded galactic centre
• 29:52
  Speaker 1: and then keep finding those same stars again and again
• 29:54
  Speaker 1: over many years. Later, Adaptive optics transform the work even further.
• 30:01
  Speaker 1: Adaptive optics is one of those technologies that sounds almost
• 30:04
  Speaker 1: unreasonable until you realise that it actually works. The system
• 30:09
  Speaker 1: measures how Earth's atmosphere is distorting in coming light, often
• 30:13
  Speaker 1: using a bright reference star or an artificial laser guide star.
• 30:17
  Speaker 1: A deformable mirror is then adjusted extremely rapidly, changing its
• 30:22
  Speaker 1: shape many times each second to compensate for the atmospheric distortion.
• 30:26
  Speaker 1: The atmosphere blurs the image. The telescope responds by reshaping
• 30:31
  Speaker 1: part of its optical system in real time. The result
• 30:34
  Speaker 1: is a much sharper view. From the ground, astronomers could
• 30:37
  Speaker 1: begin obtaining infrared images with extraordinary detail, revealing stars that
• 30:42
  Speaker 1: would otherwise blend into one another. Now the central region
• 30:46
  Speaker 1: of the Milky Way was no longer simply a glowing blur.
• 30:50
  Speaker 1: It was becoming a moving map. This is where astrometry
• 30:54
  Speaker 1: became essential. Astrometry is the precise measurement of the positions
• 30:58
  Speaker 1: and motions of objects on the sky. The idea is simple.
• 31:02
  Speaker 1: Take an image. Measure where a star is. Wait. Take
• 31:07
  Speaker 1: another image. Measure again. But near Sagittarius, a star. The
• 31:12
  Speaker 1: precision required was astonishing. A movement that looks almost meaningless
• 31:17
  Speaker 1: in a single pair of images becomes significant when observations
• 31:20
  Speaker 1: are repeated year after year. A star shifts slightly. Then
• 31:25
  Speaker 1: it shifts again. Its direction changes. Its speed changes. Eventually,
• 31:31
  Speaker 1: the points begin tracing a curve. This was the great
• 31:35
  Speaker 1: advantage of patience. No single infrared image had to reveal
• 31:39
  Speaker 1: the black hole. Astronomers could allow gravity to draw the
• 31:43
  Speaker 1: evidence slowly for them. Over the years, Reinhard Genzel's team
• 31:48
  Speaker 1: and Andrea Ghez's team repeatedly returned to the galactic centre.
• 31:53
  Speaker 1: They measured stellar positions, They measured velocities. They compared new
• 31:57
  Speaker 1: observations with old ones. What had first looked like a
• 32:01
  Speaker 1: dense cluster of bright points began showing unmistakable motion. Some
• 32:06
  Speaker 1: stars were moving much faster than expected. Others were bending
• 32:10
  Speaker 1: around a common region. The closer they approached that region,
• 32:14
  Speaker 1: the more dramatic their motion became, and all of those
• 32:17
  Speaker 1: measurements were pointing towards the same place. Sagittarius A star
• 32:22
  Speaker 1: itself remained difficult to observe directly in the way one
• 32:25
  Speaker 1: might observe an ordinary star. But direct observation was becoming
• 32:29
  Speaker 1: less important. The stars around it were beginning to act
• 32:33
  Speaker 1: like traces. Each one carried information about the gravitational field
• 32:37
  Speaker 1: through which it moved. By watching their paths with enough precision,
• 32:42
  Speaker 1: astronomers could begin reconstructing the invisible mass sitting at the
• 32:45
  Speaker 1: centre of those orbits. The dust had hidden the galactic
• 32:49
  Speaker 1: centre from ordinary sight. Infrared astronomy opened the curtain. Speckle
• 32:55
  Speaker 1: imaging sharpened the view. Adaptive optics sharpened it further. And
• 33:00
  Speaker 1: astrometry turned those increasingly clear images into measurements of motion.
• 33:05
  Speaker 1: The question was no longer simply whether something unusual was
• 33:09
  Speaker 1: hidden near Sagittarius a star. Astronomers were beginning to watch
• 33:13
  Speaker 1: the invisible object reveal itself through gravity. one moving star
• 33:17
  Speaker 1: at a time. Once astronomers could see individual stars near
• 33:21
  Speaker 1: the centre of the Milky Way clearly enough, something remarkable
• 33:25
  Speaker 1: began to emerge. The stars were moving, not with the slow,
• 33:30
  Speaker 1: almost imperceptible drift that we usually associate with the distant sky,
• 33:35
  Speaker 1: but quickly enough that their changing positions could be measured
• 33:38
  Speaker 1: over the span of months and years. Some of the
• 33:41
  Speaker 1: most important became known collectively as the S-stars. These are
• 33:45
  Speaker 1: stars packed into the innermost region surrounding Sagittarius A star,
• 33:49
  Speaker 1: occupying a place where the gravitational environment is unlike almost
• 33:53
  Speaker 1: anywhere else that astronomers can observe so directly. At first,
• 33:58
  Speaker 1: individual measurements simply showed points of infrared light shifting across
• 34:02
  Speaker 1: the sky. A star might be recorded in one position
• 34:06
  Speaker 1: during one observing season, then appear slightly farther away a
• 34:10
  Speaker 1: year later. But as the observations accumulated, those movements stopped
• 34:15
  Speaker 1: looking random. The stars were not travelling along straight paths.
• 34:19
  Speaker 1: They were curving. Several appeared to be bending around the
• 34:22
  Speaker 1: same tiny region of apparently empty space. And that immediately
• 34:27
  Speaker 1: told astronomers something profound. Gravity was there. An object does
• 34:31
  Speaker 1: not need to be visible for its gravitational influence to
• 34:34
  Speaker 1: be measured. We rely on this idea throughout astronomy. Neptune
• 34:38
  Speaker 1: was discovered in part because its gravity disturbed the orbit
• 34:42
  Speaker 1: of Uranus before Neptune itself was directly observed. Planets around
• 34:47
  Speaker 1: distant stars can sometimes be inferred because they tug gently
• 34:51
  Speaker 1: on the stars they orbit. Dark matter is mapped through
• 34:54
  Speaker 1: its gravitational influence, even though it does not produce ordinary
• 34:58
  Speaker 1: visible light. At the galactic centre, the same basic principle
• 35:03
  Speaker 1: was operating on a spectacular scale. The S-stars were responding
• 35:07
  Speaker 1: to an unseen concentration of mass. Their paths became gravitational clues.
• 35:13
  Speaker 1: The underlying logic reaches back centuries. Johannes Kepler described the
• 35:18
  Speaker 1: mathematical patterns followed by planets orbiting the Sun. His first
• 35:22
  Speaker 1: law established that planetary orbits are ellipses, with the Sun
• 35:27
  Speaker 1: lying at one focus rather than precisely at the geometrical centre.
• 35:32
  Speaker 1: His second law showed that planets move faster when they
• 35:34
  Speaker 1: are closer to the Sun, and more slowly when they
• 35:37
  Speaker 1: are farther away. His third law connected the size of
• 35:41
  Speaker 1: an orbit with the time required to complete it. Later,
• 35:45
  Speaker 1: Isaac Newton explained why those patterns exist. Newton's theory of
• 35:49
  Speaker 1: gravity showed that the same attraction, which makes an apple
• 35:53
  Speaker 1: fall towards Earth, also governs the motion of the Moon,
• 35:57
  Speaker 1: the planets, and objects throughout the heavens. Orbital motion is
• 36:02
  Speaker 1: essentially a continuous fall. An orbiting object is being pulled
• 36:06
  Speaker 1: inward by gravity, but it also possesses sideways motion. Instead
• 36:10
  Speaker 1: of falling directly towards the central mass, it continually falls
• 36:15
  Speaker 1: around it. The stronger the gravitational field and the closer
• 36:19
  Speaker 1: the orbiting object passes to the centre, the more dramatic
• 36:22
  Speaker 1: its motion can become. This meant that the stars near
• 36:26
  Speaker 1: Sagittarius A star could be used much like planets around
• 36:30
  Speaker 1: the Sun. If astronomers could measure how quickly a star moved,
• 36:35
  Speaker 1: how far it travelled from the central point, and how
• 36:38
  Speaker 1: strongly its path curved, they could work backwards and calculate
• 36:42
  Speaker 1: how much mass was required to produce that orbit. They
• 36:46
  Speaker 1: did not need to see the mass itself. Gravity was
• 36:49
  Speaker 1: effectively weighing it for them. There was an important difference, however,
• 36:53
  Speaker 1: between early observations and what eventually became possible. Simply measuring
• 36:58
  Speaker 1: a star moving quickly near the galactic centre could suggest
• 37:02
  Speaker 1: the presence of a large mass. But a velocity measured
• 37:05
  Speaker 1: at one moment gives only part of the story. A
• 37:08
  Speaker 1: complete orbit is far more informative. As the years passed,
• 37:13
  Speaker 1: astronomers watched individual stars travel through substantial portions of their paths.
• 37:18
  Speaker 1: Curves became arcs. Arcs became increasingly complete ellipses. A star
• 37:24
  Speaker 1: that had once looked as though it was simply moving
• 37:26
  Speaker 1: rapidly across an image could eventually be seen turning around.
• 37:30
  Speaker 1: accelerating towards the central region, sweeping past it and moving
• 37:35
  Speaker 1: outward again. This transformed the evidence. The centre of attraction
• 37:40
  Speaker 1: was not wandering around. Different stars, observed at different distances
• 37:45
  Speaker 1: and travelling in different directions, were all responding to the
• 37:47
  Speaker 1: same remarkably compact gravitational focus. And the S-stars were not
• 37:52
  Speaker 1: arranged like the planets of the solar system. Our planets
• 37:56
  Speaker 1: travel in roughly the same plane because they formed from
• 37:59
  Speaker 1: a flattened disk of material surrounding the young Sun. The
• 38:03
  Speaker 1: innermost stellar population around Sagittarius, A star, is much less orderly.
• 38:09
  Speaker 1: The orbital planes of individual S stars are tilted in
• 38:11
  Speaker 1: different directions. One star may pass around the central mass
• 38:15
  Speaker 1: along one orientation, while another follows an orbit sharply inclined
• 38:19
  Speaker 1: to it. From Earth, we see these motions projected onto
• 38:23
  Speaker 1: the sky. but when astronomers combine positional measurements with information
• 38:28
  Speaker 1: about motion towards and away from us, a three-dimensional picture
• 38:31
  Speaker 1: can be reconstructed. That picture resembles a swarm. Stars loop
• 38:37
  Speaker 1: around the centre on differently oriented paths, crossing above and
• 38:41
  Speaker 1: below one another in three-dimensional space, while remaining gravitationally bound
• 38:46
  Speaker 1: to the same central mass. It is a much more
• 38:49
  Speaker 1: chaotic arrangement. than the neat planetary diagram familiar from school textbooks,
• 38:55
  Speaker 1: and that disorder was useful. Because stars approached the centre
• 39:00
  Speaker 1: from different directions, astronomers could test whether they were truly
• 39:04
  Speaker 1: orbiting one common gravitational source rather than following some accidental
• 39:09
  Speaker 1: pattern produced by projection. Again and again, the answer pointed
• 39:15
  Speaker 1: towards the same location. the apparently empty point associated with
• 39:19
  Speaker 1: Sagittarius A star. There is something extraordinary about the simplicity
• 39:24
  Speaker 1: of this evidence. 26,000 light-years away, astronomers were a using
• 39:30
  Speaker 1: stars as test particles. Nature had already placed them in
• 39:34
  Speaker 1: the experiment. No spacecraft needed to be sent towards the
• 39:38
  Speaker 1: galactic centre. No probe had to survive an encounter with
• 39:42
  Speaker 1: a supermassive black hole. The stars were already there, moving
• 39:47
  Speaker 1: naturally through the gravitational field. All astronomers had to do
• 39:50
  Speaker 1: was watch carefully enough and for long enough. Every position
• 39:54
  Speaker 1: added another piece of information. Every measured velocity narrowed the possibilities.
• 39:59
  Speaker 1: Every bend in an orbit revealed the direction of the
• 40:02
  Speaker 1: gravitational pull. And the more tightly those stars curved around
• 40:06
  Speaker 1: the centre, the harder it became to explain their motion
• 40:09
  Speaker 1: without placing an enormous amount of mass into an extraordinarily
• 40:13
  Speaker 1: small region. The stars themselves were visible. The thing controlling
• 40:18
  Speaker 1: them was not. That contrast made the galactic centre one
• 40:22
  Speaker 1: of the most elegant demonstrations of a basic principle in astronomy.
• 40:26
  Speaker 1: You do not always discover an object by seeing the object.
• 40:30
  Speaker 1: Sometimes you discover it by watching everything nearby behave as
• 40:33
  Speaker 1: though it must be there. And among all of those
• 40:36
  Speaker 1: rapidly moving points of infrared light, one star would soon
• 40:40
  Speaker 1: prove especially valuable. Its orbit was short enough to follow
• 40:44
  Speaker 1: within a human lifetime. elongated enough to carry it astonishingly
• 40:48
  Speaker 1: close to the central darkness, and fast enough near its
• 40:51
  Speaker 1: closest approach to reveal just how extreme the gravity of
• 40:55
  Speaker 1: Sagittarius a star really was. Among all the stars moving
• 40:59
  Speaker 1: around the centre of the Milky Way, one has become
• 41:02
  Speaker 1: especially important. It is called S2. At first glance, there
• 41:07
  Speaker 1: is nothing particularly extraordinary about the star itself. S2 is
• 41:11
  Speaker 1: a hot, bright star, several times more massive than the Sun.
• 41:15
  Speaker 1: If it lived in a quieter part of the galaxy,
• 41:17
  Speaker 1: it might be interesting without becoming famous. What makes S2
• 41:21
  Speaker 1: remarkable is where it lives. Its orbit carries it deep
• 41:25
  Speaker 1: into the gravitational environment surrounding Sagittarius A-star, close enough that
• 41:31
  Speaker 1: its motion provides one of the clearest demonstrations of the
• 41:34
  Speaker 1: enormous invisible mass sitting at the centre of the Milky Way.
• 41:38
  Speaker 1: And unlike many astronomical processes, its journey does not require
• 41:42
  Speaker 1: thousands or millions of years to observe. SE2 completes one
• 41:47
  Speaker 1: entire orbit in roughly 16 years. That is an astonishingly
• 41:52
  Speaker 1: convenient amount of time for astronomy. A scientist can begin
• 41:56
  Speaker 1: studying the star early in a career and, with enough patience,
• 42:00
  Speaker 1: watch it travel all the way around the galactic centre
• 42:03
  Speaker 1: and return to approximately where it began. The orbit is
• 42:06
  Speaker 1: not circular. It is strongly elliptical. For much of its journey,
• 42:11
  Speaker 1: S2 remains relatively far from Sagittarius A-star. Then, as it
• 42:17
  Speaker 1: falls inward along its elongated path, gravity accelerates it. The
• 42:22
  Speaker 1: star moves faster and faster, until, at its closest approach,
• 42:28
  Speaker 1: it sweeps past the central black hole at several thousand
• 42:32
  Speaker 1: kilometres every second. Stu approaches to less than 20 billion
• 42:37
  Speaker 1: kilometres from Sagittarius, a star. That might sound like an
• 42:41
  Speaker 1: enormous distance, and by ordinary standards it certainly is, It
• 42:46
  Speaker 1: is several times the distance between the Sun and Pluto,
• 42:49
  Speaker 1: but around a supermassive black hole containing roughly 4 million
• 42:53
  Speaker 1: solar masses, it is remarkably close. At that point in
• 42:57
  Speaker 1: its orbit, S2 reaches almost 3% of the speed of light.
• 43:01
  Speaker 1: Light travels at nearly 300,000 kilometres every second. S2, for
• 43:07
  Speaker 1: a brief portion of its journey, races through the galactic
• 43:10
  Speaker 1: centre at roughly 8,000 kilometres per second. Nothing about that
• 43:14
  Speaker 1: speed is noticeable to the star itself in the dramatic
• 43:17
  Speaker 1: way we might imagine. There is no cosmic wind rushing
• 43:21
  Speaker 1: over its surface simply because it is travelling quickly through space.
• 43:25
  Speaker 1: But from Earth, the change in velocity is spectacular evidence
• 43:29
  Speaker 1: of the gravity surrounding Sagittarius A star. Compare that with
• 43:33
  Speaker 1: the Sun. Our Sun also travels around the galactic centre,
• 43:37
  Speaker 1: but it does so from roughly 26,000 light years away.
• 43:41
  Speaker 1: Its speed is a little over 200 kilometres per second,
• 43:44
  Speaker 1: and one complete journey around the Milky Way takes roughly
• 43:48
  Speaker 1: 230 million years. SS2 lives in an entirely different gravitational neighbourhood.
• 43:54
  Speaker 1: It completes its circuit not in hundreds of millions of years,
• 43:57
  Speaker 1: but in about 16. The reason is not that SS2
• 44:01
  Speaker 1: possesses some unusual engine pushing it around the centre. Gravity
• 44:05
  Speaker 1: is doing the work. As the star falls towards Sagittarius A-star,
• 44:10
  Speaker 1: Gravitational potential energy is converted into kinetic energy. S2 accelerates.
• 44:16
  Speaker 1: After it swings around the central region and begins climbing
• 44:19
  Speaker 1: outward again, it gradually slows. It is Johannes Kepler's old
• 44:24
  Speaker 1: description of elliptical orbital motion made wonderfully extreme. Fast near
• 44:29
  Speaker 1: the closest point. Slower farther away. Except the object at
• 44:34
  Speaker 1: one focus of this ellipse is not an ordinary star.
• 44:37
  Speaker 1: It is a supermassive black hole. Considering how close S2 approaches,
• 44:44
  Speaker 1: another question naturally appears. Why is the star still there?
• 44:48
  Speaker 1: Black holes are often associated with stars being torn apart
• 44:52
  Speaker 1: by enormous differences in gravity across their bodies. This can happen.
• 44:57
  Speaker 1: If a star travels sufficiently close to a massive black hole,
• 45:01
  Speaker 1: the side nearest the black hole can experience noticeably stronger
• 45:05
  Speaker 1: gravity than the side farther away. Those differences are called
• 45:09
  Speaker 1: tidal forces. If they become strong enough, they can overwhelm
• 45:13
  Speaker 1: the star's own gravity and pull it apart in what
• 45:16
  Speaker 1: astronomers call a tidal disruption event. S2 does not pass
• 45:21
  Speaker 1: close enough for that to happen. Its orbit is extraordinarily
• 45:25
  Speaker 1: tight by stellar standards, but it still remains comfortably outside
• 45:29
  Speaker 1: the distance at which Sagittarius A star would tear a
• 45:32
  Speaker 1: star like S2 apart. So the star continues on its journey.
• 45:38
  Speaker 1: orbit after orbit, providing astronomers with repeated opportunities to measure
• 45:43
  Speaker 1: the gravitational environment. And those measurements have become remarkably precise.
• 45:48
  Speaker 1: One of the most important instruments involved is the European
• 45:51
  Speaker 1: Southern Observatory instrument known as GRAVITY. It operates with the
• 45:55
  Speaker 1: Very Large Telescope in Chile and uses a technique called interferometry.
• 46:01
  Speaker 1: Rather than relying on the light gathered by only one telescope,
• 46:05
  Speaker 1: Interferometry combines observations from multiple telescopes. When the light is
• 46:10
  Speaker 1: brought together with extraordinary precision, the separate telescopes can effectively
• 46:15
  Speaker 1: work as parts of a much larger observing system. The
• 46:18
  Speaker 1: result is extremely fine positional measurement. Around Sagittarius A star,
• 46:25
  Speaker 1: that precision matters enormously. Astronomers are trying to detect tiny
• 46:30
  Speaker 1: changes in the apparent position of a star located 26,000
• 46:34
  Speaker 1: light-years away, while it travels through the gravitational field of
• 46:38
  Speaker 1: a black hole. With measurements this precise, S2 became useful
• 46:44
  Speaker 1: for more than simply demonstrating that a large mass exists.
• 46:48
  Speaker 1: It became a laboratory for Albert Einstein's general theory of relativity.
• 46:53
  Speaker 1: One important test involved gravitational redshift. According to general relativity,
• 46:59
  Speaker 1: light climbing away from a strong gravitational field loses energy
• 47:03
  Speaker 1: from the perspective of a distant observer. Its wavelength becomes longer,
• 47:09
  Speaker 1: shifting towards the red end of the spectrum. When S2
• 47:13
  Speaker 1: made a close approach to Sagittarius A Star in 2018,
• 47:17
  Speaker 1: astronomers measured its light with exceptional precision. The observed shift
• 47:22
  Speaker 1: contained the relativistic effect predicted by Einstein's theory. The star
• 47:27
  Speaker 1: was not merely moving quickly through space. its light was
• 47:30
  Speaker 1: travelling outward through strongly curved spacetime. Another effect gradually appeared
• 47:36
  Speaker 1: in the orbit itself. In simple Newtonian gravity, an isolated
• 47:41
  Speaker 1: object following an elliptical orbit would repeatedly trace approximately the
• 47:46
  Speaker 1: same ellipse. General relativity predicts something slightly different in a
• 47:51
  Speaker 1: sufficiently strong gravitational field. The orientation of the orbit slowly rotates.
• 47:57
  Speaker 1: The point of closest approach advances from one revolution to another.
• 48:01
  Speaker 1: This is known as Schwarzschild precession. A similar effect occurs
• 48:05
  Speaker 1: in the orbit of Mercury around the Sun, although the
• 48:09
  Speaker 1: relativistic contribution there is extremely small. Around Sagittarius, a star,
• 48:15
  Speaker 1: the effect is much stronger. Careful observations of S2 reveal
• 48:20
  Speaker 1: that its orbit behaves in agreement with this prediction of
• 48:22
  Speaker 1: general relativity. The ellipse does not simply close perfectly upon itself.
• 48:28
  Speaker 1: It slowly turns. That tiny shift carries information about the
• 48:32
  Speaker 1: geometry of spacetime near the galactic centre. And this is
• 48:36
  Speaker 1: what makes S2 such a powerful piece of the story. Sagittarius.
• 48:41
  Speaker 1: A star is not inferred merely because many stars seem
• 48:45
  Speaker 1: to be moving rather quickly, somewhere near the middle of
• 48:47
  Speaker 1: the Milky Way. One particular star has been watched travelling
• 48:51
  Speaker 1: around the same invisible object for decades. Its position has
• 48:55
  Speaker 1: been measured. Its velocity has been measured. Its acceleration has
• 49:00
  Speaker 1: been measured. Astronomers have watched it fall inward, race through
• 49:04
  Speaker 1: its closest approach, turn around the central mass and climb
• 49:08
  Speaker 1: outward again. They have even detected subtle relativistic effects imprinted
• 49:13
  Speaker 1: upon its light in its orbit. S2 is effectively drawing
• 49:17
  Speaker 1: the gravitational landscape of the galactic system. centre for us.
• 49:21
  Speaker 1: There is no visible 4 million solar mass object sitting
• 49:25
  Speaker 1: at the focus of its ellipse, but the star behaves
• 49:28
  Speaker 1: exactly as though an extraordinary concentration of mass is there.
• 49:33
  Speaker 1: Every 16 years it goes around again, and every journey
• 49:36
  Speaker 1: traces the same invisible centre. Once astronomers could trace the
• 49:40
  Speaker 1: orbits of stars around Sagittarius a star, the invisible object
• 49:45
  Speaker 1: at the centre of the Milky Way could finally be weighed.
• 49:48
  Speaker 1: Not on a scale, of course. Gravity was the scale.
• 49:52
  Speaker 1: An orbit contains an enormous amount of information. If astronomers
• 49:56
  Speaker 1: know how far a star is from the centre of
• 49:58
  Speaker 1: its orbit, how quickly it is moving, and how long
• 50:02
  Speaker 1: it takes to complete that orbit, they can calculate how
• 50:05
  Speaker 1: much mass must be enclosed inside. The basic idea is
• 50:09
  Speaker 1: surprisingly intuitive. A star moving slowly in a very wide
• 50:13
  Speaker 1: orbit does not require an especially intense gravitational pull to
• 50:17
  Speaker 1: keep it there. A star racing through a very small
• 50:21
  Speaker 1: orbit is different. The tighter the orbit and the greater
• 50:24
  Speaker 1: the speed, the stronger the inward gravitational acceleration must be,
• 50:29
  Speaker 1: and stronger gravity at a known distance means more mass.
• 50:34
  Speaker 1: The S-stars therefore became a kind of gravitational balance, Each
• 50:38
  Speaker 1: orbit offered another way to measure the same central object.
• 50:42
  Speaker 1: S2 was especially valuable because its orbit is both short
• 50:46
  Speaker 1: and highly elongated, allowing astronomers to see it accelerate dramatically
• 50:51
  Speaker 1: as it approaches Sagittarius A star and then slow again
• 50:55
  Speaker 1: as it moves away. But S2 was not alone. Other
• 50:59
  Speaker 1: stars traced their own path through the central region. Some
• 51:03
  Speaker 1: approached more closely. Some travelled on differently tilted orbits. Some
• 51:08
  Speaker 1: required longer observational records. Together, they allowed astronomers to ask
• 51:13
  Speaker 1: not only how much mass was present, but where that
• 51:17
  Speaker 1: mass had to be located. The answer became increasingly difficult
• 51:20
  Speaker 1: to avoid. Roughly four million times the mass of the
• 51:24
  Speaker 1: Sun had to be concentrated within an extraordinarily small region
• 51:29
  Speaker 1: surrounding Sagittarius, a star. That distinction is crucial. Finding 4
• 51:35
  Speaker 1: million solar masses somewhere near the centre of the Milky
• 51:37
  Speaker 1: Way would not, by itself, prove the existence of a
• 51:40
  Speaker 1: black hole. The central region contains an enormous population of stars.
• 51:46
  Speaker 1: A sufficiently large cluster could easily contain millions of solar
• 51:49
  Speaker 1: masses spread across a broad volume. What the stellar orbits
• 51:53
  Speaker 1: showed was that the mass was not broadly distributed. It
• 51:56
  Speaker 1: was concentrated. Very concentrated. The innermost stars behaved as though
• 52:02
  Speaker 1: almost all of the mass influencing their motion was confined
• 52:06
  Speaker 1: inside their own tiny orbits. As observations improved, the permitted
• 52:11
  Speaker 1: size of that central mass became smaller and smaller. This
• 52:15
  Speaker 1: is where density became just as important as total mass.
• 52:20
  Speaker 1: Imagine trying to replace Sagittarius A star with a cluster
• 52:23
  Speaker 1: of ordinary stars. To reproduce the same gravitational effect, Millions
• 52:28
  Speaker 1: of solar masses would have to be packed into an
• 52:31
  Speaker 1: incredibly small space. The stars would be crowded together at
• 52:35
  Speaker 1: densities far beyond anything found in normal stellar systems. Close
• 52:40
  Speaker 1: encounters would become frequent. Collisions would occur. The cluster would
• 52:45
  Speaker 1: continually exchange energy between its members. Some stars would be
• 52:49
  Speaker 1: flung outward. Others would sink inward. Over time, the system
• 52:54
  Speaker 1: would either disperse, collapse further, or evolve into something very different.
• 53:00
  Speaker 1: Perhaps the mass could instead consist mostly of stellar remnants.
• 53:05
  Speaker 1: White dwarfs are smaller than ordinary stars. Neutron stars are
• 53:09
  Speaker 1: smaller still. Stellar-mass black holes would be more compact again.
• 53:14
  Speaker 1: But packing millions of solar masses worth of these objects
• 53:17
  Speaker 1: into the required volume creates much the same problem. Gravitational
• 53:21
  Speaker 1: interactions would be violent. Objects would scatter one another. Some
• 53:26
  Speaker 1: would merge. Others would escape. A cluster dense enough to
• 53:30
  Speaker 1: imitate the measured central mass would struggle to remain stable
• 53:34
  Speaker 1: for anything approaching the age of the galactic centre. The
• 53:38
  Speaker 1: alternatives did not merely look awkward. They became physically implausible.
• 53:43
  Speaker 1: The observations were also becoming increasingly three-dimensional. Images provided positions
• 53:49
  Speaker 1: across the sky. But knowing where a star appears in
• 53:52
  Speaker 1: an image is only part of its motions. astronomers also
• 53:57
  Speaker 1: needed to know whether it was moving towards Earth or
• 53:59
  Speaker 1: away from us. That information came from spectroscopy. By spreading
• 54:05
  Speaker 1: a star's light into a spectrum and measuring shifts in
• 54:08
  Speaker 1: characteristic absorption lines, astronomers could determine its radial velocity. If
• 54:14
  Speaker 1: the lines shifted towards shorter wavelengths, the star was moving
• 54:18
  Speaker 1: towards us. If they shifted towards longer wavelengths, it was
• 54:21
  Speaker 1: moving away. combine that information with the star's changing position
• 54:25
  Speaker 1: across the sky, and its orbit can be reconstructed in
• 54:29
  Speaker 1: three dimensions. The result is far more powerful than a
• 54:33
  Speaker 1: flat picture. Astronomers can determine the orientation of the orbit,
• 54:37
  Speaker 1: its eccentricity, its closest and farthest distances from the centre,
• 54:42
  Speaker 1: and the speed of the star at different points along
• 54:45
  Speaker 1: the path. When multiple stellar orbits are solved independently and
• 54:50
  Speaker 1: all converge on essentially the same tiny gravitational focus, coincidence
• 54:55
  Speaker 1: becomes increasingly difficult to maintain. Different stars, different orbital planes,
• 55:02
  Speaker 1: different velocities, the same central mass. The picture that emerged
• 55:07
  Speaker 1: was one of roughly 4 million solar masses confined within
• 55:11
  Speaker 1: a region so small that no stable collection of ordinary
• 55:15
  Speaker 1: known objects could reasonably account for it. A supermassive black
• 55:19
  Speaker 1: hole became the natural explanation. At the heart of that
• 55:23
  Speaker 1: object lies one of the most important concepts in black
• 55:26
  Speaker 1: hole physics, the event horizon. The event horizon is not
• 55:31
  Speaker 1: a solid surface. It is a boundary in space-time. Once
• 55:36
  Speaker 1: anything crosses it inward, no signal can return to the
• 55:39
  Speaker 1: outside universe. because every possible path leading into the future
• 55:44
  Speaker 1: points farther inward. For a non-rotating black hole of roughly
• 55:48
  Speaker 1: 4 million solar masses, the event horizon would extend only
• 55:52
  Speaker 1: tens of millions of kilometres across. That is tiny compared
• 55:56
  Speaker 1: with the dimensions of the surrounding star cluster, and it
• 55:59
  Speaker 1: is important to be precise about what the stellar observations
• 56:03
  Speaker 1: actually reveal. Astronomers did not see the event horizon by
• 56:07
  Speaker 1: watching the S-stars. Those stars orbit far outside it. What
• 56:12
  Speaker 1: they measured was the gravitational field created by the compact mass.
• 56:17
  Speaker 1: The event horizon follows from the black hole interpretation of
• 56:20
  Speaker 1: that mass, not from a direct visible boundary appearing in
• 56:23
  Speaker 1: the stellar data. Most of the motion of the S-stars
• 56:27
  Speaker 1: can be understood remarkably well using the gravitational ideas developed
• 56:31
  Speaker 1: by Isaac Newton. For much of their orbits, Newtonian mechanics
• 56:35
  Speaker 1: provides an excellent approximation. But as stars move closer to
• 56:40
  Speaker 1: Sagittarius a star and reach higher speeds, the small differences
• 56:44
  Speaker 1: predicted by Albert Einstein's general theory of relativity begin to matter.
• 56:49
  Speaker 1: Gravitational redshift becomes measurable. The orientation of an orbit slowly changes.
• 56:55
  Speaker 1: Spacetime itself must be treated as curved rather than gravity
• 56:59
  Speaker 1: being described only as an ordinary force. That combination is
• 57:03
  Speaker 1: rather elegant. Centuries-old orbital mechanics provides the basic weighing scale.
• 57:10
  Speaker 1: Modern spectroscopy and precision astrometry supply the measurements. General relativity
• 57:16
  Speaker 1: adds the subtle corrections, and all of them point towards
• 57:19
  Speaker 1: the same conclusion. At the centre of the Milky Way
• 57:22
  Speaker 1: lies a mass of roughly four million suns, packed into
• 57:26
  Speaker 1: a volume so extraordinarily small that every reasonable alternative gradually
• 57:31
  Speaker 1: falls away. The object remains dark, but by this point
• 57:36
  Speaker 1: it is no longer hidden in any meaningful scientific sense.
• 57:39
  Speaker 1: Its gravity has weighed it for us. By the time
• 57:43
  Speaker 1: the stellar orbits around Sagittarius S-star had been followed for years,
• 57:47
  Speaker 1: the evidence was no longer resting on one telescope, one
• 57:51
  Speaker 1: observing technique, or one research group. Two major teams had
• 57:55
  Speaker 1: spent decades watching the same crowded region of the sky.
• 58:00
  Speaker 1: One was led by Reinhard Genzel. working with the Max
• 58:03
  Speaker 1: Planck Institute for Extraterrestrial Physics and the European Southern Observatory.
• 58:09
  Speaker 1: The other was led by Andrea Ghez at the University
• 58:12
  Speaker 1: of California, Los Angeles, using the W.M. Keck Observatory in Hawaii.
• 58:17
  Speaker 1: Their approaches were not identical. They used different telescopes, different instruments,
• 58:22
  Speaker 1: different analysis methods and different teams of researchers. That independence mattered.
• 58:29
  Speaker 1: Science becomes much stronger when separate groups, working with different
• 58:34
  Speaker 1: equipment and making their own measurements, arrive at the same
• 58:38
  Speaker 1: underlying result. And that is what happened at the centre
• 58:41
  Speaker 1: of the Milky Way. Year after year, both teams returned
• 58:45
  Speaker 1: to Sagittarius A-star. Infrared detectors improved. Speckle imaging became more refined.
• 58:52
  Speaker 1: Adaptive optics sharpened the view. Spectroscopy provided increasingly precise stellar velocities.
• 58:59
  Speaker 1: Astronomachy measured positions with astonishing accuracy, and perhaps most importantly,
• 59:05
  Speaker 1: time itself began to work in the astronomer's favour. A
• 59:09
  Speaker 1: single observation can reveal where a star is. A few
• 59:12
  Speaker 1: years can reveal that it is moving. A decade can
• 59:15
  Speaker 1: begin to reveal the shape of its orbit. Several decades
• 59:19
  Speaker 1: can transform a collection of moving points into a detailed
• 59:22
  Speaker 1: gravitational map. This long observational baseline was essential. The galactic
• 59:28
  Speaker 1: centre could not be solved in one dramatic night of
• 59:31
  Speaker 1: telescope time. It required patience. The researchers had to return
• 59:35
  Speaker 1: to the same stars again and again, measure their positions,
• 59:39
  Speaker 1: record their spectra, refine their orbital solutions, and watch as
• 59:43
  Speaker 1: the curves became more complete. Gradually the evidence converged. The
• 59:48
  Speaker 1: central object was not merely massive. It was extraordinarily compact.
• 59:54
  Speaker 1: The motions of S2 and the other S stars demanded
• 59:57
  Speaker 1: roughly 4 million solar masses concentrated into a tiny region
• 1:00:02
  Speaker 1: around Sagittarius A star. No stable cluster of ordinary stars
• 1:00:07
  Speaker 1: could account for the measurements. Collections of white dwarfs, neutron
• 1:00:11
  Speaker 1: stars or stellar mass black holes could be imagined, but
• 1:00:15
  Speaker 1: when packed densely enough to reproduce the observed gravitational field,
• 1:00:20
  Speaker 1: such systems would be dynamically unstable over long periods. They
• 1:00:24
  Speaker 1: would scatter members outward, collapse inward, collide, merge, or otherwise
• 1:00:30
  Speaker 1: fail to remain in the required state for the age
• 1:00:32
  Speaker 1: of the galactic centre. The more accurately astronomers measured the region,
• 1:00:37
  Speaker 1: the less room remained for these alternatives. This is why
• 1:00:40
  Speaker 1: the conclusion has to be phrased carefully. Astronomers did not
• 1:00:44
  Speaker 1: look directly into the black hole and inspect what was inside.
• 1:00:48
  Speaker 1: The event horizon itself was not revealed by the stellar orbits.
• 1:00:53
  Speaker 1: What the observations demonstrated was the existence of an extraordinarily massive,
• 1:00:58
  Speaker 1: extraordinarily compact object at the centre of the galaxy. According
• 1:01:03
  Speaker 1: to known physics, a supermassive black hole is the only
• 1:01:06
  Speaker 1: viable explanation. That distinction is important because it reflects how
• 1:01:12
  Speaker 1: science often works. The strongest conclusions do not always come
• 1:01:16
  Speaker 1: from seeing the object itself. Sometimes they come from ruling
• 1:01:20
  Speaker 1: out every reasonable alternative, In 2020, this work was recognised
• 1:01:25
  Speaker 1: by the Royal Swedish Academy of Sciences with the Nobel
• 1:01:29
  Speaker 1: Prize in Physics. Half of the prize was awarded jointly
• 1:01:33
  Speaker 1: to Reinhard Genzel and Andrea Ghez for the discovery of
• 1:01:37
  Speaker 1: a supermassive compact object at the centre of our galaxy.
• 1:01:41
  Speaker 1: The wording was deliberately precise. A supermassive compact object. It
• 1:01:46
  Speaker 1: reflected the observational achievement itself. Their teams had shown that
• 1:01:51
  Speaker 1: an enormous amount of mass was confined within an extremely
• 1:01:54
  Speaker 1: small region, precisely where Sagittarius A star lies. Andrea Ghez
• 1:02:00
  Speaker 1: also became only the fourth woman in history to receive
• 1:02:03
  Speaker 1: the Nobel Prize in Physics. The other half of the
• 1:02:07
  Speaker 1: 2020 prize went to mathematician and physicist Roger Penrose. His
• 1:02:11
  Speaker 1: contribution came from a very different direction. Instead of observing
• 1:02:15
  Speaker 1: stars near the galactic centre … Roger Penrose worked on
• 1:02:18
  Speaker 1: the mathematics of Albert Einstein's general theory of relativity. In
• 1:02:23
  Speaker 1: the 1960s he demonstrated that gravitational collapse could naturally produce
• 1:02:27
  Speaker 1: a singularity under very general conditions. This was important because
• 1:02:32
  Speaker 1: black holes had sometimes been treated as mathematical curiosities, perhaps
• 1:02:37
  Speaker 1: created only by unrealistically perfect assumptions. Roger Penrose showed that
• 1:02:42
  Speaker 1: black hole formation was not merely an odd special case.
• 1:02:46
  Speaker 1: It was a robust consequence of general relativity. There is
• 1:02:50
  Speaker 1: something particularly fitting about those two halves of the same
• 1:02:53
  Speaker 1: Nobel Prize. On one side was theory. Albert Einstein's description
• 1:02:59
  Speaker 1: of curved spacetime had opened the possibility of black holes,
• 1:03:04
  Speaker 1: and Roger Penrose had shown that gravitational collapse could make
• 1:03:07
  Speaker 1: them unavoidable under the right conditions. On the other side
• 1:03:11
  Speaker 1: was observation. Reinhard Genzel, Andrea Ghez and their teams had
• 1:03:16
  Speaker 1: spent decades watching stars orbit an invisible concentration of mass
• 1:03:20
  Speaker 1: at the centre of the Milky Way. One part of
• 1:03:24
  Speaker 1: the story came from equations, the other came from tiny
• 1:03:27
  Speaker 1: points of infrared light moving across images, and they met
• 1:03:30
  Speaker 1: at Sagittarius A star. Perhaps the strangest part is how
• 1:03:35
  Speaker 1: little the central object itself had to do visibly. It
• 1:03:38
  Speaker 1: did not flash brightly. It did not reveal a surface.
• 1:03:42
  Speaker 1: It did not provide astronomers with a neat outline that
• 1:03:45
  Speaker 1: could simply be photographed and measured. For years, the most
• 1:03:48
  Speaker 1: important evidence was an apparently empty patch of sky. Around
• 1:03:52
  Speaker 1: that emptiness, stars moved. They accelerated, they curved, they completed ellipses.
• 1:04:00
  Speaker 1: And by following those movements with enough care, astronomers established
• 1:04:04
  Speaker 1: the presence of one of the most extraordinary objects in
• 1:04:07
  Speaker 1: the Milky Way. The black hole remained dark. Everything around
• 1:04:12
  Speaker 1: it supplied the proof. By the time astronomers had measured
• 1:04:16
  Speaker 1: the orbits of stars around Sagittarius A star, the case
• 1:04:20
  Speaker 1: for a supermassive black hole at the centre of the
• 1:04:22
  Speaker 1: Milky Way was extraordinarily strong. But there was still something
• 1:04:27
  Speaker 1: they had never done. They had never resolved the immediate
• 1:04:30
  Speaker 1: surroundings of the event horizon itself. That required a completely
• 1:04:35
  Speaker 1: different kind of telescope. The problem was not that Sagittarius
• 1:04:39
  Speaker 1: A star was too faint in every form of light.
• 1:04:42
  Speaker 1: Hot material around the black hole produces radio emission that
• 1:04:45
  Speaker 1: can reach Earth. The problem was its apparent size. From
• 1:04:50
  Speaker 1: a distance of roughly 26,000 light years, the region immediately
• 1:04:54
  Speaker 1: surrounding the black hole occupies an almost unimaginably tiny angle
• 1:04:58
  Speaker 1: on the sky. It is rather like trying to see
• 1:05:01
  Speaker 1: something the size of a piece of fruit on the
• 1:05:03
  Speaker 1: surface of the moon. No ordinary telescope could resolve it,
• 1:05:08
  Speaker 1: so astronomers created one with a diameter comparable to Earth itself.
• 1:05:12
  Speaker 1: The project became known as the Event Horizon Telescope Collaboration.
• 1:05:17
  Speaker 1: Despite the name, the Event Horizon Telescope is not a
• 1:05:21
  Speaker 1: single instrument sitting beneath one enormous dome. It is a
• 1:05:24
  Speaker 1: network of radio observatories spread across widely separated locations on Earth. Together,
• 1:05:30
  Speaker 1: those observatories can behave, in an important sense, like parts
• 1:05:35
  Speaker 1: of one planetary-scale telescope. The technique that makes this possible
• 1:05:39
  Speaker 1: is called very long baseline interferometry. The basic idea begins
• 1:05:44
  Speaker 1: with a simple fact about resolution. When astronomers want to
• 1:05:48
  Speaker 1: distinguish extremely small details, the separation between different parts of
• 1:05:53
  Speaker 1: a telescope matters. A larger effective diameter allows finer angular
• 1:05:58
  Speaker 1: detail to be resolved. For an ordinary optical telescope, That
• 1:06:02
  Speaker 1: diameter is limited by the size of the mirror. For
• 1:06:06
  Speaker 1: an interferometer, separate telescopes can observe the same astronomical source
• 1:06:10
  Speaker 1: simultaneously and their measurements can later be combined. The distance
• 1:06:16
  Speaker 1: between the telescopes becomes part of the effective baseline. Place
• 1:06:20
  Speaker 1: observatories hundreds of kilometres apart and the baseline becomes enormous.
• 1:06:26
  Speaker 1: Place them on different continents and it becomes comparable to
• 1:06:29
  Speaker 1: the diameter of the planet. No one had to build
• 1:06:32
  Speaker 1: a radio dish 8,000 miles wide. Which is fortunate, because
• 1:06:37
  Speaker 1: finding somewhere to put it would have been awkward. Instead,
• 1:06:41
  Speaker 1: observatories across Earth watched Sagittarius A star at the same time.
• 1:06:46
  Speaker 1: Among the most important was the Atacama Large Millimeter and
• 1:06:50
  Speaker 1: Submillimeter Array, located high on the Chajnanta Plateau in northern Chile.
• 1:06:56
  Speaker 1: Other participating observatories were distributed across places including Hawaii, Mexico, Arizona,
• 1:07:03
  Speaker 1: Spain and Antarctica. The greater the geographical spread, the more
• 1:07:08
  Speaker 1: information the network could collect about the tiny structure surrounding
• 1:07:12
  Speaker 1: the black hole. The observations were made at wavelengths of
• 1:07:15
  Speaker 1: roughly 1.3 millimetres. That choice was extremely important. At much
• 1:07:21
  Speaker 1: longer radio wavelengths, the achievable resolution would not be fine
• 1:07:26
  Speaker 1: enough to reveal structures on the scale of the black
• 1:07:28
  Speaker 1: hole's shadow. At 1.3 millimetres, the wavelength is short enough
• 1:07:33
  Speaker 1: to provide extraordinary angular resolution when combined with an Earth-sized baseline.
• 1:07:40
  Speaker 1: It also has another advantage. Radiation at these wavelengths can
• 1:07:44
  Speaker 1: pass through much of the interstellar material lying between Earth
• 1:07:47
  Speaker 1: and the galactic centre. The dust that makes visible light
• 1:07:52
  Speaker 1: observations of the region so difficult becomes far less troublesome.
• 1:07:56
  Speaker 1: but combining observatories scattered across the planet is not as
• 1:08:00
  Speaker 1: simple as pointing them in the same direction. Radio waves
• 1:08:04
  Speaker 1: arriving from Sagittarius. A star reached different locations on Earth
• 1:08:09
  Speaker 1: at slightly different times. Those tiny timing differences contain the
• 1:08:13
  Speaker 1: information needed to reconstruct extremely fine detail. So the observations
• 1:08:19
  Speaker 1: must be recorded with extraordinary precision. Each participating observatory uses
• 1:08:24
  Speaker 1: an atomic clock to provide an exceptionally accurate time reference.
• 1:08:28
  Speaker 1: The radio signals are recorded along with that timing information,
• 1:08:32
  Speaker 1: allowing researchers to determine how signals detected thousands of kilometres
• 1:08:36
  Speaker 1: apart relate to one another. The amount of data involved
• 1:08:40
  Speaker 1: is enormous! Rather than simply transmitting everything across the internet
• 1:08:44
  Speaker 1: in real time, observatories have recorded vast quantities of information
• 1:08:50
  Speaker 1: onto physical storage systems. Those recordings can then be transported
• 1:08:54
  Speaker 1: to specialised computing centres where the separate observations are brought together.
• 1:09:00
  Speaker 1: In some cases, moving the data physically is simply more
• 1:09:03
  Speaker 1: practical than trying to send such enormous quantities through ordinary
• 1:09:08
  Speaker 1: communications networks. So part of the most advanced telescope ever
• 1:09:13
  Speaker 1: aimed at a black hole has depended on a method
• 1:09:15
  Speaker 1: with a pleasantly old-fashioned quality. You observe the universe with
• 1:09:19
  Speaker 1: atomic clocks and some of the most sophisticated radio equipment
• 1:09:23
  Speaker 1: on Earth. Then somebody has to move the hard drives.
• 1:09:27
  Speaker 1: Once the information arrives, the difficult work continues. The signals
• 1:09:32
  Speaker 1: from the different observatories must be synchronised and compared with
• 1:09:36
  Speaker 1: extraordinary care. Computers search for correlations between recordings made at
• 1:09:41
  Speaker 1: different sites. Those correlations allow astronomers to reconstruct information about
• 1:09:46
  Speaker 1: the distribution of radio emission on scales no individual telescope
• 1:09:50
  Speaker 1: could resolve alone. This is why the Event Horizon Telescope
• 1:09:54
  Speaker 1: is not simply a story about collecting more light. Light gathering,
• 1:09:58
  Speaker 1: power and resolution are related but distinct ideas. A large
• 1:10:02
  Speaker 1: collecting area helps a telescope detect faint signals. Resolution determines
• 1:10:07
  Speaker 1: how finely those signals can be separated in angle. A
• 1:10:11
  Speaker 1: single gigantic radio dish would certainly collect more radiation, but
• 1:10:15
  Speaker 1: unless its physical diameter approached the size of Earth, it
• 1:10:19
  Speaker 1: could not match the angular resolution produced by a planetary interferometer.
• 1:10:24
  Speaker 1: The Event Horizon Telescope solved the problem by turning geography
• 1:10:27
  Speaker 1: itself into part of the instrument. And even after all
• 1:10:31
  Speaker 1: of this, astronomers were not expecting to photograph the black
• 1:10:34
  Speaker 1: hole in the way one might photograph a planet. A
• 1:10:38
  Speaker 1: black hole is dark. Radiation crossing its event horizon cannot
• 1:10:43
  Speaker 1: come back out. What the network could detect was the
• 1:10:46
  Speaker 1: material immediately around it. Gas falling towards Sagittarius. A star
• 1:10:52
  Speaker 1: becomes extremely hot. Charged particles move through powerful magnetic fields.
• 1:10:58
  Speaker 1: The plasma produces radio emission while racing through spacetime that
• 1:11:02
  Speaker 1: is severely curved by the black hole's gravity. Some photons
• 1:11:07
  Speaker 1: escape towards Earth. Others are bent onto dramatic paths. Some
• 1:11:11
  Speaker 1: disappear across the event horizon. General relativity predicts that this
• 1:11:15
  Speaker 1: interaction between light and curved spacetime should produce a dark
• 1:11:19
  Speaker 1: central region, surrounded by bright emission. The dark area is
• 1:11:24
  Speaker 1: commonly called the black hole's shadow. It is not the
• 1:11:27
  Speaker 1: event horizon itself. The shadow appears larger than the horizon
• 1:11:31
  Speaker 1: because the black hole's gravity bends the trajectories of nearby light.
• 1:11:36
  Speaker 1: That was the structure astronomers hoped to resolve. Not a
• 1:11:40
  Speaker 1: solid black surface. Not a photograph of whatever might somehow
• 1:11:44
  Speaker 1: lie inside. But the imprint that a black hole leaves
• 1:11:48
  Speaker 1: upon radiation passing through the space-time around it. For decades,
• 1:11:53
  Speaker 1: Sagittarius A-star had been revealed through the behaviour of objects
• 1:11:57
  Speaker 1: much farther away. Gas moved around it. Stars orbited it.
• 1:12:03
  Speaker 1: Their velocities and trajectories gave away its enormous mass. Now
• 1:12:08
  Speaker 1: astronomers were attempting something far more intimate. They were trying
• 1:12:12
  Speaker 1: to resolve glowing plasma on scales, comparable to the black
• 1:12:16
  Speaker 1: hole itself. To do it, they synchronised radio observatories across continents,
• 1:12:22
  Speaker 1: tied them together with atomic clocks, recorded immense quantities of data,
• 1:12:28
  Speaker 1: and used mathematics to make the planet behave as though
• 1:12:31
  Speaker 1: it were one enormous telescope. Earth itself had become part
• 1:12:35
  Speaker 1: of the instrument. In April of 2017, radio telescopes scattered
• 1:12:41
  Speaker 1: across Earth turned towards the centre of the Milky Way.
• 1:12:45
  Speaker 1: They were all watching the same tiny region of sky.
• 1:12:49
  Speaker 1: They were all recording radiation at almost exactly the same time.
• 1:12:53
  Speaker 1: And although each observatory was separated from the others by
• 1:12:56
  Speaker 1: thousands of kilometres, together they were attempting something that had
• 1:13:00
  Speaker 1: never been done before. They were trying to resolve the
• 1:13:03
  Speaker 1: immediate surroundings of Sagittarius A star. The observations lasted only
• 1:13:08
  Speaker 1: a matter of days. The result took years. The Event
• 1:13:13
  Speaker 1: Horizon Telescope collaboration had already demonstrated what its planetary network
• 1:13:17
  Speaker 1: could do when, in 2019, it released the first image
• 1:13:22
  Speaker 1: of the supermassive black hole in the galaxy Messier 87.
• 1:13:27
  Speaker 1: That image became instantly recognisable. A dark central region. a
• 1:13:32
  Speaker 1: glowing ring around it, a shape predicted by general relativity
• 1:13:36
  Speaker 1: and reconstructed from radio signals recorded across an entire planet.
• 1:13:41
  Speaker 1: But Sagittarius A-star was different. It was our black hole.
• 1:13:46
  Speaker 1: It sat at the centre of our own galaxy, only
• 1:13:49
  Speaker 1: around 26,000 light-years away rather than tens of millions of
• 1:13:53
  Speaker 1: light-years away. And yet, strangely, it proved more difficult to image.
• 1:13:59
  Speaker 1: observations gathered in 2017 had to be combined with enormous care.
• 1:14:05
  Speaker 1: Signals recorded at different observatories had to be synchronised using
• 1:14:09
  Speaker 1: atomic clocks. The data had to be correlated. Atmospheric effects
• 1:14:13
  Speaker 1: had to be accounted for. Different reconstruction methods had to
• 1:14:17
  Speaker 1: be tested. And most importantly, astronomers had to deal with
• 1:14:20
  Speaker 1: the fact that the object they were trying to image
• 1:14:22
  Speaker 1: would not sit still. Eventually, in May of 2022, The
• 1:14:27
  Speaker 1: Event Horizon Telescope collaboration unveiled the first horizon-scale image of Sagittarius,
• 1:14:34
  Speaker 1: a star. The image looked both strange and familiar. A
• 1:14:39
  Speaker 1: glowing orange ring surrounded a darker central region. But it
• 1:14:43
  Speaker 1: is important to understand what that picture actually shows. It
• 1:14:48
  Speaker 1: is not a conventional photograph of a black hole. There
• 1:14:51
  Speaker 1: is no camera in space looking directly at a dark sphere.
• 1:14:55
  Speaker 1: The orange colouring is not the natural colour of the object.
• 1:14:58
  Speaker 1: Radio waves do not have a colour that human eyes
• 1:15:01
  Speaker 1: can see. The image is a representation of radio emission
• 1:15:05
  Speaker 1: coming from extremely hot plasma surrounding the black hole. The
• 1:15:09
  Speaker 1: central darkness is what astronomers call the black hole shadow,
• 1:15:13
  Speaker 1: and even that shadow should not be confused with the
• 1:15:15
  Speaker 1: event horizon itself. The event horizon is the boundary beyond
• 1:15:19
  Speaker 1: which light can no longer escape to the outside universe.
• 1:15:23
  Speaker 1: The shadow is larger. It is produced because the black
• 1:15:26
  Speaker 1: hole's gravity bends the paths of nearby photons so strongly
• 1:15:32
  Speaker 1: that a dark region appears against the brighter radiation around it.
• 1:15:36
  Speaker 1: This bending of light is gravitational lensing. According to Albert
• 1:15:41
  Speaker 1: Einstein's general theory of relativity, mass and energy curve spacetime.
• 1:15:47
  Speaker 1: Light follows the geometry of that curve spacetime. Near a
• 1:15:50
  Speaker 1: supermassive black hole, the effect becomes extreme. Photons emitted by
• 1:15:55
  Speaker 1: hot plasma can be deflected around the black hole. Some
• 1:15:59
  Speaker 1: may travel around part of it before escaping. Some may
• 1:16:02
  Speaker 1: be bent dramatically towards us. Others disappear across the event
• 1:16:06
  Speaker 1: horizon and never reach the outside universe again. The result
• 1:16:10
  Speaker 1: is a ring-like concentration of light around a central shadow.
• 1:16:15
  Speaker 1: The apparent size of that ring is almost unimaginably small.
• 1:16:19
  Speaker 1: The Event Horizon Telescope measured it at roughly 52 millionths
• 1:16:23
  Speaker 1: of an arcsecond across. An arcsecond is already only 1
• 1:16:27
  Speaker 1: 3600th of a degree. Now divide that tiny angle by
• 1:16:31
  Speaker 1: a million. Then take only about 52 of those pieces.
• 1:16:36
  Speaker 1: That is approximately how large the ring around Sagittarius A
• 1:16:39
  Speaker 1: star appears from Earth. It is one of the reasons
• 1:16:42
  Speaker 1: the achievement required an Earth-sized interferometer. A conventional telescope, no
• 1:16:47
  Speaker 1: matter how carefully built, simply could not resolve something that
• 1:16:51
  Speaker 1: small from 26,000 light years away. When the image was released,
• 1:16:56
  Speaker 1: one of the most striking things was how similar Sagittarius
• 1:17:00
  Speaker 1: A star looked to the earlier image from Messier 87.
• 1:17:05
  Speaker 1: The two black holes are profoundly different in scale. The
• 1:17:08
  Speaker 1: black hole in Messier 87 contains billions of times the
• 1:17:12
  Speaker 1: mass of the Sun. Sagittarius A star contains roughly 4
• 1:17:16
  Speaker 1: million solar masses. The difference in mass exceeds a factor
• 1:17:20
  Speaker 1: of 1000, and yet both images show the same broad
• 1:17:24
  Speaker 1: structure– a dark shadow, a surrounding ring, the same basic
• 1:17:29
  Speaker 1: behaviour of light in strongly curved spacetime. That similarity is
• 1:17:33
  Speaker 1: exactly what general relativity predicts. The laws governing black holes
• 1:17:38
  Speaker 1: do not suddenly change because one black hole is dramatically
• 1:17:41
  Speaker 1: larger than another. Their physical scales change, their timescales change.
• 1:17:47
  Speaker 1: but the geometry around them follows the same underlying principles.
• 1:17:51
  Speaker 1: And this difference in timescale is part of the reason
• 1:17:54
  Speaker 1: Sagittarius A star was so difficult to image. At first,
• 1:17:58
  Speaker 1: being closer sounds as though it should have made the
• 1:18:00
  Speaker 1: job easier. But Sagittarius A star is much less massive
• 1:18:05
  Speaker 1: than the black hole in Messier 87. That means the
• 1:18:08
  Speaker 1: characteristic timescales of motion close to it are much shorter.
• 1:18:13
  Speaker 1: Gas and plasma near Sagittarius A star can change noticeably
• 1:18:16
  Speaker 1: within minutes. Material can move through a substantial fraction of
• 1:18:20
  Speaker 1: an orbit while observations are still being collected. By contrast,
• 1:18:24
  Speaker 1: the much larger black hole in Messier 87 changes more slowly.
• 1:18:29
  Speaker 1: During a night's observations, its immediate surroundings remain relatively stable.
• 1:18:34
  Speaker 1: Sagittarius A star does not have that courtesy. The challenge
• 1:18:38
  Speaker 1: is a little like trying to make a long exposure
• 1:18:40
  Speaker 1: photograph of an object that keeps changing shape before the
• 1:18:44
  Speaker 1: exposure is finished. except this was not a conventional long
• 1:18:48
  Speaker 1: exposure photograph. The image had to be reconstructed statistically from
• 1:18:52
  Speaker 1: measurements collected by radio observatories across Earth. Researchers therefore had
• 1:18:57
  Speaker 1: to develop methods capable of handling a source that was
• 1:19:00
  Speaker 1: variable while still extracting the stable underlying features shared across
• 1:19:04
  Speaker 1: the data. The released image represents that deeper structure rather
• 1:19:08
  Speaker 1: than one frozen instant, and the result provided an important
• 1:19:12
  Speaker 1: consistency check. Long before the image existed, astronomers had already
• 1:19:17
  Speaker 1: measured the mass of Sagittarius A star using the orbits
• 1:19:21
  Speaker 1: of nearby stars. They also knew its distance from Earth,
• 1:19:24
  Speaker 1: with increasing precision. Those two quantities allowed general relativity to
• 1:19:29
  Speaker 1: predict approximately how large the black hole shadow should appear.
• 1:19:33
  Speaker 1: The observed ring closely matched those expectations. That agreement mattered.
• 1:19:39
  Speaker 1: The stellar orbits had measured the gravitational influence of the
• 1:19:42
  Speaker 1: central mass on scales, much larger than the Event Horizon.
• 1:19:47
  Speaker 1: The Event Horizon telescope was now probing scales close to
• 1:19:50
  Speaker 1: the black hole itself. Two completely different types of observation
• 1:19:54
  Speaker 1: were pointing towards the same object, the same mass, the
• 1:19:58
  Speaker 1: same location, the same general relativistic description. there is a
• 1:20:03
  Speaker 1: satisfying progression to the story. In 1974, Bruce Ballack and
• 1:20:09
  Speaker 1: Robert Brown identified an unusually compact radio source at the
• 1:20:12
  Speaker 1: centre of the Milky Way. For years, its true nature
• 1:20:16
  Speaker 1: remained uncertain. Then, infrared astronomy revealed stars moving around an
• 1:20:21
  Speaker 1: invisible concentration of mass. Their orbits showed that roughly 4
• 1:20:25
  Speaker 1: million solar masses had been compressed into a tiny region.
• 1:20:30
  Speaker 1: S2 traced that gravitational field in extraordinary detail. General relativity
• 1:20:35
  Speaker 1: appeared in the subtle behavior of its light and its orbit.
• 1:20:39
  Speaker 1: And then, nearly half a century after Sagittarius, a star,
• 1:20:42
  Speaker 1: was first identified as a compact radio source, astronomers finally
• 1:20:47
  Speaker 1: resolved the glowing material immediately around the black hole itself.
• 1:20:51
  Speaker 1: The object had gone from a mysterious radio point to
• 1:20:54
  Speaker 1: an unseen mass, to a measured gravitational source. and finally
• 1:20:59
  Speaker 1: to a dark shadow surrounded by light. The black hole
• 1:21:02
  Speaker 1: itself was still invisible. In a sense, that was the
• 1:21:06
  Speaker 1: whole point. What astronomers had finally imaged was the way
• 1:21:10
  Speaker 1: light behaves when it reaches the edge of that darkness.
• 1:21:14
  Speaker 1: The image of Sagittarius Earth Star gives the centre of
• 1:21:16
  Speaker 1: the Milky Way an almost peaceful appearance, a dark region
• 1:21:20
  Speaker 1: surrounded by a soft ring of glowing material. The reality
• 1:21:24
  Speaker 1: is considerably busier. The space surrounding the black hole contains
• 1:21:29
  Speaker 1: one of the most crowded and energetic environments anywhere in
• 1:21:32
  Speaker 1: our galaxy. Stars rush through the central region at extraordinary speeds.
• 1:21:38
  Speaker 1: Gas drifts between them. Dust is heated and disturbed. Magnetic
• 1:21:43
  Speaker 1: fields twist through hot plasma. Powerful radiation appears and fades
• 1:21:48
  Speaker 1: on short timescales. And at the centre of it all
• 1:21:51
  Speaker 1: sits Sagittarius A star. Immediately surrounding the black hole is
• 1:21:56
  Speaker 1: the dense nuclear star cluster. Millions of stars occupy the
• 1:22:00
  Speaker 1: wider central region, with stellar densities rising dramatically towards the
• 1:22:04
  Speaker 1: galactic centre. Some of these stars are young and massive.
• 1:22:09
  Speaker 1: Others are much older. Stellar winds blow material into the
• 1:22:13
  Speaker 1: surrounding space, particularly from large, hot stars that lose substantial
• 1:22:18
  Speaker 1: amounts of gas during their lives. That gas provides one
• 1:22:22
  Speaker 1: possible source of food for Sagittarius A star. But black
• 1:22:26
  Speaker 1: holes do not eat in quite the way their reputation
• 1:22:29
  Speaker 1: sometimes suggests. The familiar picture of a black hole acting
• 1:22:33
  Speaker 1: like a gigantic vacuum cleaner is misleading. Gravity does not
• 1:22:37
  Speaker 1: suddenly acquire a special sucking ability merely because an object
• 1:22:40
  Speaker 1: has become a black hole. Material near Sagittarius A star
• 1:22:46
  Speaker 1: usually possesses sideways motion. It has angular momentum. Rather than
• 1:22:51
  Speaker 1: falling directly inward, much of it moves around the black hole,
• 1:22:55
  Speaker 1: just as planets orbit the Sun rather than immediately falling
• 1:22:59
  Speaker 1: into it. For gas to move significantly closer, it generally
• 1:23:03
  Speaker 1: has to lose some of that orbital energy and angular momentum.
• 1:23:07
  Speaker 1: This process is called accretion. Particles within the gas interact.
• 1:23:12
  Speaker 1: Magnetic fields become tangled. Turbulence develops. Energy and angular momentum
• 1:23:19
  Speaker 1: can be redistributed through the flow. A small fraction of
• 1:23:22
  Speaker 1: the material gradually moves inward, while other material can carry
• 1:23:26
  Speaker 1: angular momentum farther outward. The result around Sagittarius A star
• 1:23:31
  Speaker 1: is not necessarily the thin, bright, orderly accretion disk often
• 1:23:36
  Speaker 1: shown in illustrations of actively feeding black holes. Its accretion
• 1:23:40
  Speaker 1: flow is comparatively tenuous, hot, turbulent and inefficient at producing radiation.
• 1:23:48
  Speaker 1: As material moves closer to the black hole, conditions become
• 1:23:51
  Speaker 1: increasingly extreme. Gas can be heated to enormous temperatures. Electrons
• 1:23:58
  Speaker 1: accelerate through strong magnetic fields. The plasma becomes turbulent. Magnetic
• 1:24:04
  Speaker 1: field lines can stretch, twist and reconnect. These processes allow
• 1:24:10
  Speaker 1: the material outside the event horizon to produce radiation across
• 1:24:14
  Speaker 1: a broad range of wavelengths. Radio waves come from the
• 1:24:18
  Speaker 1: central region. Infrared emission can brighten and fade. X-rays occasionally
• 1:24:23
  Speaker 1: flare dramatically. The black hole itself still emits no ordinary
• 1:24:28
  Speaker 1: light from inside its event horizon. The radiation comes from
• 1:24:32
  Speaker 1: matter that has not yet crossed that boundary. This distinction
• 1:24:36
  Speaker 1: is useful because the brightest black hole systems in the
• 1:24:39
  Speaker 1: universe are not bright because their black holes glow. They
• 1:24:43
  Speaker 1: are bright because material surrounding the black holes is releasing
• 1:24:46
  Speaker 1: enormous amounts of energy before disappearing from view. And Sagittarius
• 1:24:50
  Speaker 1: Astar is surprisingly bad at this. For a black hole
• 1:24:54
  Speaker 1: containing roughly 4 million solar masses, it is extraordinarily dim.
• 1:25:00
  Speaker 1: Other supermassive black holes can be surrounded by vast supplies
• 1:25:03
  Speaker 1: of infalling material. When the feeding rate becomes high enough,
• 1:25:08
  Speaker 1: The central region of a galaxy can become what astronomers
• 1:25:11
  Speaker 1: call an active galactic nucleus. The most luminous examples are quasars.
• 1:25:17
  Speaker 1: Some quasars shine so intensely that the region around a
• 1:25:20
  Speaker 1: single black hole can outshine the combined starlight of an
• 1:25:24
  Speaker 1: entire galaxy. Sagittarius. A star is nothing like that today.
• 1:25:33
  Speaker 1: It is remarkably underfed. Only a small amount of the
• 1:25:36
  Speaker 1: gas available in the central region ultimately reaches the immediate
• 1:25:40
  Speaker 1: vicinity of the event horizon, and much of the energy
• 1:25:43
  Speaker 1: within the accretion flow is not efficiently converted into escaping radiation.
• 1:25:49
  Speaker 1: Measurements by the National Aeronautics and Space Administration have described
• 1:25:54
  Speaker 1: Sagittarius A-star as being roughly a billion times fainter than
• 1:25:58
  Speaker 1: it could be if it were accreting vigorously. For something
• 1:26:02
  Speaker 1: with 4 million solar masses, it is putting on a
• 1:26:05
  Speaker 1: remarkably modest performance. That does not mean it's completely inactive.
• 1:26:11
  Speaker 1: Sagittarius A star flickers. Its brightness changes continuously, particularly at
• 1:26:19
  Speaker 1: infrared wavelengths. Occasionally these variations become much stronger and appear
• 1:26:24
  Speaker 1: as flares. Infrared observations have revealed sudden brightenings close to
• 1:26:29
  Speaker 1: the black hole, while X-ray telescopes have recorded more energetic bursts.
• 1:26:34
  Speaker 1: These events can unfold over relatively short periods, sometimes changing
• 1:26:39
  Speaker 1: significantly within minutes or hours. One important mechanism thought to
• 1:26:44
  Speaker 1: contribute is magnetic reconnection. Plasma around the black hole carries
• 1:26:48
  Speaker 1: magnetic fields. As those fields are stretched and tangled by
• 1:26:52
  Speaker 1: turbulent motion, regions with different magnetic orientations can be pushed together.
• 1:26:58
  Speaker 1: The magnetic configuration can suddenly reorganise, releasing stored magnetic energy
• 1:27:04
  Speaker 1: and accelerating particles. A related process occurs much close to
• 1:27:08
  Speaker 1: home during energetic activity on the Sun, around Sagittarius. A star, however,
• 1:27:16
  Speaker 1: it takes place within hot plasma moving through the powerful
• 1:27:20
  Speaker 1: gravitational field of a supermassive black hole. Accelerated electrons can
• 1:27:26
  Speaker 1: then produce bursts of infrared and X-ray radiation. The result
• 1:27:31
  Speaker 1: is a central black hole that is quiet by galactic
• 1:27:34
  Speaker 1: standards but far from motionless. It flickers, flashes and changes continually.
• 1:27:41
  Speaker 1: There are also clues that Sagittarius A-star has not always
• 1:27:46
  Speaker 1: been quite so subdued. Large molecular clouds surrounding the galactic
• 1:27:51
  Speaker 1: centre have been observed glowing in X-rays in ways that
• 1:27:55
  Speaker 1: suggest they are reflecting radiation from much brighter events in
• 1:27:58
  Speaker 1: the past. The idea is similar to an echo. A
• 1:28:02
  Speaker 1: burst of radiation leaves the region around Sagittarius A star.
• 1:28:06
  Speaker 1: It travels outward. Eventually it strikes a distant cloud of
• 1:28:10
  Speaker 1: gas and causes that cloud to produce or scatter X-rays.
• 1:28:15
  Speaker 1: Because the cloud may lie hundreds of light-years from the
• 1:28:18
  Speaker 1: black hole, the reflected radiation can reach us long after
• 1:28:22
  Speaker 1: the original outburst has faded. These X-ray echoes suggest that
• 1:28:26
  Speaker 1: Sagittarius Astar experienced episodes of substantially greater activity within the
• 1:28:31
  Speaker 1: relatively recent astronomical past. So when the black hole is
• 1:28:35
  Speaker 1: described as sleeping, the phrase needs a little care. Sagittarius
• 1:28:39
  Speaker 1: a star is not asleep in any physical sense. Its
• 1:28:42
  Speaker 1: gravity does not switch off. Its event horizon does not disappear.
• 1:28:47
  Speaker 1: Nearby stars continue to orbit. Gas continues to move. Spacetime
• 1:28:53
  Speaker 1: remains strongly curved. What changes is the amount of material
• 1:28:57
  Speaker 1: being delivered towards the black hole. and the amount of
• 1:29:00
  Speaker 1: radiation produced by that material before it falls in. A
• 1:29:04
  Speaker 1: quiet black hole is simply a poorly supplied black hole.
• 1:29:08
  Speaker 1: Give the same object a large and sustained flow of gas,
• 1:29:12
  Speaker 1: and its surroundings could become enormously luminous. For now, the
• 1:29:16
  Speaker 1: centre of the Milky Way is comparatively calm, and most
• 1:29:20
  Speaker 1: objects there remain perfectly capable of avoiding the black hole altogether.
• 1:29:25
  Speaker 1: Stars orbit it. Gas clouds pass around it. Material can
• 1:29:30
  Speaker 1: remain gravitationally bound without ever crossing the event horizon. Sagittarius
• 1:29:36
  Speaker 1: A star is not continually swallowing everything that enters the
• 1:29:40
  Speaker 1: galactic centre. Its influence is powerful, but gravity still obeys
• 1:29:44
  Speaker 1: the ordinary rules of orbital motion. What makes the region
• 1:29:48
  Speaker 1: extraordinary is not constant destruction. It is the combination of density, speed, temperature, magnetism,
• 1:29:57
  Speaker 1: radiation and and gravity concentrated into one remarkably small part
• 1:30:02
  Speaker 1: of the Milky Way. At its centre sits an object
• 1:30:05
  Speaker 1: of four million solar masses. Around it, stars race through
• 1:30:09
  Speaker 1: the darkness. Hot plasma twists through magnetic fields. Flares appear
• 1:30:15
  Speaker 1: and disappear. Gas slowly exchanges energy and angular momentum, and
• 1:30:20
  Speaker 1: only a tiny fraction of all that surrounding material ever
• 1:30:24
  Speaker 1: completes the final journey inward. Sagittarius. A star may feel
• 1:30:30
  Speaker 1: unusual because it belongs to our own galaxy, but it
• 1:30:33
  Speaker 1: is not unusual in the wider universe. As astronomers studied
• 1:30:37
  Speaker 1: more and more galaxies, a remarkable pattern began to emerge.
• 1:30:42
  Speaker 1: Almost every large galaxy examined in sufficient detail appears to
• 1:30:46
  Speaker 1: contain a supermassive black hole near its centre. The Milky
• 1:30:50
  Speaker 1: Way has Sagittarius A star. The Andromeda galaxy has a
• 1:30:54
  Speaker 1: much more massive central black hole. The giant elliptical galaxy
• 1:30:59
  Speaker 1: Messier 87 contains an even larger one, with a mass
• 1:31:03
  Speaker 1: measured in billions of suns. Across the observable universe, these
• 1:31:08
  Speaker 1: objects seem to be a normal part of large galaxies
• 1:31:10
  Speaker 1: rather than rare cosmic accidents. The term supermassive black hole
• 1:31:15
  Speaker 1: generally refers to black holes containing millions, hundreds of millions,
• 1:31:19
  Speaker 1: or even billions of times the mass of the sun.
• 1:31:23
  Speaker 1: There is no perfectly sharp boundary separating every category of
• 1:31:26
  Speaker 1: black hole, And astronomical terminology can vary slightly depending on context.
• 1:31:32
  Speaker 1: But the important point is scale. A black hole formed
• 1:31:35
  Speaker 1: from the collapse of a massive star might contain several
• 1:31:38
  Speaker 1: or several tens of solar masses. Sagittarius, a star, contains
• 1:31:42
  Speaker 1: roughly four million. The black hole in Messier 87 contains
• 1:31:46
  Speaker 1: several billion. Some of the largest known central black holes
• 1:31:50
  Speaker 1: may contain tens of billions of solar masses. They are
• 1:31:53
  Speaker 1: not simply slightly enlarged versions of stellar black holes. They
• 1:31:57
  Speaker 1: occupy an entirely different range of mass, and yet, despite
• 1:32:01
  Speaker 1: their enormous gravity, many of them can be remarkably difficult
• 1:32:06
  Speaker 1: to notice. Sagittarius, A star, is a good example. If
• 1:32:11
  Speaker 1: we had not carefully measured the movements of nearby stars,
• 1:32:14
  Speaker 1: its 4 million solar masses would hardly announce themselves through
• 1:32:18
  Speaker 1: spectacular brightness. The black hole is relatively quiet because very
• 1:32:23
  Speaker 1: little material is currently reaching the innermost accretion flow. Other
• 1:32:27
  Speaker 1: galaxies are very different. When large quantities of gas fall
• 1:32:31
  Speaker 1: towards a central supermassive black hole, the surrounding region can
• 1:32:35
  Speaker 1: become extraordinarily luminous. The black hole itself remains dark, but
• 1:32:40
  Speaker 1: material outside the event horizon can release immense amounts of energy.
• 1:32:45
  Speaker 1: Gas spiralling inward becomes compressed and heated. Magnetic fields become tangled.
• 1:32:51
  Speaker 1: Particles accelerate. Radiation pours outward across the electromagnetic spectrum. When
• 1:32:58
  Speaker 1: this activity becomes powerful enough, astronomers describe the central region
• 1:33:03
  Speaker 1: as an active galactic nucleus. Some active galactic nuclei produce
• 1:33:08
  Speaker 1: enormous jets extending far beyond their host galaxies. Others radiate
• 1:33:14
  Speaker 1: intensely in X-rays, ultraviolet light, visible light, infrared radiation and
• 1:33:21
  Speaker 1: radio waves. And among the brightest examples are quasars. A
• 1:33:26
  Speaker 1: quasar is not a different kind of black hole. It
• 1:33:29
  Speaker 1: is an exceptionally luminous form of active galactic nucleus. At
• 1:33:34
  Speaker 1: the centre sits a supermassive black hole, just as one
• 1:33:37
  Speaker 1: sits at the centre of the Milky Way. The difference
• 1:33:40
  Speaker 1: is the supply of material. A well-fed supermassive black hole
• 1:33:44
  Speaker 1: can be surrounded by an accretion flow so luminous that
• 1:33:48
  Speaker 1: the central region outshines the combined starlight of the entire
• 1:33:51
  Speaker 1: galaxy around it. That is an extraordinary amount of energy
• 1:33:55
  Speaker 1: coming from a region that, on galactic scales, is extremely small.
• 1:34:01
  Speaker 1: Quasars can therefore be detected across enormous cosmic distances. And
• 1:34:06
  Speaker 1: that created one of the great puzzles of modern astronomy.
• 1:34:10
  Speaker 1: When astronomers look deeply into space, they're also looking backwards
• 1:34:14
  Speaker 1: in time. Light from very distant galaxies may have travelled
• 1:34:18
  Speaker 1: for more than 12 or 13 billion years before reaching us.
• 1:34:22
  Speaker 1: Some quasars are seen at times when the universe was
• 1:34:25
  Speaker 1: less than a billion years old. Yet several of those
• 1:34:28
  Speaker 1: early quasars already contained black holes with masses reaching hundreds
• 1:34:32
  Speaker 1: of millions or billions of suns. Somehow, enormous black holes
• 1:34:36
  Speaker 1: had formed and grown remarkably quickly. That sounds simple until
• 1:34:40
  Speaker 1: the numbers are considered. A black hole does not gain
• 1:34:44
  Speaker 1: mass instantly. It must begin from something. Then it must grow.
• 1:34:49
  Speaker 1: One obvious possibility is that the first supermassive black holes
• 1:34:52
  Speaker 1: began as the remnants of the earliest generations of massive stars.
• 1:34:57
  Speaker 1: The first stars formed from gas containing almost no elements
• 1:35:00
  Speaker 1: heavier than hydrogen and helium. Some may have grown to
• 1:35:04
  Speaker 1: very large masses. When sufficiently massive stars died, they could
• 1:35:08
  Speaker 1: have left black holes behind. Those early black holes might
• 1:35:11
  Speaker 1: then have grown by consuming gas and merging with other
• 1:35:14
  Speaker 1: black holes. This is often called a light seed scenario
• 1:35:18
  Speaker 1: because the initial black hole begins relatively small compared with
• 1:35:22
  Speaker 1: the supermassive object it eventually becomes. The difficulty is time.
• 1:35:27
  Speaker 1: If the seed begins with only tens or hundreds of
• 1:35:29
  Speaker 1: solar masses, it must grow by an enormous factor to
• 1:35:33
  Speaker 1: reach a billion solar masses within the first several hundred
• 1:35:36
  Speaker 1: million years of cosmic history. Another possibility begins with a
• 1:35:41
  Speaker 1: much larger seed, under certain conditions in the early universe.
• 1:35:45
  Speaker 1: An enormous cloud of primordial gas might collapse directly rather
• 1:35:49
  Speaker 1: than first fragmenting into an ordinary population of stars. This
• 1:35:54
  Speaker 1: could potentially produce a black hole with thousands, tens of thousands,
• 1:35:59
  Speaker 1: or perhaps even more solar masses from the beginning. A
• 1:36:02
  Speaker 1: heavier seed would have less growing to do. Dense clusters
• 1:36:06
  Speaker 1: of stars provide another possible route in an extremely crowded
• 1:36:10
  Speaker 1: stellar environment. Repeated collisions and mergers might build a very
• 1:36:15
  Speaker 1: massive star or a central compact object that eventually collapses
• 1:36:19
  Speaker 1: into a substantial black hole seed. It is also possible
• 1:36:23
  Speaker 1: that nature used several routes rather than choosing only one.
• 1:36:27
  Speaker 1: Whatever the starting point, growth requires material, and that introduces
• 1:36:32
  Speaker 1: another important idea. The Eddington Limit. As gas falls towards
• 1:36:37
  Speaker 1: a black hole and becomes hot, it produces radiation. That
• 1:36:41
  Speaker 1: radiation exerts pressure on surrounding material. Gravity is pulling gas inward.
• 1:36:47
  Speaker 1: Radiation pressure is pushing outward. At sufficiently high luminosity, the
• 1:36:53
  Speaker 1: outward pressure can begin to resist the continuing inflow. The
• 1:36:57
  Speaker 1: characteristic balance between these effects is called the Eddington limit.
• 1:37:01
  Speaker 1: It is not an impenetrable cosmic speed limit for black
• 1:37:04
  Speaker 1: hole growth. Real accretion flows can be complicated. Geometry matters.
• 1:37:11
  Speaker 1: magnetic fields matter, radiation can escape more easily in some
• 1:37:16
  Speaker 1: directions than others. Short periods of growth above the conventional
• 1:37:21
  Speaker 1: Eddington rate may be possible. But the concept reveals why
• 1:37:25
  Speaker 1: building a billion-solar-mass black hole very quickly is not as
• 1:37:29
  Speaker 1: simple as placing a large cloud nearby and allowing gravity
• 1:37:33
  Speaker 1: to do the rest. As the black hole feeds, the
• 1:37:37
  Speaker 1: accretion flow itself can fight against further feeding. This is
• 1:37:41
  Speaker 1: why astronomers consider periods of rapid or super-Eddington accretion when
• 1:37:45
  Speaker 1: trying to explain the earliest quasars. Perhaps some early black
• 1:37:49
  Speaker 1: holes grew much faster than a simple steady model would suggest.
• 1:37:53
  Speaker 1: Perhaps their seeds were already unusually massive. Perhaps mergers helped.
• 1:37:58
  Speaker 1: Perhaps dense gas environments in young galaxies allowed several favourable
• 1:38:02
  Speaker 1: processors to operate together. Observations from the James Webb Space
• 1:38:07
  Speaker 1: Telescope have made the question even more interesting by revealing
• 1:38:11
  Speaker 1: active black holes and unusual compact galaxies at very early
• 1:38:15
  Speaker 1: cosmic times. Some of these objects appear surprisingly mature given
• 1:38:20
  Speaker 1: how little time had passed since the Big Bang. But
• 1:38:24
  Speaker 1: the overall formation story remains unsettled. Astronomers do not yet
• 1:38:28
  Speaker 1: have one universally accepted sequence, explaining how the first supermassive
• 1:38:33
  Speaker 1: black holes appeared and reached enormous masses so rapidly. That
• 1:38:38
  Speaker 1: uncertainty is important. The existence of supermassive black holes is
• 1:38:42
  Speaker 1: no longer the mystery it once was. We can measure them.
• 1:38:46
  Speaker 1: We can watch gas falling around them, We can observe
• 1:38:49
  Speaker 1: their jets. We can detect quasars across most of cosmic history.
• 1:38:54
  Speaker 1: We have even resolved the shadow-scale environments of Sagittarius A-star
• 1:38:59
  Speaker 1: and the black hole in Messier 87. The deeper mystery
• 1:39:03
  Speaker 1: is how these objects came to exist so early and
• 1:39:05
  Speaker 1: became so large. Sagittarius A-star, quiet and comparatively modest at
• 1:39:12
  Speaker 1: 4 million solar masses, is only one member of an
• 1:39:15
  Speaker 1: enormous population. Across the universe, other galaxies contain central black
• 1:39:21
  Speaker 1: holes thousands of times more massive. Some sleep quietly, some
• 1:39:26
  Speaker 1: feed slowly, some blaze brightly enough to be seen from
• 1:39:29
  Speaker 1: almost the beginning of cosmic history. And somewhere in those
• 1:39:34
  Speaker 1: earliest generations of galaxies, the first seeds of these enormous
• 1:39:38
  Speaker 1: objects were already beginning to grow. Once astronomers began measuring
• 1:39:42
  Speaker 1: supermassive black holes in many different galaxies, they noticed something unexpected.
• 1:39:48
  Speaker 1: The black holes did not seem to be completely independent
• 1:39:51
  Speaker 1: of the galaxies around them. Certain properties of the central
• 1:39:55
  Speaker 1: black hole appear to correlate with properties of the galaxy's
• 1:39:58
  Speaker 1: central stellar population. One of the most important examples involves
• 1:40:03
  Speaker 1: what astronomers call the galactic bulge. In many galaxies, the
• 1:40:08
  Speaker 1: bulge is the dense, rounded concentration of stars surrounding the
• 1:40:11
  Speaker 1: central region. It can extend for thousands of light years
• 1:40:15
  Speaker 1: and contain billions of stars moving in many different directions.
• 1:40:20
  Speaker 1: When astronomers compared these bulges with the black holes sitting
• 1:40:23
  Speaker 1: at their centres, a pattern appeared. Galaxies with more massive
• 1:40:28
  Speaker 1: central bulges tended, on average, to contain more massive supermassive
• 1:40:32
  Speaker 1: black holes. Another relationship involved the speeds of the stars themselves.
• 1:40:38
  Speaker 1: Astronomers can measure how rapidly stars in a galactic bulge
• 1:40:41
  Speaker 1: move by studying the way their combined light is broadened
• 1:40:45
  Speaker 1: and shifted. The spread in those stellar velocities is called
• 1:40:49
  Speaker 1: the stellar velocity dispersion. A larger velocity dispersion generally means
• 1:40:54
  Speaker 1: that the stars are moving around more rapidly within the
• 1:40:57
  Speaker 1: gravitational environment of the bulge. And when astronomers compared this
• 1:41:02
  Speaker 1: quantity with central black hole mass, they found a striking trend.
• 1:41:06
  Speaker 1: Galaxies whose bulge stars move more quickly tend statistically to
• 1:41:10
  Speaker 1: contain more massive black holes. This relationship is often described
• 1:41:15
  Speaker 1: as the connection between black hole mass and stellar velocity dispersion.
• 1:41:19
  Speaker 1: It is not a perfect rule. Galaxies do not line
• 1:41:22
  Speaker 1: up with mathematical neatness. Each black hole assigned one exact
• 1:41:26
  Speaker 1: mass for one exact stellar velocity. There is scatter. Some
• 1:41:31
  Speaker 1: galaxies lie above the average relationship. Others lie below it.
• 1:41:35
  Speaker 1: Measurements have uncertainties and galaxies have complicated individual histories. But
• 1:41:41
  Speaker 1: across large populations, the correlation is clear enough to demand
• 1:41:44
  Speaker 1: an explanation. Related patterns appear when black hole mass is
• 1:41:49
  Speaker 1: compared with the mass or luminosity of the surrounding bulge. Again,
• 1:41:54
  Speaker 1: the details vary from galaxy to galaxy. But the overall
• 1:41:57
  Speaker 1: message is difficult to ignore. Somehow the growth of the
• 1:42:01
  Speaker 1: object occupying the tiniest central region appears connected with the
• 1:42:05
  Speaker 1: growth of structures extending across thousands of light years. That
• 1:42:09
  Speaker 1: is surprising when the difference in scale is considered. The
• 1:42:13
  Speaker 1: event horizon of a supermassive black hole is minuscule compared
• 1:42:16
  Speaker 1: with its host galaxy. Even the region immediately dominated by
• 1:42:20
  Speaker 1: the black hole's gravity is tiny beside the full stellar bulge.
• 1:42:24
  Speaker 1: A galactic bulge may extend for thousands of light years.
• 1:42:29
  Speaker 1: The black hole's event horizon may span only a few
• 1:42:31
  Speaker 1: astronomical units. or perhaps tens or hundreds depending on its mass.
• 1:42:36
  Speaker 1: It is a little like discovering that the size of
• 1:42:38
  Speaker 1: a grain of sand sitting at the centre of a
• 1:42:40
  Speaker 1: city is somehow related to the average speed of the
• 1:42:44
  Speaker 1: traffic across the whole metropolitan area. The two scales seem
• 1:42:48
  Speaker 1: almost absurdly disconnected, and yet the observations say they are related.
• 1:42:53
  Speaker 1: One broad idea used to describe this is co-evolution. The
• 1:42:57
  Speaker 1: black hole and the galaxy may not grow as two
• 1:43:00
  Speaker 1: entirely separate systems. Instead, they can be influenced by some
• 1:43:04
  Speaker 1: of the same processes. Gas is a particularly important part
• 1:43:08
  Speaker 1: of this picture. When a galaxy contains a large supply
• 1:43:12
  Speaker 1: of cold gas, that material can form stars. Some of
• 1:43:16
  Speaker 1: the same gas can also lose angular momentum and move
• 1:43:19
  Speaker 1: inward towards the centre. Part of it may feed the
• 1:43:23
  Speaker 1: supermassive black hole. So a period that builds a galaxy's
• 1:43:27
  Speaker 1: central stellar population can also provide fuel for its black hole.
• 1:43:32
  Speaker 1: A galaxy growing a denser bulge may therefore create conditions
• 1:43:35
  Speaker 1: that encourage the black hole to grow as well. Galaxy
• 1:43:39
  Speaker 1: mergers can strengthen this connection. When two galaxies interact, their
• 1:43:44
  Speaker 1: gravitational fields disturb stars and gas. Gas that had previously
• 1:43:49
  Speaker 1: followed relatively stable orbits can be redirected towards the centre.
• 1:43:53
  Speaker 1: That inflow can trigger bursts of star formation. At the
• 1:43:57
  Speaker 1: same time, it can feed one or both central black holes.
• 1:44:02
  Speaker 1: If the galaxies eventually merge completely, their black holes may
• 1:44:06
  Speaker 1: later form a binary and potentially merge as well. Over
• 1:44:10
  Speaker 1: billions of years, repeated interactions like these can build both
• 1:44:14
  Speaker 1: galaxies and their central black holes together. But the existence
• 1:44:18
  Speaker 1: of a correlation does not automatically reveal the cause. That
• 1:44:22
  Speaker 1: distinction matters. Astronomers know that black hole mass and bulge
• 1:44:26
  Speaker 1: properties are related statistically. They are still working to understand
• 1:44:30
  Speaker 1: exactly why. One possibility is black hole feedback. A vigorously
• 1:44:36
  Speaker 1: accreting black hole can release enormous amounts of energy through radiation,
• 1:44:40
  Speaker 1: winds and sometimes relativistic jets. That energy can heat gas
• 1:44:45
  Speaker 1: or drive it away from the central region, potentially limiting
• 1:44:49
  Speaker 1: both further star formation and further black hole growth. In
• 1:44:53
  Speaker 1: that picture, the black hole may help regulate the galaxy
• 1:44:56
  Speaker 1: surrounding it. Another possibility is that the connection arises partly
• 1:45:00
  Speaker 1: because both black hole growth and bulge growth depend on
• 1:45:03
  Speaker 1: the same supply of gas. The black hole would not
• 1:45:06
  Speaker 1: necessarily need to regulate the galaxy directly in every case.
• 1:45:11
  Speaker 1: Both could simply be responding to the same underlying conditions.
• 1:45:15
  Speaker 1: Hierarchical growth may also matter. Large galaxies are assembled gradually
• 1:45:20
  Speaker 1: through the merging and accumulation of smaller systems. As galaxies combine,
• 1:45:25
  Speaker 1: their stellar populations grow. Their black holes can grow through
• 1:45:28
  Speaker 1: accretion and mergers as well. Repeated generations of this process
• 1:45:34
  Speaker 1: could naturally help establish broad statistical relationships between the two.
• 1:45:38
  Speaker 1: The real universe is probably not obligated to choose just
• 1:45:42
  Speaker 1: one explanation. Common gas supply, mergers, black hole feedback, star
• 1:45:48
  Speaker 1: formation and long-term galactic evolution may all contribute in different proportions.
• 1:45:54
  Speaker 1: Sagittarius A star provides a nearby example of this broader connection.
• 1:45:59
  Speaker 1: At roughly 4 million solar masses, It is undoubtedly supermassive,
• 1:46:04
  Speaker 1: but compared with the black holes found in some enormous
• 1:46:07
  Speaker 1: elliptical galaxies, it is fairly modest. Some of those systems
• 1:46:12
  Speaker 1: contain central black holes with several billion times the mass
• 1:46:16
  Speaker 1: of the Sun. Their galaxies also tend to possess far
• 1:46:20
  Speaker 1: larger and more massive central stellar structures. The Milky Way
• 1:46:25
  Speaker 1: fits into the larger pattern without sitting at its most
• 1:46:28
  Speaker 1: extreme end. that makes Sagittarius a star particularly useful. Because
• 1:46:34
  Speaker 1: it is close enough for individual stellar orbits to be
• 1:46:37
  Speaker 1: measured in extraordinary detail, astronomers can study one supermassive black
• 1:46:42
  Speaker 1: hole with a precision impossible in most distant galaxies. Then
• 1:46:47
  Speaker 1: they can compare what they learn here with broader statistical
• 1:46:50
  Speaker 1: patterns across the universe. The result is a curious change
• 1:46:54
  Speaker 1: in perspective. A black hole might seem like the ultimate
• 1:46:57
  Speaker 1: isolated object, separated from everything beyond its event horizon. But
• 1:47:03
  Speaker 1: on galactic scales, its history may be deeply connected with
• 1:47:06
  Speaker 1: everything around it. The stars of the bulge, the gas
• 1:47:11
  Speaker 1: flowing through the centre, the mergers that shape the galaxy,
• 1:47:15
  Speaker 1: the periods of intense star formation, the episodes of accretion
• 1:47:19
  Speaker 1: that made the black hole grow. The black hole at
• 1:47:22
  Speaker 1: the centre of a galaxy is tiny compared with the
• 1:47:25
  Speaker 1: galaxy itself, but the two do not appear to have
• 1:47:29
  Speaker 1: lived entirely separate lives. A supermassive black hole occupies an
• 1:47:34
  Speaker 1: almost absurdly small region, compared with the galaxy surrounding it.
• 1:47:39
  Speaker 1: Its event horizon may span a region measured in millions
• 1:47:42
  Speaker 1: or billions of kilometres. Its host galaxy can stretch across
• 1:47:46
  Speaker 1: tens or hundreds of thousands of light-years, and yet, under
• 1:47:50
  Speaker 1: the right conditions, activity near that tiny central region can
• 1:47:54
  Speaker 1: influence gas on vastly larger scales. The process is known
• 1:47:59
  Speaker 1: as black hole feedback. The name does not mean that
• 1:48:02
  Speaker 1: the black hole itself is somehow reaching across the galaxy
• 1:48:06
  Speaker 1: and pushing things around directly. The important energy comes from
• 1:48:10
  Speaker 1: material falling towards it. As gas spirals inward, gravitational potential
• 1:48:16
  Speaker 1: energy is converted into heat, radiation, and motion. The material
• 1:48:22
  Speaker 1: can become extraordinarily hot before crossing the event horizon. Magnetic
• 1:48:28
  Speaker 1: fields can become twisted and compressed. Particles can be accelerated
• 1:48:33
  Speaker 1: to enormous speeds, and in some systems, A small fraction
• 1:48:37
  Speaker 1: of the energy associated with the black hole's rotation may
• 1:48:41
  Speaker 1: also be tapped by magnetic fields threading the region around it.
• 1:48:45
  Speaker 1: The result can be spectacular. Some active galactic nuclei launch
• 1:48:50
  Speaker 1: narrow relativistic jets extending thousands or even millions of light years.
• 1:48:55
  Speaker 1: Other systems produce powerful winds and high-speed outflows. Radiation from
• 1:49:00
  Speaker 1: the accretion flow can flood the surrounding galaxy. All of
• 1:49:04
  Speaker 1: this happens outside the event horizon. Nothing is escaping from
• 1:49:08
  Speaker 1: inside the black hole. Instead, the energy is released by
• 1:49:13
  Speaker 1: matter that has not yet fallen beyond the point of
• 1:49:15
  Speaker 1: no return. That distinction is important because the environment around
• 1:49:20
  Speaker 1: a black hole can be astonishingly bright even though the
• 1:49:23
  Speaker 1: black hole itself remains dark. If enough gas is supplied
• 1:49:28
  Speaker 1: … The accretion process can convert a remarkable fraction of
• 1:49:32
  Speaker 1: the material's mass into outward flowing energy. That energy can
• 1:49:36
  Speaker 1: then interact with the rest of the galaxy. Gas is
• 1:49:40
  Speaker 1: especially vulnerable. Stars are comparatively difficult to push around once
• 1:49:45
  Speaker 1: they have formed. A star is compact and gravitationally bound.
• 1:49:50
  Speaker 1: Diffuse gas is much easier to heat, compress or drive away.
• 1:49:55
  Speaker 1: And because new stars form from cold, dense gas, Anything
• 1:49:59
  Speaker 1: that changes the condition of that gas can change the
• 1:50:02
  Speaker 1: future rate of star formation. Suppose an active galactic nucleus
• 1:50:08
  Speaker 1: launches a powerful wind through the central regions. The outflow
• 1:50:12
  Speaker 1: can slam into surrounding gas. Some of that gas may
• 1:50:16
  Speaker 1: be accelerated outward. If enough energy is transferred, material can
• 1:50:20
  Speaker 1: be removed from the central region entirely. That means there
• 1:50:24
  Speaker 1: is less cold gas available to collapse into new stars.
• 1:50:28
  Speaker 1: Even when the gas is not expelled heating alone can matter.
• 1:50:33
  Speaker 1: For a cloud to form stars efficiently parts of it
• 1:50:36
  Speaker 1: usually need to cool so that gravity can overcome internal
• 1:50:39
  Speaker 1: pressure and allow collapse. If energy from an active galactic
• 1:50:44
  Speaker 1: nucleus keeps the gas hot that cooling can be delayed.
• 1:50:48
  Speaker 1: The galaxy may still contain large quantities of gas but
• 1:50:51
  Speaker 1: the gas can remain too warm or too turbulent to
• 1:50:54
  Speaker 1: collapse easily. In that sense Black hole feedback can reduce
• 1:50:59
  Speaker 1: star formation without physically blowing every cloud out of the galaxy.
• 1:51:04
  Speaker 1: This is one reason active galactic nuclei are often included
• 1:51:07
  Speaker 1: in modern models of galaxy evolution. Without some form of feedback,
• 1:51:12
  Speaker 1: simulations can produce galaxies whose central regions keep accumulating too
• 1:51:17
  Speaker 1: much cold gas and forming too many stars. Energy from
• 1:51:22
  Speaker 1: accreting black holes provides one physically plausible way to regulate
• 1:51:26
  Speaker 1: that process. but the effect is not always purely suppressive.
• 1:51:31
  Speaker 1: Under some circumstances, an outflow can compress gas rather than
• 1:51:35
  Speaker 1: simply disperse it. A shockwave travelling through a cloud can
• 1:51:39
  Speaker 1: squeeze part of the material to higher density. If that
• 1:51:42
  Speaker 1: compression is strong enough, and if the gas can cool efficiently,
• 1:51:46
  Speaker 1: collapse may become easier rather than harder. In those cases,
• 1:51:52
  Speaker 1: black hole activity can potentially encourage star formation in particular regions.
• 1:51:57
  Speaker 1: This is sometimes called positive feedback, in contrast with the
• 1:52:01
  Speaker 1: more familiar negative feedback that suppresses star formation. The names
• 1:52:05
  Speaker 1: are useful, but the reality is not as tidy as
• 1:52:08
  Speaker 1: the labels suggest. The same outflow could heat one region,
• 1:52:12
  Speaker 1: clear another, and compress a third. Galaxies are complicated places.
• 1:52:18
  Speaker 1: That complexity is one reason astronomers are careful when describing
• 1:52:22
  Speaker 1: the role of black holes in galactic evolution. There is
• 1:52:26
  Speaker 1: strong evidence that active galactic nuclei can drive winds, jets
• 1:52:31
  Speaker 1: and large-scale disturbances. There is also strong evidence that supermassive
• 1:52:35
  Speaker 1: black hole masses correlate statistically with properties of the galactic
• 1:52:40
  Speaker 1: bulges around them. Together, these observations suggest that black holes
• 1:52:45
  Speaker 1: and galaxies have influenced one another over cosmic time, but
• 1:52:49
  Speaker 1: the exact chain of cause and effect can be difficult
• 1:52:52
  Speaker 1: to untangle. A massive galaxy may feed its black hole
• 1:52:56
  Speaker 1: more effectively. The growing black hole may then influence the
• 1:52:59
  Speaker 1: surrounding gas. Galaxy mergers can send fresh material inward while
• 1:53:04
  Speaker 1: also disturbing star formation. Periods of black hole activity may
• 1:53:09
  Speaker 1: come and go. The final relationship observed billions of years
• 1:53:13
  Speaker 1: later can therefore be the accumulated result of many interconnected processes.
• 1:53:19
  Speaker 1: This is why black hole feedback is best thought of
• 1:53:22
  Speaker 1: as an important physical mechanism rather than a single master control.
• 1:53:28
  Speaker 1: It can matter enormously. But it does not operate in
• 1:53:32
  Speaker 1: exactly the same way in every galaxy. Some systems host
• 1:53:36
  Speaker 1: dramatic jets. Others are dominated by broad winds. Some experience
• 1:53:41
  Speaker 1: long quiet periods. Others go through repeated bursts of activity.
• 1:53:47
  Speaker 1: Sagittarius A star provides a useful contrast. At the present time,
• 1:53:51
  Speaker 1: the black hole at the centre of the Milky Way
• 1:53:53
  Speaker 1: is not behaving like a powerful quasar. Its feeding rate
• 1:53:57
  Speaker 1: is low. Its surrounding accretion flow is faint. It does
• 1:54:01
  Speaker 1: not currently dominate the energy output of the galaxy. The
• 1:54:04
  Speaker 1: stars of the Milky Way are not sitting beneath a
• 1:54:07
  Speaker 1: central beacon bright enough to outshine them all. Our black
• 1:54:10
  Speaker 1: hole is comparatively quiet. But there are signs that it
• 1:54:15
  Speaker 1: has not always been so quiet. Evidence from X-ray echoes
• 1:54:19
  Speaker 1: in molecular clouds suggest that Sagittarius A star experienced brighter
• 1:54:24
  Speaker 1: episodes in the recent astronomical past. Other structures around the
• 1:54:28
  Speaker 1: galactic centre may also preserve traces of earlier energetic events.
• 1:54:33
  Speaker 1: This fits comfortably with what astronomers see elsewhere. Galactic nuclei
• 1:54:38
  Speaker 1: can change. A black hole that is faint today may
• 1:54:42
  Speaker 1: have been far more active in the past. A galaxy
• 1:54:45
  Speaker 1: that currently looks peaceful may once have hosted a luminous
• 1:54:49
  Speaker 1: active nucleus. The availability of gas changes. Mergers occur. Stellar
• 1:54:55
  Speaker 1: winds feed central regions. Gas is consumed, expelled or rearranged.
• 1:55:01
  Speaker 1: The black hole responds to whatever supply reaches it. So
• 1:55:05
  Speaker 1: the influence of a supermassive black hole is not fixed.
• 1:55:09
  Speaker 1: It can vary across millions and billions of years. At
• 1:55:13
  Speaker 1: some times, the central object may have almost no effect
• 1:55:16
  Speaker 1: beyond its immediate surroundings. At others, accretion can release enough
• 1:55:21
  Speaker 1: energy to reshape gas across a substantial fraction of the galaxy,
• 1:55:26
  Speaker 1: and this may help explain why the tiniest central region
• 1:55:29
  Speaker 1: is statistically linked to the much larger stellar structure surrounding it.
• 1:55:33
  Speaker 1: A black hole does not need to physically dominate an
• 1:55:36
  Speaker 1: entire galaxy with its gravity in order to matter. It
• 1:55:40
  Speaker 1: only needs to control what happens to enough gas. The
• 1:55:43
  Speaker 1: event horizon may be tiny. The consequences of feeding the
• 1:55:47
  Speaker 1: object inside it can be anything but. The reassuring answer
• 1:55:51
  Speaker 1: is simple. No, Sagittarius A star poses no plausible gravitational
• 1:55:57
  Speaker 1: threat to Earth. It lies roughly 26,000 light-years away, far
• 1:56:03
  Speaker 1: across the Milky Way, and although it contains about 4
• 1:56:06
  Speaker 1: million times the mass of the Sun, distance matters enormously
• 1:56:11
  Speaker 1: when gravity is concerned. A black hole can sound uniquely
• 1:56:15
  Speaker 1: dangerous because of the language we use around it. We
• 1:56:18
  Speaker 1: talk about event horizons. We talk about matter being trapped.
• 1:56:22
  Speaker 1: We talk about stars being torn apart. All of that
• 1:56:25
  Speaker 1: is real in the right circumstances, but a black hole
• 1:56:29
  Speaker 1: is not a special kind of gravitational vacuum cleaner. From
• 1:56:33
  Speaker 1: far away, gravity behaves according to mass and distance. If
• 1:56:38
  Speaker 1: you could somehow replace Sagittarius A star with another compact
• 1:56:42
  Speaker 1: object containing exactly the same 4 million solar masses, then
• 1:56:46
  Speaker 1: at our location in the galaxy, the gravitational effect would
• 1:56:50
  Speaker 1: be essentially the same. The fact that the mass is
• 1:56:53
  Speaker 1: inside a black hole does not give it some extra
• 1:56:56
  Speaker 1: long-range pulling power. The danger only becomes extraordinary when you
• 1:57:00
  Speaker 1: travel extraordinarily close. This is easiest to understand by thinking
• 1:57:04
  Speaker 1: about ordinary orbits. Earth does not fall directly into the
• 1:57:08
  Speaker 1: Sun because it has sideways motion, The Sun's gravity continuously
• 1:57:12
  Speaker 1: pulls Earth inward, but Earth is also moving across space.
• 1:57:17
  Speaker 1: The result is an orbit. The same principle applies around
• 1:57:20
  Speaker 1: a black hole. If an object is moving with the
• 1:57:23
  Speaker 1: right speed and direction, it can remain in orbit perfectly well.
• 1:57:28
  Speaker 1: Stars orbit Sagittarius, a star. Gas moves around it. Even
• 1:57:34
  Speaker 1: very close to the event horizon, material can spend time
• 1:57:37
  Speaker 1: moving in orbit. before losing enough energy to spiral inward.
• 1:57:41
  Speaker 1: A black hole does not simply drag everything nearby directly
• 1:57:44
  Speaker 1: towards its centre, and the Milky Way as a whole
• 1:57:47
  Speaker 1: is certainly not draining into Sagittarius A-star. That idea becomes
• 1:57:52
  Speaker 1: even clearer when we compare the mass of the central
• 1:57:54
  Speaker 1: black hole with the mass of the galaxy. Sagittarius A-star
• 1:57:58
  Speaker 1: contains roughly 4 million solar masses. That sounds enormous, and
• 1:58:03
  Speaker 1: it is, but the Milky Way contains vastly more mass overall.
• 1:58:07
  Speaker 1: There are hundreds of billions of stars. enormous quantities of
• 1:58:12
  Speaker 1: gas and dust, and an extended halo of dark matter
• 1:58:15
  Speaker 1: whose mass greatly exceeds that of the visible material. The
• 1:58:20
  Speaker 1: black hole is therefore only a tiny fraction of the
• 1:58:23
  Speaker 1: total mass associated with the galaxy. Very close to Sagittarius,
• 1:58:28
  Speaker 1: A star is Its gravity dominates. The S-stars are a
• 1:58:32
  Speaker 1: good example. Their motion is controlled overwhelmingly by the central
• 1:58:36
  Speaker 1: black hole because they travel through the inner region, where
• 1:58:39
  Speaker 1: that mass is concentrated. But the solar system is nowhere
• 1:58:43
  Speaker 1: near that environment. We lie roughly 26,000 light-years from the centre.
• 1:58:49
  Speaker 1: At this distance, our orbit through the galaxy is governed
• 1:58:52
  Speaker 1: mainly by the combined gravitational effect of all the matter
• 1:58:56
  Speaker 1: lying within and around our path. Stars matter. Gas matters.
• 1:59:00
  Speaker 1: Dark matter matters. The large-scale distribution of mass throughout the
• 1:59:04
  Speaker 1: Milky Way matters. Sagittarius A-star is part of that gravitational system,
• 1:59:10
  Speaker 1: but it is not the dominant influence on us. The
• 1:59:13
  Speaker 1: solar system has been travelling around the galaxy for billions
• 1:59:16
  Speaker 1: of years without being pulled catastrophically inward. That long history
• 1:59:21
  Speaker 1: is itself a useful reminder that the present arrangement is
• 1:59:24
  Speaker 1: stable on human timescales. Radiation is not a serious threat either.
• 1:59:29
  Speaker 1: Supermassive black holes can become associated with extraordinarily energetic environments
• 1:59:35
  Speaker 1: when large amounts of matter fall towards them. Active galactic
• 1:59:39
  Speaker 1: nuclei can produce intense radiation, powerful winds and enormous jets.
• 1:59:45
  Speaker 1: Quasars can shine across billions of light years. Sagittarius A
• 1:59:50
  Speaker 1: star is not doing anything remotely like that today. It
• 1:59:54
  Speaker 1: is currently very faint compared with an actively feeding supermassive
• 1:59:57
  Speaker 1: black hole. Only a small amount of material reaches the
• 2:00:01
  Speaker 1: innermost accretion flow, and the central region produces far less
• 2:00:05
  Speaker 1: radiation than it could if the black hole were being
• 2:00:07
  Speaker 1: supplied heavily with gas. Even if Sagittarius A star were
• 2:00:11
  Speaker 1: more active, we are still 26,000 light-years away. Radiation spreads
• 2:00:17
  Speaker 1: out as it travels. Distance dramatically reduces the intensity reaching
• 2:00:22
  Speaker 1: any particular location. At Earth, The present-day emissions from Sagittarius
• 2:00:27
  Speaker 1: A star are simply not a meaningful danger. The situation
• 2:00:32
  Speaker 1: would be very different if you moved much closer. Near
• 2:00:35
  Speaker 1: the galactic centre, the environment becomes considerably less friendly. Stars
• 2:00:40
  Speaker 1: pass through the region at thousands of kilometres per second.
• 2:00:44
  Speaker 1: Hot plasma produces energetic radiation. Magnetic fields become intense and turbulent,
• 2:00:52
  Speaker 1: close enough to the black holes. Tidal gravity would create
• 2:00:55
  Speaker 1: increasingly severe differences in gravitational pull across an object. And
• 2:01:00
  Speaker 1: closer still, space-time itself becomes so strongly curved that ordinary
• 2:01:06
  Speaker 1: intuition begins to fail. A spacecraft heading deep towards Sagittarius
• 2:01:11
  Speaker 1: a star would eventually face an environment no sensible travel
• 2:01:15
  Speaker 1: brochure would recommend. But we are not there. We are
• 2:01:18
  Speaker 1: separated from it by an enormous distance. From Earth, Sagittarius
• 2:01:23
  Speaker 1: a star is not a looming threat. It is better
• 2:01:26
  Speaker 1: thought of as a gravitational landmark. It marks the centre
• 2:01:30
  Speaker 1: of our galaxy. It provides astronomers with a natural laboratory
• 2:01:35
  Speaker 1: for testing general relativity. It allows stars nearby to trace
• 2:01:39
  Speaker 1: out the structure of an extreme gravitational field, and it
• 2:01:42
  Speaker 1: reminds us that the Milky Way is not simply a
• 2:01:45
  Speaker 1: glowing band of distant stars crossing the night sky. It
• 2:01:48
  Speaker 1: is a vast gravitational system, and we are travelling through it.
• 2:01:53
  Speaker 1: The black hole at its centre has been there throughout
• 2:01:55
  Speaker 1: the entire history of humanity. It was there before Earth formed.
• 2:02:00
  Speaker 1: It was there while life evolved. It was there while
• 2:02:03
  Speaker 1: the solar system completed earlier journeys around the Milky Way.
• 2:02:07
  Speaker 1: And it remains far away, quiet by black hole standards,
• 2:02:11
  Speaker 1: and safely separated from us by tens of thousands of
• 2:02:14
  Speaker 1: light years. Sagittarius A-star is one of the most extreme
• 2:02:19
  Speaker 1: objects in our galaxy. From here, It is also one
• 2:02:23
  Speaker 1: of the least urgent things to worry about. Sagittarius. A
• 2:02:27
  Speaker 1: star has existed at the centre of the Milky Way
• 2:02:29
  Speaker 1: for an immense span of cosmic history. And if the
• 2:02:33
  Speaker 1: universe continues unfolding in anything like the way our present
• 2:02:36
  Speaker 1: understanding suggests, it will remain there long after the solar
• 2:02:39
  Speaker 1: system has become almost unrecognisable. But the Milky Way itself
• 2:02:44
  Speaker 1: will not remain unchanged forever. Beyond our galaxy lies another
• 2:02:49
  Speaker 1: enormous spiral system, The Andromeda Galaxy is the nearest large
• 2:02:53
  Speaker 1: galaxy comparable in scale to the Milky Way. It sits
• 2:02:57
  Speaker 1: roughly 2.5 million light-years away and contains hundreds of billions
• 2:03:01
  Speaker 1: of stars, along with its own central supermassive black hole.
• 2:03:05
  Speaker 1: For many years, one of the most familiar predictions in
• 2:03:08
  Speaker 1: astronomy was that Andromeda and the Milky Way were heading
• 2:03:11
  Speaker 1: towards an almost inevitable collision. Measurements showed that Andromeda is
• 2:03:16
  Speaker 1: approaching us overall rather than receding with the general expansion
• 2:03:20
  Speaker 1: of the universe. Older models suggested that the first major
• 2:03:24
  Speaker 1: encounter between the two galaxies could occur roughly four and
• 2:03:28
  Speaker 1: a half billion years from now. The traditional picture was dramatic.
• 2:03:33
  Speaker 1: The two great spiral galaxies would approach one another. Their
• 2:03:37
  Speaker 1: gravitational fields would distort their disks. Long streams of stars
• 2:03:42
  Speaker 1: might be pulled outward. Gas clouds could be compressed, perhaps
• 2:03:46
  Speaker 1: triggering new episodes of star formation. The galaxies would pass
• 2:03:51
  Speaker 1: through one another, separate again, and then return for further encounters. Eventually,
• 2:03:57
  Speaker 1: after billions of years of gravitational interaction, the two systems
• 2:04:01
  Speaker 1: might settle into one much larger merged galaxy. It is
• 2:04:05
  Speaker 1: important to understand what a galactic collision actually means. galaxies
• 2:04:10
  Speaker 1: are full of stars, but they are also overwhelmingly full
• 2:04:13
  Speaker 1: of empty space. Even if the Milky Way and Andromeda
• 2:04:16
  Speaker 1: passed directly through one another, individual stars would be extremely
• 2:04:21
  Speaker 1: unlikely to collide. The stars are simply too small compared
• 2:04:25
  Speaker 1: with the enormous distances separating them. The gravitational structures of
• 2:04:30
  Speaker 1: the galaxies, however, would interact strongly. Orbits would be rearranged,
• 2:04:35
  Speaker 1: gas would be disturbed, dark matter, halos would overlap. the
• 2:04:40
  Speaker 1: shapes of both galaxies could change profoundly. For a long time,
• 2:04:44
  Speaker 1: this general merger scenario was treated as the likely future
• 2:04:48
  Speaker 1: of the local group. But astronomy rarely rewards too much
• 2:04:52
  Speaker 1: confidence about events several billion years away. In 2025, a
• 2:04:57
  Speaker 1: study led by astronomer Til Sawala revisited the future of
• 2:05:01
  Speaker 1: the Milky Way and Andromeda using improved information from the
• 2:05:05
  Speaker 1: European Space Agency's Gaia mission and the Hubble Space Telescope.
• 2:05:10
  Speaker 1: The result made the future considerably less certain. Rather than
• 2:05:14
  Speaker 1: finding an inevitable merger, the researchers concluded that there is
• 2:05:18
  Speaker 1: roughly a 50-50 chance that the Milky Way and Andromeda
• 2:05:23
  Speaker 1: will not merge within the next 10 billion years. That
• 2:05:27
  Speaker 1: is a very different picture from the familiar idea of
• 2:05:29
  Speaker 1: a collision, scheduled rather neatly for four and a half
• 2:05:33
  Speaker 1: billion years from now. The older prediction was not foolish.
• 2:05:38
  Speaker 1: It was based on the best measurements available at the time.
• 2:05:42
  Speaker 1: The difficulty is that predicting the future motion of entire
• 2:05:45
  Speaker 1: galaxies requires extraordinarily precise knowledge of how they are moving
• 2:05:50
  Speaker 1: in three dimensions. The motion directly towards or away from
• 2:05:55
  Speaker 1: us can be measured relatively well. The sideways motion across
• 2:05:59
  Speaker 1: the sky is much harder to determine because these galaxies
• 2:06:03
  Speaker 1: are so distant. A tiny uncertainty today can become an
• 2:06:07
  Speaker 1: enormous difference after billions of years. There are also other
• 2:06:11
  Speaker 1: members of the Local Group to consider. The Milky Way
• 2:06:14
  Speaker 1: and Andromeda are not travelling through an empty universe containing
• 2:06:17
  Speaker 1: only each other. Two smaller galaxies are particularly important. One
• 2:06:23
  Speaker 1: is the Large Magellanic Cloud, a substantial satellite galaxy currently
• 2:06:28
  Speaker 1: interacting with the Milky Way. The other is the Triangulum Galaxy,
• 2:06:33
  Speaker 1: another large member of the local group associated more closely
• 2:06:36
  Speaker 1: with Andromeda. Their gravity matters. The Large Magellanic Cloud is
• 2:06:41
  Speaker 1: massive enough to affect the motion of the Milky Way itself.
• 2:06:45
  Speaker 1: The Triangulum Galaxy can alter Andromeda's trajectory. When these additional
• 2:06:50
  Speaker 1: gravitational influences are included, along with the uncertainties in the
• 2:06:54
  Speaker 1: masses and motions of the galaxies, the range of possible
• 2:06:58
  Speaker 1: futures becomes much broader. Some simulated histories still produce a
• 2:07:02
  Speaker 1: major Milky Way and Andromeda encounter, after roughly four and
• 2:07:06
  Speaker 1: a half billion years. Others produce a more distant passage,
• 2:07:12
  Speaker 1: some delay a merger substantially, and some avoid a merger
• 2:07:15
  Speaker 1: for at least 10 billion years. So the famous collision
• 2:07:19
  Speaker 1: remains a genuine possibility. It is simply no longer something
• 2:07:23
  Speaker 1: we can describe as inevitable. If a major merger does
• 2:07:27
  Speaker 1: eventually occur, Sagittarius A-star would enter an extraordinary new phase
• 2:07:32
  Speaker 1: of its existence. Andromeda has its own central supermassive black hole,
• 2:07:38
  Speaker 1: considerably more massive than Sagittarius A-star. As the galaxies began
• 2:07:43
  Speaker 1: to intermingle, the two black holes would not immediately crash together.
• 2:07:48
  Speaker 1: At first, they would remain separated by enormous distances while
• 2:07:52
  Speaker 1: the stars, gas and dark matter of their host galaxies
• 2:07:55
  Speaker 1: were rearranged around them, Over time, however, a process called
• 2:08:00
  Speaker 1: dynamical friction could gradually bring them closer. Dynamical friction is
• 2:08:05
  Speaker 1: not friction in the everyday sense of two surfaces rubbing together.
• 2:08:09
  Speaker 1: A massive object moving through a sea of stars and
• 2:08:12
  Speaker 1: dark matter gravitationally disturbs the material around it. It creates
• 2:08:17
  Speaker 1: a slight overdensity behind itself, rather like a gravitational wake.
• 2:08:23
  Speaker 1: That wake pulls backwards on the moving object. The massive
• 2:08:26
  Speaker 1: object gradually loses orbital energy. In a merged galaxy, this
• 2:08:31
  Speaker 1: process could cause both central black holes to sink towards
• 2:08:35
  Speaker 1: the new common centre. Eventually, they might form a gravitationally
• 2:08:39
  Speaker 1: bound pair, a supermassive black hole binary. At first, the
• 2:08:44
  Speaker 1: two black holes could orbit one another from relatively large distances.
• 2:08:48
  Speaker 1: Interactions with surrounding stars and gas might gradually remove energy
• 2:08:52
  Speaker 1: from the binary and shrink the orbit. If the pair
• 2:08:56
  Speaker 1: became sufficiently close, another mechanism would begin to dominate. Gravitational
• 2:09:02
  Speaker 1: waves Albert Einstein's general theory of relativity predicts that accelerating
• 2:09:08
  Speaker 1: masses can produce ripples in spacetime. a pair of supermassive
• 2:09:12
  Speaker 1: black holes orbiting one another would radiate gravitational energy. As
• 2:09:16
  Speaker 1: that energy escaped, their orbit would shrink, they would revolve
• 2:09:20
  Speaker 1: around each other more rapidly, the gravitational waves would grow stronger,
• 2:09:24
  Speaker 1: and if the process continued all the way to coalescence,
• 2:09:28
  Speaker 1: the two event horizons would eventually become one. The final
• 2:09:32
  Speaker 1: merger would release an extraordinary amount of energy in gravitational
• 2:09:35
  Speaker 1: waves and The resulting black hole would contain the combined
• 2:09:39
  Speaker 1: mass of the original objects, minus the portion of their
• 2:09:43
  Speaker 1: energy carried away through those ripples in spacetime. But even
• 2:09:47
  Speaker 1: this distant scenario comes with uncertainty. Getting two supermassive black
• 2:09:52
  Speaker 1: holes from galactic scales all the way to final coalescence
• 2:09:57
  Speaker 1: involves complicated interactions with stars, gas and the changing gravitational
• 2:10:02
  Speaker 1: structure of the merged galaxy. the timescale could be extremely long.
• 2:10:08
  Speaker 1: The exact outcome would depend on the details of the
• 2:10:11
  Speaker 1: encounter itself, and before any of that can happen, the
• 2:10:15
  Speaker 1: Milky Way and Andromeda first have to meet in the
• 2:10:18
  Speaker 1: right way. That is no longer guaranteed. This is one
• 2:10:22
  Speaker 1: of the stranger lessons of trying to predict the deep future.
• 2:10:26
  Speaker 1: Gravity follows precise physical laws, but a system containing several
• 2:10:31
  Speaker 1: enormous galaxies, their satellites, Vast dark matter halos and uncertain
• 2:10:38
  Speaker 1: three-dimensional motions can still have a future that is difficult
• 2:10:42
  Speaker 1: to determine. A tiny difference in present velocity can become
• 2:10:47
  Speaker 1: a difference of hundreds of thousands of light-years after billions
• 2:10:50
  Speaker 1: of years. So Sagittarius A-star has no appointment written into
• 2:10:55
  Speaker 1: the cosmos. Perhaps it will one day find itself orbiting
• 2:10:59
  Speaker 1: another supermassive black hole at the centre of a newly
• 2:11:02
  Speaker 1: merged galaxy. Perhaps the Milky Way and Andromeda will pass
• 2:11:06
  Speaker 1: at a greater distance and continue evolving separately for far longer.
• 2:11:10
  Speaker 1: Perhaps later encounters will eventually bring them together anyway. The
• 2:11:15
  Speaker 1: physics is not uncertain because the laws of gravity are vague.
• 2:11:19
  Speaker 1: The uncertainty lies in the starting conditions and the immense
• 2:11:22
  Speaker 1: amount of time over which those conditions will unfold. For now,
• 2:11:27
  Speaker 1: Sagittarius Astar remains where it has been throughout human history,
• 2:11:30
  Speaker 1: at the centre of the Milky Way. form million solar
• 2:11:34
  Speaker 1: masses surrounded by stars, gas, dust and darkness. Its distant
• 2:11:39
  Speaker 1: future is tied to the future of the galaxy around it.
• 2:11:43
  Speaker 1: And even after studying the heavens with extraordinary precision, we
• 2:11:47
  Speaker 1: cannot yet say exactly what that future will be. 26,000
• 2:11:52
  Speaker 1: light years away, in the direction of Sagittarius, there is
• 2:11:56
  Speaker 1: a region of space that looks almost completely ordinary from here.
• 2:12:00
  Speaker 1: No enormous black circle hangs visibly in the sky. No
• 2:12:04
  Speaker 1: great funnel appears among the stars. Nothing announces that this
• 2:12:08
  Speaker 1: is the centre of the Milky Way. And yet, hidden there,
• 2:12:13
  Speaker 1: Sagittarius a star contains roughly four million times the mass
• 2:12:17
  Speaker 1: of the Sun compressed into an extraordinarily compact region. It
• 2:12:21
  Speaker 1: is one of the most extreme objects in our galaxy,
• 2:12:25
  Speaker 1: and for most of human history, we had no idea
• 2:12:28
  Speaker 1: it was there. That alone makes the story rather remarkable.
• 2:12:32
  Speaker 1: Human beings looked towards the centre of the Milky Way
• 2:12:34
  Speaker 1: for thousands of years without knowing what occupied it. The
• 2:12:38
  Speaker 1: stars were visible. The bright band of the galaxy was visible.
• 2:12:43
  Speaker 1: The constellation Sagittarius was visible. But thick clouds of interstellar
• 2:12:49
  Speaker 1: dust blocked much of the central region from ordinary sight.
• 2:12:53
  Speaker 1: The black hole itself produced no visible surface light. So
• 2:12:57
  Speaker 1: the centre of our own galaxy remained hidden, even while
• 2:13:01
  Speaker 1: astronomers were beginning to understand galaxies far beyond it. The
• 2:13:05
  Speaker 1: first major clues did not arrive through visible light at all.
• 2:13:09
  Speaker 1: Radio waves passed through the dust far more easily. And
• 2:13:13
  Speaker 1: in 1974, Bruce Ballack and Robert Brown identified an exceptionally
• 2:13:19
  Speaker 1: compact radio source at almost exactly the position of the
• 2:13:23
  Speaker 1: galactic centre. That source became known as Sagittarius A-star. At
• 2:13:29
  Speaker 1: the time, it was intriguing. It was compact. It was
• 2:13:33
  Speaker 1: bright at radio wavelengths. It sat in exactly the sort
• 2:13:37
  Speaker 1: of place where something important might be expected. But it
• 2:13:41
  Speaker 1: was not yet proof of a black hole. That proof
• 2:13:44
  Speaker 1: had to be assembled slowly. Infrared astronomy opened a new
• 2:13:49
  Speaker 1: view through the dust, with increasingly powerful telescopes, improved detectors,
• 2:13:54
  Speaker 1: speckle imaging, adaptive optics, and extraordinarily precise astrometry, astronomers began
• 2:14:00
  Speaker 1: resolving individual stars near the very centre of the galaxy.
• 2:14:05
  Speaker 1: Then they waited. Months passed. Years passed. And the stars moved.
• 2:14:12
  Speaker 1: Some moved quickly. Some curved. Some began tracing unmistakable orbits
• 2:14:18
  Speaker 1: around the same apparently empty point Independent teams led by
• 2:14:23
  Speaker 1: Reinhard Gensel and Andrea Ghez followed those motions for decades.
• 2:14:27
  Speaker 1: The measurements became more precise, the orbital paths became clearer,
• 2:14:33
  Speaker 1: and the amount of hidden mass required to produce them
• 2:14:35
  Speaker 1: became increasingly difficult to explain in any other way. Roughly
• 2:14:40
  Speaker 1: four million solar masses had to be packed into a
• 2:14:42
  Speaker 1: tiny central region, not spread comfortably across a large star cluster,
• 2:14:48
  Speaker 1: not distributed throughout the centre. concentrated. The more tightly the
• 2:14:53
  Speaker 1: stars orbited, the smaller that central region had to be.
• 2:14:57
  Speaker 1: Alternative explanations became increasingly strained. A sufficiently dense collection of
• 2:15:03
  Speaker 1: ordinary stars would not remain stable. Neither would a fantastically
• 2:15:08
  Speaker 1: crowded cluster of neutron stars or stellar-mass black holes. Objects
• 2:15:13
  Speaker 1: would collide, scatter, merge or escape. Eventually, the supermassive black
• 2:15:20
  Speaker 1: hole explanation was not simply the most dramatic option. It
• 2:15:24
  Speaker 1: was the physically viable one. And perhaps no object made
• 2:15:28
  Speaker 1: that conclusion more intuitive than S2. S2 is a real star.
• 2:15:33
  Speaker 1: It can be observed. Its position can be measured. Its
• 2:15:37
  Speaker 1: spectrum can be studied. And every roughly 16 years, it
• 2:15:41
  Speaker 1: completes an orbit around the unseen centre of the Milky Way.
• 2:15:46
  Speaker 1: Near its closest approach, It travels at several thousand kilometres
• 2:15:50
  Speaker 1: every second. Almost 3% of the speed of light. There
• 2:15:55
  Speaker 1: is something wonderfully simple about that evidence. You cannot see
• 2:15:59
  Speaker 1: the central mass directly. But you can watch a star
• 2:16:03
  Speaker 1: race around it. You can see the star accelerate as
• 2:16:06
  Speaker 1: it falls inward. You can watch it swing around the
• 2:16:09
  Speaker 1: hidden focus. You can measure the way its light is
• 2:16:12
  Speaker 1: shifted by gravity. You can even detect the subtle turning
• 2:16:16
  Speaker 1: of its orbit predicted by Albert Einstein's general theory of relativity.
• 2:16:21
  Speaker 1: The black hole remains invisible. S2 draws its presence across
• 2:16:25
  Speaker 1: the sky. And then came another extraordinary step. The Event
• 2:16:29
  Speaker 1: Horizon Telescope collaboration linked radio observatories across Earth to create
• 2:16:35
  Speaker 1: the resolving power of a telescope comparable in scale to
• 2:16:38
  Speaker 1: the planet itself. At wavelengths of roughly 1.3 millimetres, astronomers
• 2:16:44
  Speaker 1: could see through much of the intervening galactic material and
• 2:16:48
  Speaker 1: resolve structures on scales comparable to the black hole's shadow.
• 2:16:52
  Speaker 1: In 2022, the first horizon scale image associated with Sagittarius
• 2:16:58
  Speaker 1: A star was unveiled. Once again, the black hole itself
• 2:17:01
  Speaker 1: remained dark. What appeared was a glowing ring of radio
• 2:17:06
  Speaker 1: emission from hot plasma surrounding a central shadow. Light was
• 2:17:10
  Speaker 1: being bent by strongly curved spacetime. Photons were being redirected
• 2:17:14
  Speaker 1: around the black hole. Some escaped. Others disappeared across the
• 2:17:18
  Speaker 1: event horizon. The image was not a photograph of a
• 2:17:22
  Speaker 1: solid black object. It was something more interesting. It was
• 2:17:25
  Speaker 1: a picture of how light behaves around an invisible concentration
• 2:17:29
  Speaker 1: of mass so extreme that spacetime itself becomes dramatically curved.
• 2:17:35
  Speaker 1: And each part of this story required a different way
• 2:17:37
  Speaker 1: of seeing. Radio waves penetrated the dusty galactic plane. and
• 2:17:42
  Speaker 1: reveal the compact central source. Infrared observations allowed individual stars
• 2:17:48
  Speaker 1: to be tracked through the crowded nuclear star cluster. Spectroscopy
• 2:17:53
  Speaker 1: measured their velocities. X-ray observations revealed energetic flares and echoes
• 2:17:59
  Speaker 1: from past activity. Millimeter radio interferometry resolved the glowing structure
• 2:18:04
  Speaker 1: immediately around the black hole. No single wavelength told the
• 2:18:09
  Speaker 1: whole story. The Milky Way had to be studied almost
• 2:18:12
  Speaker 1: like a puzzle, with different forms of electromagnetic radiation revealing
• 2:18:17
  Speaker 1: different pieces, and the picture that emerged is strangely balanced.
• 2:18:21
  Speaker 1: Sagittarius A star is extreme. Four million solar masses are
• 2:18:27
  Speaker 1: concentrated into a region tiny compared with the galaxy. Nearby
• 2:18:32
  Speaker 1: stars travel at thousands of kilometres per second. Space-time is
• 2:18:36
  Speaker 1: strongly curved. Hot plasma moves through magnetic fields. Infrared and
• 2:18:42
  Speaker 1: X-ray flares appear from the surrounding accretion flow. Anything travelling
• 2:18:46
  Speaker 1: close enough would enter an environment utterly unlike the peaceful
• 2:18:49
  Speaker 1: space around Earth. And yet Sagittarius A-star is also surprisingly quiet.
• 2:18:57
  Speaker 1: It is not presently behaving like a quasar. It is
• 2:19:00
  Speaker 1: not consuming enormous quantities of gas. It does not outshine
• 2:19:04
  Speaker 1: the Milky Way. Compared with vigorously feeding supermassive black holes
• 2:19:08
  Speaker 1: in other galaxies, it is remarkably faint. Most nearby matter
• 2:19:13
  Speaker 1: does not simply fall in. Stars remain in orbit. Gas
• 2:19:17
  Speaker 1: can circle the centre. Only a small fraction of the
• 2:19:21
  Speaker 1: material wandering into the region ultimately loses enough energy and
• 2:19:24
  Speaker 1: angular momentum to continue inward. The black hole is enormous,
• 2:19:29
  Speaker 1: but it is not ravenous in the way popular imagination
• 2:19:32
  Speaker 1: sometimes suggests. Sagittarius A-star is not unique simply because it
• 2:19:37
  Speaker 1: is supermassive. Central black holes appear to be a normal
• 2:19:41
  Speaker 1: feature of large galaxies. Across the universe, astronomers find black
• 2:19:45
  Speaker 1: holes containing millions or billions of solar masses sitting inside
• 2:19:50
  Speaker 1: galactic centres. Some are quiet, some are active. Some power
• 2:19:55
  Speaker 1: luminous quasars visible across billions of light years. And somehow,
• 2:20:00
  Speaker 1: Their growth appears connected with the growth of the galaxies
• 2:20:03
  Speaker 1: around them. Black hole mass correlates with properties of galactic bulges.
• 2:20:09
  Speaker 1: Accretion can produce winds, jets and radiation capable of influencing
• 2:20:14
  Speaker 1: gas far beyond the immediate central region. Galaxy mergers can
• 2:20:19
  Speaker 1: feed black holes while reshaping their host galaxies. The story
• 2:20:23
  Speaker 1: of a central black hole and the story of its
• 2:20:26
  Speaker 1: galaxy seem to be intertwined. Our own Milky Way is
• 2:20:30
  Speaker 1: part of that larger cosmic pattern. But Sagittarius A star
• 2:20:34
  Speaker 1: is not a threat to us. At our distance, its
• 2:20:37
  Speaker 1: 4 million solar masses are only one small contribution to
• 2:20:41
  Speaker 1: the total gravitational structure of the galaxy. The Sun is
• 2:20:45
  Speaker 1: not orbiting the black hole in the simple way Earth
• 2:20:48
  Speaker 1: orbits the Sun. Our motion is controlled by the combined
• 2:20:51
  Speaker 1: gravity of the immense distribution of stars, gas and dark
• 2:20:55
  Speaker 1: matter throughout the Milky Way. Sagittarius, a star, dominates only
• 2:21:00
  Speaker 1: in the innermost central region. Out here, roughly 26,000 light
• 2:21:05
  Speaker 1: years away, we are safely part of a much larger system.
• 2:21:09
  Speaker 1: The solar system has been travelling through that system for
• 2:21:12
  Speaker 1: billions of years. One complete journey around the galactic centre
• 2:21:17
  Speaker 1: takes roughly 230 million years. That number should not be
• 2:21:22
  Speaker 1: treated as the orbital period of every star in the galaxy.
• 2:21:26
  Speaker 1: Different stars occupy different distances from the centre. They move
• 2:21:30
  Speaker 1: along different paths. They travel at different speeds. The Milky
• 2:21:34
  Speaker 1: Way does not rotate like a solid wheel. But for
• 2:21:37
  Speaker 1: the Sun, roughly 230 million years gives the scale of
• 2:21:42
  Speaker 1: one galactic orbit. Think about what that means. During a
• 2:21:46
  Speaker 1: single trip, entire groups of organisms can appear, evolve and disappear.
• 2:21:53
  Speaker 1: Continents move. Oceans open and close. Mountains rise and erode.
• 2:22:00
  Speaker 1: The night sky changes. And through all of it, the
• 2:22:04
  Speaker 1: solar system continues its slow journey around the galaxy. Every
• 2:22:08
  Speaker 1: ordinary night, when you look at the stars, almost every
• 2:22:12
  Speaker 1: point of light visible to the unaided eye belongs to
• 2:22:15
  Speaker 1: this same enormous system. Those stars are moving too. Each
• 2:22:20
  Speaker 1: follows its own path through the Milky Way. Some are
• 2:22:23
  Speaker 1: closer to the centre. Some are farther out, some travel
• 2:22:27
  Speaker 1: on unusually tilted or elongated orbits, but together they belong
• 2:22:32
  Speaker 1: to the same vast gravitational structure. So do we. The
• 2:22:37
  Speaker 1: ground beneath you does not feel as though it is
• 2:22:39
  Speaker 1: moving around a galactic centre. The sun does not appear
• 2:22:44
  Speaker 1: to be racing through space. The night sky seems calm,
• 2:22:48
  Speaker 1: but the stillness is an illusion created by scale. Earth turns.
• 2:22:54
  Speaker 1: Earth orbits the Sun. The Sun carries the planets through
• 2:22:58
  Speaker 1: the Milky Way. The Milky Way itself moves through the
• 2:23:02
  Speaker 1: local group, and somewhere in the direction of Sagittarius, buried
• 2:23:06
  Speaker 1: behind stars and dust, sits the dark central object whose
• 2:23:10
  Speaker 1: existence took generations of astronomers to uncover. Sagittarius. A star
• 2:23:18
  Speaker 1: emits no visible surface light of its own. We know
• 2:23:22
  Speaker 1: it is there because the universe around it refuses to
• 2:23:24
  Speaker 1: keep the secret. Radio waves reveal its location. Hot plasma
• 2:23:29
  Speaker 1: outlines its shadow. Gas announces its activity. Stars trace its gravity.
• 2:23:35
  Speaker 1: S2 repeatedly races around it. Everything nearby points back towards
• 2:23:40
  Speaker 1: the same invisible place. And so, the Milky Way continues turning.
• 2:23:46
  Speaker 1: Hundreds of billions of stars moving through a structure so
• 2:23:49
  Speaker 1: large that a single orbit can outlast entire geological eras,
• 2:23:54
  Speaker 1: a vast, quiet carousel drifting through darkness, and at its heart,
• 2:23:59
  Speaker 1: almost perfectly unseen, sits a black hole four million times
• 2:24:04
  Speaker 1: the mass of the sun, not pulling us towards disaster,
• 2:24:08
  Speaker 1: not waiting to swallow the galaxy, simply occupying the dark
• 2:24:13
  Speaker 1: centre of a system that has been carrying us around
• 2:24:16
  Speaker 1: it all along.