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100 Mind-Blowing Facts About the Milky Way to Help You Fall Asleep · Luke's English Podcast

• 0:00
  Speaker 1: Hello, and welcome back. If you're here for the first time,
• 0:04
  Speaker 1: I'm so glad you found your way. And if you've
• 0:07
  Speaker 1: been here before, then you already know what this is.
• 0:11
  Speaker 1: A quiet place at the end of a long day,
• 0:14
  Speaker 1: where the universe opens up just enough to let your
• 0:17
  Speaker 1: mind wander somewhere beautiful before it drifts off to sleep. Tonight,
• 0:23
  Speaker 1: we're going somewhere vast. Somewhere ancient. Somewhere that's been turning slowly,
• 0:29
  Speaker 1: in the dark, for over 13 billion years, long before
• 0:34
  Speaker 1: there were eyes to see it, long before there was
• 0:37
  Speaker 1: anyone to wonder about it at all. Tonight, we're going home,
• 0:42
  Speaker 1: not to the house or the apartment or the room
• 0:45
  Speaker 1: you're lying in right now, but to the larger home,
• 0:49
  Speaker 1: the one that contains everything you've ever known, every star
• 0:53
  Speaker 1: you've ever wished upon, every night sky you've ever stood
• 0:57
  Speaker 1: beneath in silence. We're going to the Milky Way. Over
• 1:02
  Speaker 1: the next little while, I'll share with you 100 things
• 1:05
  Speaker 1: about our galaxy. Truths, mysteries, wonders, and quiet impossibilities that
• 1:12
  Speaker 1: live right above our heads every single night. Some of
• 1:16
  Speaker 1: these facts will be familiar, like a name you half
• 1:19
  Speaker 1: remember from a dream. Others will surprise you. A few
• 1:24
  Speaker 1: of them might even change the way you see the sky.
• 1:28
  Speaker 1: And all of them, I promise, are best experienced with
• 1:31
  Speaker 1: the lights low, your body warm, and your breath just
• 1:35
  Speaker 1: a little slower than it was an hour ago, so
• 1:37
  Speaker 1: find a comfortable position. Let your shoulders drop, let your
• 1:41
  Speaker 1: jaw unclench. And let's begin. The Milky Way is old.
• 1:47
  Speaker 1: Not old the way a building is old, or even
• 1:50
  Speaker 1: old the way a mountain is old. The Milky Way
• 1:54
  Speaker 1: began forming less than a billion years after the Big Bang,
• 1:57
  Speaker 1: which means it has been quietly turning in the dark
• 2:00
  Speaker 1: for roughly 13 billion years. Think about that for a moment.
• 2:06
  Speaker 1: 13 billion years. The Earth itself has only existed for
• 2:11
  Speaker 1: about 4.5 billion of those years, which means that for
• 2:16
  Speaker 1: the first 8.5 billion years of the Milky Way's life,
• 2:21
  Speaker 1: Our planet simply did not exist. There was no ground
• 2:24
  Speaker 1: to stand on, no ocean to sail, no sky to
• 2:28
  Speaker 1: look up at. The galaxy was already ancient by the
• 2:31
  Speaker 1: time the sun ignited for the first time, and yet
• 2:34
  Speaker 1: here we are, late arrivals in a story that began long,
• 2:39
  Speaker 1: long before us. But here is something that might gently
• 2:43
  Speaker 1: shift the way you're thinking right now. You are made
• 2:46
  Speaker 1: of this galaxy, not just living inside it, not just
• 2:50
  Speaker 1: orbiting within it. The atoms of your body, the iron
• 2:54
  Speaker 1: in your blood, the calcium in your bones, the carbon
• 2:58
  Speaker 1: woven through every cell, were forged inside stars. Stars that
• 3:03
  Speaker 1: lived and died billions of years before the Earth existed.
• 3:08
  Speaker 1: Stars that exploded their guts into space, scattered their material
• 3:12
  Speaker 1: across thousands of light years, and eventually seeded the clouds
• 3:17
  Speaker 1: that collapsed into our sun and our world. The galaxy
• 3:21
  Speaker 1: didn't just make stars. It made you. Now let me
• 3:25
  Speaker 1: ask you something, because I think it will help you
• 3:28
  Speaker 1: settle in. Have you ever looked at the night sky
• 3:31
  Speaker 1: on a truly dark night, far from cities, far from streetlights,
• 3:37
  Speaker 1: and felt, even briefly, like something was looking back? That
• 3:41
  Speaker 1: feeling isn't irrational. It isn't poetic exaggeration. Because, as you're
• 3:47
  Speaker 1: about to discover, The Milky Way is not a static backdrop.
• 3:51
  Speaker 1: It is alive with motion, with history, with violence, with patience,
• 3:56
  Speaker 1: and with a kind of slow, enormous creativity that takes
• 4:00
  Speaker 1: billions of years to complete a single thought. Let's spend
• 4:05
  Speaker 1: the night exploring it. Our galaxy contains somewhere between 100
• 4:10
  Speaker 1: billion and 400 billion stars. The range itself tells you
• 4:15
  Speaker 1: something important. Even counting the stars in our own galaxy
• 4:20
  Speaker 1: is something we haven't fully figured out. We can't photograph
• 4:25
  Speaker 1: it from above. We can't fly outside and look back.
• 4:29
  Speaker 1: We are sitting in the middle of the Milky Way,
• 4:32
  Speaker 1: like a person trying to describe a cathedral from inside
• 4:36
  Speaker 1: its walls, unable to see the full shape of the
• 4:39
  Speaker 1: building they're standing in. The estimates come from measuring brightness, density,
• 4:46
  Speaker 1: and extrapolating outward. But the truth is, we are still
• 4:50
  Speaker 1: learning to count our own home, and the stars we
• 4:53
  Speaker 1: can see with the naked eye, on the clearest, darkest
• 4:58
  Speaker 1: night imaginable, represent roughly a few thousand of those hundred
• 5:03
  Speaker 1: billion to four hundred billion suns. You could stand in
• 5:08
  Speaker 1: a field in the middle of nowhere, tilt your head back,
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  Speaker 1: and look at every single point of light you're capable
• 5:14
  Speaker 1: of seeing, and you would be seeing approximately 0.00003% of
• 5:21
  Speaker 1: the Milky Way's stellar population. Not even the surface of
• 5:26
  Speaker 1: the ocean, more like a single drop of seawater cupped
• 5:30
  Speaker 1: in your hand. Have you ever wondered why the Milky
• 5:33
  Speaker 1: Way looks like a streak of pale light across the sky,
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  Speaker 1: rather than a glowing disk you could see in full?
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  Speaker 1: The answer is simple and a little dizzying. You're inside it.
• 5:46
  Speaker 1: When you look at the smeared band of light that
• 5:49
  Speaker 1: crosses the night sky, you are looking along the flat
• 5:53
  Speaker 1: plane of the galaxy's disk, peering through billions of overlapping
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  Speaker 1: stars the way you'd look down a long corridor. and
• 6:02
  Speaker 1: see the walls and ceiling blur together in the distance.
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  Speaker 1: The Milky Way doesn't look like a spiral from Earth
• 6:09
  Speaker 1: because we're embedded in the spiral itself. We're like fish
• 6:14
  Speaker 1: trying to describe the shape of the ocean. Now, stay
• 6:18
  Speaker 1: with me here, because this next part is one of
• 6:21
  Speaker 1: my favourites. Our solar system, the Sun, the Earth, every planet,
• 6:27
  Speaker 1: every moon, every asteroid and comet in our neighbourhood, is
• 6:31
  Speaker 1: orbiting the center of the Milky Way, right now as
• 6:34
  Speaker 1: you lie there, as you breathe, as your eyes grow heavy.
• 6:39
  Speaker 1: We are moving. The solar system travels at roughly 828,
• 6:42
  Speaker 1: soon kilometers per hour, through space. That's about 230 kilometers
• 6:45
  Speaker 1: every single second, and still it takes approximately 225 to
• 6:48
  Speaker 1: 250 million years to complete one full orbit around the
• 6:50
  Speaker 1: galactic center. This journey is called a galactic year. In
• 7:05
  Speaker 1: all of the 4.6 billion years the Sun has existed,
• 7:08
  Speaker 1: it has completed this enormous loop fewer than 25 times.
• 7:12
  Speaker 1: 25 galactic years. We think of the Earth as ancient,
• 7:18
  Speaker 1: and it is. But on a galactic timescale, it's still
• 7:22
  Speaker 1: relatively new. The last time the solar system was in
• 7:26
  Speaker 1: the section of its orbit it currently occupies The supercontinent
• 7:30
  Speaker 1: Pangea had only just begun to break apart. The first
• 7:34
  Speaker 1: dinosaurs were just appearing. And the time before that, two
• 7:39
  Speaker 1: galactic years ago, the first complex animals were evolving in
• 7:43
  Speaker 1: the seas of a young Earth. The galaxy is not
• 7:47
  Speaker 1: counting in years. It's counting in eons. Now here's something
• 7:51
  Speaker 1: that might make your mind do a quiet somersault. What
• 7:55
  Speaker 1: if I told you that some of the stars in
• 7:58
  Speaker 1: the Milky Way are older than the Milky Way itself.
• 8:02
  Speaker 1: It sounds impossible. And yet it's true. Scattered throughout the
• 8:06
  Speaker 1: galaxy's outer halo, that vast, diffuse sphere of stars and
• 8:12
  Speaker 1: dark matter that surrounds the flat disk, there are ancient
• 8:16
  Speaker 1: stars with almost no heavy elements in their composition, no iron,
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  Speaker 1: no calcium, almost nothing beyond hydrogen and helium. This chemical
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  Speaker 1: signature marks them as some of the very first stars
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  Speaker 1: ever born in the universe. They did not form inside
• 8:34
  Speaker 1: the Milky Way as we know it. They formed in tiny,
• 8:38
  Speaker 1: primordial galaxies that existed long before our own, galaxies that
• 8:43
  Speaker 1: eventually collided with and were absorbed by the proto-Milky Way
• 8:47
  Speaker 1: billions of years ago. These stars are essentially refugees from
• 8:52
  Speaker 1: an older universe, wandering in the halo of our own galaxies,
• 8:57
  Speaker 1: carrying light from a time we can barely imagine. And
• 9:01
  Speaker 1: speaking of wandering, let me ask you something. What do
• 9:04
  Speaker 1: you think it would mean for a star to be rogue?
• 9:07
  Speaker 1: Not a rogue planet or a rogue asteroid, a rogue star.
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  Speaker 1: A full-blown, hydrogen-fusing, light-emitting star, hurtling through intergalactic space with
• 9:19
  Speaker 1: no galaxy to call home, no stellar neighbourhood around it,
• 9:24
  Speaker 1: Just emptiness in every direction for millions of light years.
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  Speaker 1: Does that sound like the beginning of a science fiction story?
• 9:32
  Speaker 1: Because it's not. It's real. These are called hypervelocity stars,
• 9:38
  Speaker 1: and they are among the strangest objects we've found, drifting
• 9:43
  Speaker 1: beyond the spiral arms. Here's how they happen. Near the
• 9:47
  Speaker 1: centre of the Milky Way, there is a supermassive black hole.
• 9:52
  Speaker 1: We'll return to it in detail soon. and sometimes a
• 9:55
  Speaker 1: pair of stars ventures too close. In the chaos of
• 9:59
  Speaker 1: the encounter, one star is captured, spiraling inward toward its doom,
• 10:04
  Speaker 1: but the other star is slingshot outward with such extraordinary
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  Speaker 1: force that it achieves escape velocity, leaving the galaxy entirely.
• 10:14
  Speaker 1: It travels alone. At more than a million kilometers per hour,
• 10:19
  Speaker 1: It crosses the outer edge of the Milky Way and
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  Speaker 1: flies into the dark between galaxies. It will never return.
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  Speaker 1: It will never find another home. It is a star,
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  Speaker 1: burning faithfully, lighting nothing but the void. These rogue stars
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  Speaker 1: are the galaxy's exiles. And if there were a planet
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  Speaker 1: orbiting one, though that itself is unlikely, the night sky
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  Speaker 1: from that world would show nothing but darkness. No neighbor stars.
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  Speaker 1: No galactic band of light. Just an utterly black and
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  Speaker 1: empty cosmos, lit only by the glow of the star
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  Speaker 1: they orbit. Imagine that. Imagine growing up under a sky
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  Speaker 1: with no Milky Way, no constellations, no sense of belonging
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  Speaker 1: to something larger. The Exile Star and its hypothetical world,
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  Speaker 1: utterly alone. Now let's drift inward. Let's move toward the
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  Speaker 1: center of the galaxy. because that is where things get
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  Speaker 1: strange and magnificent, and, if we're honest, a little frightening.
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  Speaker 1: At the very heart of the Milky Way, hidden behind
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  Speaker 1: curtains of gas and dust so thick that no visible
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  Speaker 1: light can penetrate them, lies an object called Sagittarius A-star.
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  Speaker 1: It is a supermassive black hole, and when I say supermassive,
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  Speaker 1: I mean that almost casually, because the actual scale of
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  Speaker 1: it is the kind of thing that resists being held
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  Speaker 1: in the mind. Sagittarius A-star contains a mass equivalent to
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  Speaker 1: more than four million suns. Four million suns. Compressed into
• 11:58
  Speaker 1: a region of space smaller than the orbit of Mercury
• 12:01
  Speaker 1: around our own sun, we cannot see it directly. Black holes,
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  Speaker 1: by definition, do not emit light, but we can see
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  Speaker 1: the effect it has on everything around it. Stars near
• 12:13
  Speaker 1: the center of the galaxy, stars that are otherwise completely normal,
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  Speaker 1: have been observed completing full orbits around the central black
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  Speaker 1: hole in as little as 12 years. 12 years. Those
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  Speaker 1: stars are moving so fast, under such incredible gravitational pressure,
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  Speaker 1: that they complete an entire orbit in the time it
• 12:37
  Speaker 1: might take a child to go from kindergarten to high
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  Speaker 1: school graduation. And the thing pulling them around it is invisible.
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  Speaker 1: But here is what keeps the story interesting. And please
• 12:48
  Speaker 1: stay awake just a little longer for this one. Because
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  Speaker 1: it's worth it. Sagittarius A-star is, by the standards of
• 12:57
  Speaker 1: supermassive black holes, remarkably quiet. It is not currently consuming
• 13:02
  Speaker 1: large amounts of material. It is not actively flaring or blazing.
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  Speaker 1: If you could somehow observe the Milky Way from a
• 13:11
  Speaker 1: distant galaxy... our core would look dark and subdued compared
• 13:16
  Speaker 1: to galaxies with active nuclei. But there is strong evidence
• 13:21
  Speaker 1: that this was not always the case. The Fermi bubbles,
• 13:25
  Speaker 1: two enormous lobes of high-energy gamma-ray light extending tens of
• 13:30
  Speaker 1: thousands of light-years above and below the galactic centre, are
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  Speaker 1: almost certainly the echoes of a past outburst. Something enormous
• 13:40
  Speaker 1: happened at the core of our galaxy. Perhaps two to
• 13:44
  Speaker 1: four million years ago, some scientists believe the central black
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  Speaker 1: hole briefly ignited as a quasar, one of the most
• 13:52
  Speaker 1: luminous objects in the universe, devouring gas and radiating energy
• 13:58
  Speaker 1: visible across millions of light years. If that happened, our
• 14:02
  Speaker 1: galaxy would have looked very different from the outside. A
• 14:06
  Speaker 1: brilliant burning heart in the center of a spiral arm.
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  Speaker 1: That time has passed. What remains are the fermi bubbles,
• 14:15
  Speaker 1: ghostly monuments to an ancient fire, invisible to the naked eye,
• 14:20
  Speaker 1: but glowing with the residue of an explosion that happened
• 14:24
  Speaker 1: before our earliest human ancestors walked the earth. Are you
• 14:29
  Speaker 1: still with me? Good. Then let me ask you something
• 14:33
  Speaker 1: a little quieter, a little more personal. Have you ever
• 14:37
  Speaker 1: looked up at the sky and wondered how many of
• 14:40
  Speaker 1: those stars have worlds around them? How many of those
• 14:44
  Speaker 1: tiny specks of light are actually suns, like ours, with
• 14:49
  Speaker 1: planets orbiting them? With moons? With maybe, somewhere, a surface
• 14:54
  Speaker 1: where something has learned to wonder about its own sky?
• 14:58
  Speaker 1: Because here is what we know now. Planetary systems are
• 15:02
  Speaker 1: not rare. They are the default. Almost every star we
• 15:06
  Speaker 1: study carefully turns out to have something orbiting it. Some
• 15:10
  Speaker 1: have gas giants the size of Jupiter. Others have rocky
• 15:14
  Speaker 1: planets smaller than Earth. Some are wrapped in dozens of moons.
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  Speaker 1: Others orbit in pairs caught in gravitational dances around binary suns.
• 15:25
  Speaker 1: Across the Milky Way, the number of planets likely runs
• 15:29
  Speaker 1: into the trillions. Trillions. And many of these planets sit
• 15:33
  Speaker 1: within what astronomers call the habitable zone. Not too hot.
• 15:38
  Speaker 1: Not too cold for liquid water on the surface. Water.
• 15:42
  Speaker 1: which seems to be one of life's most persistent prerequisites.
• 15:46
  Speaker 1: We have not yet found life elsewhere, but the math
• 15:50
  Speaker 1: of the galaxy makes the question feel increasingly less like fantasy.
• 15:56
  Speaker 1: And the Milky Way itself has something like a habitable zone, too.
• 16:01
  Speaker 1: Not for individual planets, but for the galaxy as a whole.
• 16:05
  Speaker 1: The Inner Regions of the Galaxy packed with stars, bathed
• 16:09
  Speaker 1: in radiation from the active core, swept by frequent supernova explosions,
• 16:15
  Speaker 1: are harsh places. The outer edges of the disk, too
• 16:19
  Speaker 1: far from the centre for the chemistry of heavy elements
• 16:22
  Speaker 1: to accumulate, are spare and cold. But in between, there's
• 16:27
  Speaker 1: a wide band sometimes called the galactic habitable zone, where
• 16:32
  Speaker 1: star density is moderate, where radiation storms are less common.
• 16:37
  Speaker 1: where the elements necessary for complex chemistry, carbon, oxygen, nitrogen, iron,
• 16:44
  Speaker 1: are plentiful enough to build living things, and our sun,
• 16:47
  Speaker 1: your sun, sits comfortably in this zone. We are not
• 16:51
  Speaker 1: too close to the chaos of the core, and not
• 16:55
  Speaker 1: too far from the richness of the center. We're in
• 16:58
  Speaker 1: exactly the right neighborhood. Doesn't it feel like we were
• 17:02
  Speaker 1: placed here on purpose? That's the kind of question that
• 17:06
  Speaker 1: keeps people awake at night. But the quieter, stranger answer
• 17:11
  Speaker 1: is that the galaxy did not need to place us
• 17:13
  Speaker 1: here intentionally. It just needed time, and it had 13
• 17:18
  Speaker 1: billion years. Now, do you know what dark matter is?
• 17:23
  Speaker 1: You've probably heard the term. But do you have a
• 17:26
  Speaker 1: sense of what it actually means to say that most
• 17:29
  Speaker 1: of the Milky Way is made of something we cannot detect,
• 17:33
  Speaker 1: cannot see, cannot measure directly, and cannot yet explain. Here
• 17:39
  Speaker 1: is the situation. When astronomers look at how stars orbit
• 17:43
  Speaker 1: the centre of our galaxy, they expect the same behaviour
• 17:47
  Speaker 1: we see in our solar system. Planets farther from the
• 17:51
  Speaker 1: Sun orbit more slowly than planets closer in because they
• 17:55
  Speaker 1: experience less gravitational pull. If the Milky Way worked the
• 18:00
  Speaker 1: same way, stars in the outer disk should orbit more
• 18:04
  Speaker 1: slowly than stars near the center. But they don't. Stars
• 18:08
  Speaker 1: in the outer regions orbit almost as quickly as those
• 18:12
  Speaker 1: near the middle. This is mathematically wrong, given the mass
• 18:16
  Speaker 1: we can see. The galaxy should be flying apart. The
• 18:20
  Speaker 1: visible matter, all the stars, all the gas, all the dust,
• 18:25
  Speaker 1: is not nearly enough to hold it together at those speeds,
• 18:29
  Speaker 1: and yet it holds. The only explanation that fits the
• 18:33
  Speaker 1: data is that there is an enormous amount of additional
• 18:36
  Speaker 1: mass that we simply cannot see. This unseen substance, dark matter,
• 18:42
  Speaker 1: is estimated to make up roughly 85% of the Milky
• 18:44
  Speaker 1: Way's total mass. Not 5%, not 30%. 85. The galaxy
• 18:46
  Speaker 1: we see The beautiful spiral of glowing stars and luminous
• 18:57
  Speaker 1: gas is essentially a thin veneer floating on top of
• 19:01
  Speaker 1: an enormous dark structure we can't directly observe. The Milky
• 19:06
  Speaker 1: Way's true shape is not defined by its stars. It's
• 19:09
  Speaker 1: defined by dark matter. And we do not know what
• 19:12
  Speaker 1: dark matter is. We've been looking for decades. We've built
• 19:17
  Speaker 1: detectors buried miles underground to catch dark matter particles. We've
• 19:22
  Speaker 1: built particle accelerators, hoping to produce them. So far, nothing.
• 19:28
  Speaker 1: The answer is out there, somewhere, in the dark. Here
• 19:32
  Speaker 1: is a gentler mystery, a softer one, before we go
• 19:36
  Speaker 1: a little further. You know how the Milky Way looks
• 19:39
  Speaker 1: in photographs. That magnificent, sweeping spiral, like a giant pinwheel
• 19:45
  Speaker 1: of stars. Here's what those photographs don't show. The Milky
• 19:49
  Speaker 1: Way is warped. It's not flat. The outer edges of
• 19:53
  Speaker 1: the galactic disk bend upward on one side and downward
• 19:57
  Speaker 1: on the other, like a vinyl record left in the sun,
• 20:01
  Speaker 1: or a wave moving across the surface of a spinning disk.
• 20:05
  Speaker 1: The warp is subtle. You wouldn't notice it if you
• 20:08
  Speaker 1: were standing on a star in the outer regions, but
• 20:11
  Speaker 1: it's real. And astronomers only discovered its full extent when
• 20:16
  Speaker 1: they mapped hydrogen gas far beyond the visible stars using
• 20:21
  Speaker 1: radio telescopes. The leading explanation is that the large and
• 20:25
  Speaker 1: small Magellanic clouds, two satellite galaxies orbiting the Milky Way,
• 20:32
  Speaker 1: are gravitationally tugging on its edges, bending the disk as
• 20:36
  Speaker 1: they pass. A pair of dwarf galaxies, millions of light
• 20:40
  Speaker 1: years smaller than our own, gently warping the shape of
• 20:44
  Speaker 1: our home, and the Magellanic clouds themselves have a story
• 20:48
  Speaker 1: worth telling. For a long time, astronomers assumed they were
• 20:53
  Speaker 1: long-standing companions of the Milky Way, galaxies that had been
• 20:58
  Speaker 1: orbiting us for billions of years in comfortable, familiar loops.
• 21:04
  Speaker 1: But new measurements of their velocities suggest something remarkable. They
• 21:09
  Speaker 1: may be moving too fast to be gravitationally bound to
• 21:13
  Speaker 1: the Milky Way in the traditional sense. They might be visitors.
• 21:18
  Speaker 1: Strangers passing through on their first close approach to our galaxy,
• 21:23
  Speaker 1: pulled in from the larger cosmic web by gravity, and
• 21:27
  Speaker 1: currently making their nearest pass before being either captured or
• 21:31
  Speaker 1: flung away. If this is true, It means the Magellanic
• 21:36
  Speaker 1: Clouds are not old friends. They're newcomers, and they've already
• 21:41
  Speaker 1: left a mark. Not just the warp in the disk,
• 21:44
  Speaker 1: but the Magellanic Stream, a vast ribbon of hydrogen gas
• 21:48
  Speaker 1: hundreds of thousands of light-years long, trailing behind them across
• 21:53
  Speaker 1: the sky like a tail. stripped away by tidal forces.
• 21:58
  Speaker 1: This stream will eventually fall into the Milky Way, adding
• 22:02
  Speaker 1: fresh gas that may fuel a new generation of star formation.
• 22:07
  Speaker 1: Even a passing encounter has consequences that last for billions
• 22:12
  Speaker 1: of years. Can I tell you something about star formation?
• 22:16
  Speaker 1: Because it's one of the most quietly beautiful processes in
• 22:20
  Speaker 1: the galaxy, and I think it deserves a slow, careful telling.
• 22:25
  Speaker 1: Stars are not born from nothing. They are born from cold,
• 22:29
  Speaker 1: dark clouds of hydrogen gas that drift through the galaxy's
• 22:33
  Speaker 1: spiral arms. Clouds so enormous they would dwarf any structure
• 22:38
  Speaker 1: you've ever imagined. Some of these molecular clouds span hundreds
• 22:43
  Speaker 1: of light years across. They are, in the visible light
• 22:47
  Speaker 1: we're accustomed to, essentially invisible. They look like holes in
• 22:52
  Speaker 1: the star field, Dark patches where the distant stars are
• 22:56
  Speaker 1: blotted out by the clouds' dense interior. But in infrared
• 23:00
  Speaker 1: and radio wavelengths, they glow with complex chemistry. Water, ammonia,
• 23:06
  Speaker 1: carbon monoxide, and even organic molecules like simple sugars and alcohols.
• 23:12
  Speaker 1: The chemical precursors to biology. Within these clouds, something begins.
• 23:18
  Speaker 1: Maybe a shockwave from a nearby supernova. nudges a region
• 23:22
  Speaker 1: of the cloud slightly denser than its surroundings. Maybe the
• 23:26
  Speaker 1: slow sweep of a spiral arm compresses the gas. Whatever
• 23:31
  Speaker 1: the trigger, gravity begins to take hold. A small region
• 23:35
  Speaker 1: starts to collapse. As it falls inward, it heats. It
• 23:39
  Speaker 1: spins faster, the way a figure skater pulls their arms
• 23:43
  Speaker 1: in to speed up. The collapsing cloud flattens into a disk,
• 23:47
  Speaker 1: a swirling protoplanetary disk. with a dense, glowing core at
• 23:52
  Speaker 1: its centre. That core, as it grows denser and hotter,
• 23:56
  Speaker 1: eventually reaches the critical pressure and temperature for hydrogen fusion
• 24:00
  Speaker 1: to ignite. And in that moment, a star is born,
• 24:04
  Speaker 1: a star, from nothing but cold gas and gravity and time,
• 24:09
  Speaker 1: and as it blazes to life, the remaining material in
• 24:13
  Speaker 1: the disk around it The gas and dust that didn't
• 24:16
  Speaker 1: fall into the star itself begins to clump and coalesce.
• 24:21
  Speaker 1: Small particles stick together. Clumps become pebbles, pebbles become boulders,
• 24:27
  Speaker 1: boulders become planetesimals, and over millions of years, gravity organizes
• 24:32
  Speaker 1: all of this into planets, moons, asteroid belts, comets, an
• 24:37
  Speaker 1: entire solar system assembled from the leftovers of a star's birth.
• 24:42
  Speaker 1: It happened here, 4.6 billion years ago, around a middling
• 24:47
  Speaker 1: yellow star in the Orion spur of the Milky Way.
• 24:51
  Speaker 1: This exact process played out, and here you are. Now
• 24:55
  Speaker 1: let me ask you something that might keep your mind
• 24:58
  Speaker 1: moving gently for another few minutes. When a massive star,
• 25:03
  Speaker 1: something 20 or 30 times the mass of the Sun,
• 25:06
  Speaker 1: reaches the end of its life, what do you imagine happens?
• 25:10
  Speaker 1: A gradual fading. A quiet dimming? No. What happens is
• 25:15
  Speaker 1: one of the most violent events in the observable universe.
• 25:19
  Speaker 1: The star exhausts its nuclear fuel. The core, which spent
• 25:24
  Speaker 1: millions of years fusing lighter elements into heavier ones, has
• 25:28
  Speaker 1: built up a core of iron. And iron is the
• 25:31
  Speaker 1: end of the road for stellar fusion. It takes more
• 25:34
  Speaker 1: energy to fuse iron than the reaction releases. So the
• 25:38
  Speaker 1: fusion stops. Suddenly, the outward pressure that held the star
• 25:43
  Speaker 1: against gravity's inward pull vanishes. And the star collapses. In
• 25:49
  Speaker 1: less than a second, a core the size of Earth
• 25:52
  Speaker 1: is compressed to the size of a city, roughly 20
• 25:56
  Speaker 1: kilometers across. The density becomes so extreme that protons and
• 26:01
  Speaker 1: electrons are forced together to form neutrons. This is a
• 26:05
  Speaker 1: neutron star. and the outer layers of the massive star,
• 26:10
  Speaker 1: collapsing inward and then rebounding off the suddenly incompressibly dense core,
• 26:16
  Speaker 1: explode outward in a supernova, an event so bright it
• 26:19
  Speaker 1: can outshine an entire galaxy for weeks. The energy released
• 26:24
  Speaker 1: in a few seconds is comparable to what the sun
• 26:27
  Speaker 1: will emit across its entire 10 billion year lifetime. The
• 26:32
  Speaker 1: shockwave expands outward at thousands of kilometers per second, crashing
• 26:38
  Speaker 1: into the surrounding interstellar medium, compressing nearby gas clouds, triggering
• 26:44
  Speaker 1: new rounds of star formation and scattering the heavy elements
• 26:48
  Speaker 1: forged in the dying star's core. Silicon, iron, nickel, gold, platinum, uranium.
• 26:56
  Speaker 1: Across light-years of space, these elements will eventually be incorporated
• 27:00
  Speaker 1: into new stars, new planets, new living things. The supernova
• 27:06
  Speaker 1: is not just an ending. It is the galaxy's most
• 27:09
  Speaker 1: dramatic act of recycling, the way the Milky Way converts
• 27:13
  Speaker 1: one generation of stars into the raw material for the next.
• 27:18
  Speaker 1: And something remarkable happens when two neutron stars, those ultra-dense
• 27:23
  Speaker 1: city-sized remnants of dead massive stars, spiral together and collide.
• 27:29
  Speaker 1: This event, called a kilonova, is even more exotic than
• 27:33
  Speaker 1: a supernova. In the final fractions of a second of
• 27:37
  Speaker 1: the merger, The conditions of extreme temperature and density forge
• 27:42
  Speaker 1: enormous quantities of the heaviest elements, gold, platinum, iridium, and more.
• 27:49
  Speaker 1: The kilonova sprays these materials into space in a spray
• 27:52
  Speaker 1: of radioactive light. Every atom of gold on Earth, every
• 27:57
  Speaker 1: wedding ring, every piece of jewelry, every gold coin, was
• 28:01
  Speaker 1: forged in the collision of neutron stars, not in the sun,
• 28:05
  Speaker 1: not in the Earth's mantle. in the death spiral of
• 28:09
  Speaker 1: two compact stellar remnants that lived and merged billions of
• 28:13
  Speaker 1: years before our solar system existed. You are wearing the
• 28:18
  Speaker 1: wreckage of ancient catastrophes. Let your breath slow for a moment.
• 28:23
  Speaker 1: We're about halfway through, and there's so much more to explore.
• 28:28
  Speaker 1: Did you know that the Milky Way is currently in
• 28:31
  Speaker 1: the process of eating another galaxy? Not metaphorically. Literally. The
• 28:36
  Speaker 1: Sagittarius dwarf spheroidal galaxy, a small, faint companion galaxy, is
• 28:42
  Speaker 1: being torn apart by the Milky Way's tidal forces right
• 28:45
  Speaker 1: now at this moment as you lie in your bed.
• 28:48
  Speaker 1: Its stars are being stripped away in long, looping streams
• 28:53
  Speaker 1: that wrap around the galactic disk multiple times. Some of
• 28:57
  Speaker 1: the stars that you see in the night sky, ordinary-looking
• 29:01
  Speaker 1: stars scattered across the halo, are Sagittarius dwarf stars. ripped
• 29:07
  Speaker 1: from their home galaxy and absorbed into ours. Astronomers have
• 29:11
  Speaker 1: mapped several of these tidal streams, tracing the ghost of
• 29:15
  Speaker 1: the Sagittarius dwarf across the sky. In a billion or
• 29:20
  Speaker 1: so years, nothing recognisable will remain of it. The Milky
• 29:24
  Speaker 1: Way will have consumed it entirely, and its stars will
• 29:28
  Speaker 1: simply be Milky Way stars, their origins forgotten. And the
• 29:33
  Speaker 1: Milky Way has done this many times before. One of
• 29:37
  Speaker 1: the most significant such mergers called the Gaia Sausage or
• 29:42
  Speaker 1: Gaia Enceladus merger, happened roughly 8 to 10 billion years ago,
• 29:47
  Speaker 1: when a dwarf galaxy roughly a quarter of the Milky
• 29:50
  Speaker 1: Way's mass spiraled inward and collided with our young galaxy.
• 29:56
  Speaker 1: The merger was enormous by cosmic standards. It likely triggered
• 30:00
  Speaker 1: massive waves of star formation. It scattered new stars throughout
• 30:05
  Speaker 1: the halo. It may have helped shape the early structure
• 30:09
  Speaker 1: of the disk and the inner stellar populations we see today.
• 30:14
  Speaker 1: We know about this event not because we observed it happening.
• 30:18
  Speaker 1: We weren't here. But because the chemical fingerprints and orbital
• 30:23
  Speaker 1: patterns of stars in the halo still carry the signature
• 30:28
  Speaker 1: of that ancient disruption. The galaxy remembers its past in
• 30:32
  Speaker 1: the motion and chemistry of its stars. This is one
• 30:36
  Speaker 1: of the most beautiful things about astronomy. The universe keeps records,
• 30:42
  Speaker 1: not in books or databases, but in the orbits of stars,
• 30:46
  Speaker 1: in the abundances of elements, in the direction and speed
• 30:49
  Speaker 1: of ancient stellar streams. Every merger, every supernova, Every collision
• 30:56
  Speaker 1: is archived in the fabric of what came after. An
• 30:59
  Speaker 1: astronomer studying the halo of the Milky Way is, in
• 31:03
  Speaker 1: a very real sense, reading history. Not history written down.
• 31:08
  Speaker 1: History written in light. Now, have you ever considered how
• 31:12
  Speaker 1: the spiral arms of the Milky Way actually work? Because
• 31:16
  Speaker 1: I think many people picture them as fixed structures. Solid,
• 31:21
  Speaker 1: curving lanes of stars and gas rotating around the centre
• 31:25
  Speaker 1: the way the spokes of a wheel rotate. But that's
• 31:29
  Speaker 1: not quite right. The spiral arms of the Milky Way
• 31:32
  Speaker 1: are not made of fixed stars. They are patterns, density waves.
• 31:37
  Speaker 1: Imagine a slow-moving traffic jam on a highway. The jam
• 31:41
  Speaker 1: itself moves along the road, even though the cars that
• 31:45
  Speaker 1: form it are constantly changing. As a car enters the jam,
• 31:49
  Speaker 1: it slows down. As it exits, it speeds up again.
• 31:53
  Speaker 1: The cars are moving. But the pattern, the jam, has
• 31:57
  Speaker 1: a position and a motion of its own. The spiral
• 32:00
  Speaker 1: arms of the Milky Way work exactly this way. The stars, gas,
• 32:05
  Speaker 1: and dust are constantly moving through the arms. As they
• 32:09
  Speaker 1: enter a region of higher density, they slow down, pile up,
• 32:13
  Speaker 1: become compressed. The increased compression in gas clouds triggers the
• 32:18
  Speaker 1: collapse and birth of new stars. These new stars, bright
• 32:23
  Speaker 1: and young and burning hot, light up the arm with
• 32:26
  Speaker 1: blue-white radiance. Eventually they drift out of the arm, and
• 32:31
  Speaker 1: the arm moves on, finding new material to compress. The
• 32:35
  Speaker 1: arm is not the stars. The arm is a wave
• 32:38
  Speaker 1: moving through the stars. And we are orbiting inside one
• 32:41
  Speaker 1: of these waves. our sun slowly drifting through the Orion spur,
• 32:47
  Speaker 1: a smaller structure between the larger Sagittarius and Perseus arms,
• 32:52
  Speaker 1: which brings me to something worth considering carefully. Our location
• 32:57
  Speaker 1: in the galaxy may not be random. We orbit at
• 33:00
  Speaker 1: a distance from the center where the rotation speed of
• 33:04
  Speaker 1: the spiral arms happens to roughly match the orbital speed
• 33:08
  Speaker 1: of individual stars. This is called the Corrotation Radius. Stars
• 33:14
  Speaker 1: near this distance don't frequently cross through the spiral arms,
• 33:19
  Speaker 1: which means they're not as often swept by the shock
• 33:22
  Speaker 1: waves and radiation that accompany the dense, star-forming regions of
• 33:28
  Speaker 1: the arms. Fewer close passages through stellar nurseries means fewer
• 33:33
  Speaker 1: nearby supernova. Fewer supernova means fewer extinction-level radiation events. Our
• 33:40
  Speaker 1: position in the galaxy, in this particular orbit, in this
• 33:44
  Speaker 1: particular quiet spur, may have given life on Earth a
• 33:48
  Speaker 1: stable enough environment, over billions of years, to evolve from
• 33:53
  Speaker 1: simple chemistry into the complexity we see today. We may
• 33:57
  Speaker 1: be alive, in part, because of where we live in
• 34:01
  Speaker 1: the galaxy. Now let me slow things down a little.
• 34:04
  Speaker 1: because we've been moving through enormous ideas, and I want
• 34:08
  Speaker 1: to give you a moment to breathe and simply float.
• 34:12
  Speaker 1: Close your eyes if they aren't already closed. Imagine you're
• 34:16
  Speaker 1: looking at the Milky Way from above, far outside the galaxy,
• 34:21
  Speaker 1: high above the galactic plane, looking down on the spiral.
• 34:26
  Speaker 1: You can see the arms curving outward from the central bar.
• 34:30
  Speaker 1: You can see the warm yellow glow of the core,
• 34:33
  Speaker 1: Thick with ancient stars, dense with mass and history, you
• 34:38
  Speaker 1: can see the faint shimmer of the outer disk where
• 34:41
  Speaker 1: star formation is still happening. But more slowly now, and
• 34:46
  Speaker 1: beyond the visible disk, you can sense, even if you
• 34:50
  Speaker 1: can't see, the enormous dark halo, extending far beyond the glow,
• 34:55
  Speaker 1: holding the galaxy together with invisible gravity. This is your
• 34:59
  Speaker 1: home seen from outside, a spiral in the dark, one
• 35:03
  Speaker 1: of 200 billion galaxies in the observable universe. And yet,
• 35:08
  Speaker 1: as far as we know, the only one that contains
• 35:11
  Speaker 1: a civilization looking back at it. Let me return to
• 35:16
  Speaker 1: the center, because there's more there that deserves your attention.
• 35:20
  Speaker 1: When astronomers look toward the galactic core using infrared telescopes,
• 35:25
  Speaker 1: which can penetrate the dust clouds that block visible light,
• 35:30
  Speaker 1: they don't see a simple round ball of stars. they
• 35:33
  Speaker 1: see a bar, a thick, elongated structure of stars crossing
• 35:38
  Speaker 1: the central region of the galaxy, rotating slowly, acting like
• 35:43
  Speaker 1: a stirring spoon in the inner disk. This bar is
• 35:46
  Speaker 1: not an accident. It forms when stars in the inner
• 35:50
  Speaker 1: regions drift inward and pile up along a central axis,
• 35:54
  Speaker 1: reinforcing each other's gravity and creating a stable, elongated structure.
• 36:00
  Speaker 1: The bar then influences everything around it, It channels gas
• 36:05
  Speaker 1: inward toward the supermassive black hole at the center. It
• 36:09
  Speaker 1: shapes the inner spiral arms, and it creates a distinctive
• 36:13
  Speaker 1: peanut-shaped bulge visible in cross-section. The core of the Milky Way,
• 36:19
  Speaker 1: viewed from the side, would look like a swollen, lumpy
• 36:22
  Speaker 1: hourglass of stars. And within that bar, there may be
• 36:27
  Speaker 1: something even stranger. Hints of a second, smaller bar, tilted
• 36:32
  Speaker 1: at a different angle, rotating independently. We can't see it directly.
• 36:37
  Speaker 1: The dust is too thick, the distance too great. But
• 36:41
  Speaker 1: its gravitational fingerprint shows up in the motion of surrounding
• 36:45
  Speaker 1: stars and gas. The centre of our galaxy is not
• 36:49
  Speaker 1: one structure. It's a nest of structures, rotating and interacting
• 36:55
  Speaker 1: and feeding material down into the black hole at the middle.
• 36:59
  Speaker 1: It's astonishingly complex for something we can't even look at directly.
• 37:05
  Speaker 1: Is star formation slowing down across the galaxy? Yes. Long ago,
• 37:10
  Speaker 1: perhaps 10 or 11 billion years ago, the Milky Way
• 37:14
  Speaker 1: was exploding with new stars. The gas was dense, the
• 37:18
  Speaker 1: conditions were ripe, and stars were forming at rates far
• 37:21
  Speaker 1: higher than today. But gradually, over billions of years, The
• 37:26
  Speaker 1: available raw material has been consumed or heated or scattered.
• 37:31
  Speaker 1: Many of the dense gas clouds have already collapsed into stars.
• 37:36
  Speaker 1: The interstellar medium, while still present, is thinner and warmer
• 37:40
  Speaker 1: in many regions than it once was. Today, the Milky
• 37:45
  Speaker 1: Way forms new stars at perhaps a few solar masses
• 37:48
  Speaker 1: per year, a slow, steady trickle compared to the stellar
• 37:52
  Speaker 1: flood of the past. This doesn't mean the galaxy is dying.
• 37:57
  Speaker 1: It means it's maturing. We are living in a quieter
• 38:00
  Speaker 1: chapter of the Milky Way's life. A time of gradual
• 38:04
  Speaker 1: consolidation rather than explosive creation. The galaxy is settling into
• 38:09
  Speaker 1: a kind of middle age. And what about its future?
• 38:13
  Speaker 1: Where is the Milky Way going? Not metaphorically. Literally. Where
• 38:18
  Speaker 1: is it physically travelling in space? The answer is strange
• 38:23
  Speaker 1: and interesting. The Milky Way, along with the Andromeda Galaxy
• 38:28
  Speaker 1: and about 50 other smaller galaxies, belongs to a gravitationally
• 38:32
  Speaker 1: bound group called the Local Group. This collection of galaxies
• 38:38
  Speaker 1: is drifting together through space, pulled by the gravity of
• 38:42
  Speaker 1: larger structures beyond it. But within the Local Group, two
• 38:46
  Speaker 1: galaxies dominate, the Milky Way and Andromeda, and they are
• 38:51
  Speaker 1: approaching each other. Right now, Andromeda is over two million
• 38:56
  Speaker 1: light years away. To the naked eye on a clear
• 39:00
  Speaker 1: dark night in the northern hemisphere, it appears as a
• 39:03
  Speaker 1: faint oval smudge near the constellation Cassiopeia. But it is
• 39:08
  Speaker 1: moving toward us at roughly 110 kilometers per second. In
• 39:13
  Speaker 1: about four and a half billion years, roughly the current
• 39:17
  Speaker 1: age of the solar system, the Milky Way and Andromeda
• 39:21
  Speaker 1: will collide. not immediately merge, but collide. The two spiral
• 39:27
  Speaker 1: galaxies will begin to pass through each other their stars
• 39:31
  Speaker 1: so sparsely distributed that individual stellar collisions will be rare.
• 39:37
  Speaker 1: But gravity will warp both galaxies into long tidal arms
• 39:41
  Speaker 1: and streams. The shapes will be unrecognizable. After hundreds of
• 39:46
  Speaker 1: millions of years of ongoing interaction, The two galaxies will
• 39:50
  Speaker 1: merge into a single larger structure, sometimes called Milkemeda, likely
• 39:56
  Speaker 1: an elliptical galaxy, its spiral arms long since disrupted by
• 40:01
  Speaker 1: the merger. What happens to the solar system in all
• 40:04
  Speaker 1: of this? Almost certainly nothing dramatic. The Sun, the Earth,
• 40:10
  Speaker 1: the other planets, all of them will continue orbiting, undisturbed
• 40:14
  Speaker 1: by the stellar dance around them. The night sky will
• 40:18
  Speaker 1: look very different. The stars will be rearranged. Perhaps the
• 40:23
  Speaker 1: Andromeda galaxy's core will arc brilliantly across the sky during
• 40:28
  Speaker 1: the merger. But the Earth, if it still exists by then,
• 40:32
  Speaker 1: if it hasn't already been engulfed by an expanding sun,
• 40:36
  Speaker 1: will still orbit. The merger of two galaxies is a slow,
• 40:40
  Speaker 1: gravitational dance, not a demolition. Now let me ask you
• 40:44
  Speaker 1: something before we go further. When you hear about stellar streams,
• 40:50
  Speaker 1: those faint ribbons of stars scattered across the sky, the
• 40:54
  Speaker 1: remnants of destroyed dwarf galaxies, does it feel like loss?
• 40:59
  Speaker 1: Because in some ways, it is. A galaxy that existed
• 41:03
  Speaker 1: for billions of years, containing its own population of stars,
• 41:07
  Speaker 1: its own history, its own dark matter halo, has been
• 41:11
  Speaker 1: gradually pulled apart by the tidal forces of the Milky
• 41:15
  Speaker 1: Way and scattered into a thread of light thousands of
• 41:19
  Speaker 1: light years long. Each star in that stream was once
• 41:23
  Speaker 1: part of a different galactic neighborhood. It had stellar neighbors
• 41:28
  Speaker 1: that are now gone, scattered across different quadrants of the
• 41:32
  Speaker 1: Milky Way's halo. The dwarf galaxy's identity has been erased.
• 41:38
  Speaker 1: Its stars now orbit the Milky Way as if they
• 41:40
  Speaker 1: were always part of it. And yet, The Milky Way
• 41:44
  Speaker 1: is built from these destructions. Every merger, every consumed dwarf galaxy,
• 41:50
  Speaker 1: every tidal stream has added to the mass and richness
• 41:53
  Speaker 1: of our own galaxy. Without these billions of years of accumulation,
• 41:59
  Speaker 1: without the stars, the gas, the dark matter, the heavy
• 42:03
  Speaker 1: elements donated by absorbed galaxies, the Milky Way would be
• 42:07
  Speaker 1: a much smaller, much less complex place. We are the
• 42:12
  Speaker 1: sum of many destructions. Perhaps that's true of most beautiful things.
• 42:17
  Speaker 1: Do you ever think about what the galaxy looks like
• 42:19
  Speaker 1: in light we can't see? Because the Milky Way we
• 42:23
  Speaker 1: observe with our eyes is only one channel of a
• 42:27
  Speaker 1: much richer broadcast. In radio wavelengths, the galaxy glows with
• 42:32
  Speaker 1: the signals of hydrogen gas, pulsars, and supernova remnants. In infrared,
• 42:38
  Speaker 1: it reveals its hidden star-forming regions the warm dust cloaking
• 42:43
  Speaker 1: stellar nurseries that would otherwise be invisible. In X-rays, it
• 42:47
  Speaker 1: blazes with the extreme temperatures of neutron star binaries and
• 42:52
  Speaker 1: hot coronal gas. In gamma rays, it shimmers with the
• 42:57
  Speaker 1: energy of cosmic ray interactions and the enormous Fermi bubbles
• 43:02
  Speaker 1: above and below the core. Each wavelength reveals a different structure,
• 43:08
  Speaker 1: a different population of objects, a different chapter in the
• 43:11
  Speaker 1: galaxy's story. We tend to think of the Milky Way
• 43:15
  Speaker 1: as what we see on a dark night, that pale
• 43:18
  Speaker 1: river of overlapping stars. But the full picture is infinitely richer.
• 43:24
  Speaker 1: The galaxy is broadcasting in dozens of frequencies simultaneously, and
• 43:30
  Speaker 1: we've only learned to receive a few of them. Let
• 43:33
  Speaker 1: me tell you about pulsars, because they are among the
• 43:36
  Speaker 1: strangest and most precise objects we've ever discovered. When a
• 43:41
  Speaker 1: massive star dies in a supernova, the core left behind,
• 43:45
  Speaker 1: a neutron star, may be spinning rapidly, very rapidly. Some
• 43:51
  Speaker 1: neutron stars rotate hundreds of times per second, and as
• 43:55
  Speaker 1: they spin, they emit beams of radio waves from their
• 43:58
  Speaker 1: magnetic poles. If one of those beams sweeps past the Earth,
• 44:03
  Speaker 1: we detect a regular pulse, a precise repeating radio signal.
• 44:08
  Speaker 1: blinking as reliably as the most accurate clock ever made.
• 44:12
  Speaker 1: These are pulsars, and their regularity is extraordinary. Some are
• 44:17
  Speaker 1: so precisely timed that they rival atomic clocks in stability.
• 44:22
  Speaker 1: Astronomers use pulsars to measure distances across the galaxy, to
• 44:27
  Speaker 1: test theories of gravity, and to search for gravitational waves.
• 44:32
  Speaker 1: They are the galaxy's metronomes, left behind by death blinking
• 44:37
  Speaker 1: steadily in the dark. And since we're talking about things
• 44:40
  Speaker 1: that pass through the galaxy invisibly, let me tell you
• 44:44
  Speaker 1: about neutrinos. Every second, trillions of neutrinos, ghostly, nearly massless
• 44:52
  Speaker 1: particles produced in nuclear reactions, pass through your body. Most
• 44:57
  Speaker 1: of them come from the sun, but some come from
• 45:00
  Speaker 1: much farther away. Supernova explosions produce enormous bursts of neutrinos.
• 45:06
  Speaker 1: so intense that in the brief seconds of the stellar collapse,
• 45:11
  Speaker 1: more neutrinos are produced than the Sun will emit across
• 45:15
  Speaker 1: its entire billion-year lifetime. Some of these ancient neutrinos, produced
• 45:21
  Speaker 1: in supernova explosions from the far reaches of the galaxy,
• 45:25
  Speaker 1: are passing through you as you lie there, through your body,
• 45:30
  Speaker 1: through the Earth, through everything. They interact with ordinary matter
• 45:35
  Speaker 1: so rarely that billions of them cross the planet every second,
• 45:39
  Speaker 1: and the planet barely notices. Special detectors buried deep underground
• 45:45
  Speaker 1: or frozen in Antarctic ice wait patiently to catch just
• 45:50
  Speaker 1: a few. The rare flash of light from a neutrino's
• 45:53
  Speaker 1: chance collision with an atom. Neutrinos carry information from the
• 45:58
  Speaker 1: galaxy's most violent events, and most of the time, They
• 46:03
  Speaker 1: carry it right through us without stopping to say hello.
• 46:07
  Speaker 1: Let's talk about globular clusters. These are some of the
• 46:11
  Speaker 1: oldest and most beautiful structures in the Milky Way. Compact
• 46:16
  Speaker 1: spheres of stars, sometimes containing hundreds of thousands of suns,
• 46:21
  Speaker 1: packed into a volume just a few dozen light-years across.
• 46:26
  Speaker 1: The Milky Way has more than 150 of them, orbiting
• 46:30
  Speaker 1: the galactic centre out in the halo, Like ancient satellites
• 46:35
  Speaker 1: around an ancient world, the stars inside globular clusters are
• 46:40
  Speaker 1: among the oldest in the galaxy, many more than 12
• 46:43
  Speaker 1: billion years old, some possibly older than the Milky Way's
• 46:48
  Speaker 1: disk itself. Their chemistry tells the story of the early universe,
• 46:53
  Speaker 1: when heavy elements were still scarce, when only the lightest
• 46:57
  Speaker 1: atoms had yet been forged and scattered. In a globular cluster,
• 47:02
  Speaker 1: If you stood on a planet orbiting one of its stars,
• 47:06
  Speaker 1: the night sky would be stunning beyond anything imaginable from Earth.
• 47:11
  Speaker 1: Stars packed so close together that the night would never
• 47:15
  Speaker 1: truly be dark. Multiple stars near enough to shine as
• 47:19
  Speaker 1: bright as a full moon. A sky blazing with ancient light,
• 47:24
  Speaker 1: but there's a catch. Globular clusters, being so old and
• 47:28
  Speaker 1: so dense, may not be the most hospitable places for life.
• 47:32
  Speaker 1: The high stellar density means frequent gravitational encounters between stars,
• 47:38
  Speaker 1: which can destabilize planetary orbits. Radiation from nearby stars can
• 47:44
  Speaker 1: strip atmospheres. The dense central regions may see stellar collisions,
• 47:50
  Speaker 1: two stars merging into one, creating what are called blue
• 47:54
  Speaker 1: straggler stars, objects that appear younger than their neighbors. because
• 47:59
  Speaker 1: the merger has essentially recharged their fusion fuel. Globular clusters
• 48:05
  Speaker 1: are magnificent time capsules from the early universe, but they
• 48:10
  Speaker 1: might be difficult places to evolve in peace. Now, gently,
• 48:15
  Speaker 1: let's talk about what lives between the stars. The space
• 48:18
  Speaker 1: between the stars is not empty. This is one of
• 48:21
  Speaker 1: the most important corrections in our understanding of the galaxy.
• 48:26
  Speaker 1: The interstellar medium, The thin mixture of gas, dust, and
• 48:31
  Speaker 1: plasma that fills the space between stars is complex, structured,
• 48:36
  Speaker 1: and astonishingly active. It includes cold, dense molecular clouds where
• 48:42
  Speaker 1: stars are born. It includes warm, diffuse hydrogen gas glowing
• 48:46
  Speaker 1: faintly in red light. It includes hot, ionized plasma heated
• 48:51
  Speaker 1: by nearby supernova and stellar winds. It includes magnetic fields
• 48:57
  Speaker 1: threading through the disk, guiding the motion of charged particles
• 49:01
  Speaker 1: and influencing how clouds collapse and how cosmic rays travel.
• 49:07
  Speaker 1: It includes shock waves from supernova, travelling outward at thousands
• 49:11
  Speaker 1: of kilometres per second, compressing the medium, triggering new star formation,
• 49:17
  Speaker 1: heating gas to millions of degrees. The interstellar medium is
• 49:21
  Speaker 1: not a void. It's a turbulent, multi-phase ecosystem, constantly stirred
• 49:27
  Speaker 1: and reshaped by the stars embedded within it. And within
• 49:31
  Speaker 1: that interstellar medium, within its coldest and densest clouds, something
• 49:37
  Speaker 1: remarkable has been found. organic molecules, complex carbon-based compounds, polycyclic
• 49:44
  Speaker 1: aromatic hydrocarbons, simple sugars, amino acid precursors, the basic building
• 49:50
  Speaker 1: blocks of chemistry that, on Earth, led to life. These
• 49:54
  Speaker 1: molecules form in the atmospheres of dying stars and within
• 49:59
  Speaker 1: the cold interiors of molecular clouds, shaped by cosmic rays
• 50:04
  Speaker 1: and ultraviolet radiation. They drift through the galaxy on microscopic
• 50:09
  Speaker 1: dust grains, traveling billions of kilometers, settling into the gas
• 50:14
  Speaker 1: clouds that collapse into new stars and new planetary systems.
• 50:19
  Speaker 1: Some of these molecules may have arrived on the early Earth,
• 50:23
  Speaker 1: carried by comets and asteroids, contributing to the chemical inventory
• 50:29
  Speaker 1: from which life emerged. The galaxy is not just making stars.
• 50:34
  Speaker 1: It is making chemistry. and perhaps, through that chemistry, making life.
• 50:40
  Speaker 1: Can I tell you something about the Milky Way's magnetic field?
• 50:44
  Speaker 1: It's easy to forget, because we rarely think about magnetism
• 50:48
  Speaker 1: at galactic scales. But the Milky Way is laced with
• 50:52
  Speaker 1: a magnetic field that extends across the entire disk, rising
• 50:57
  Speaker 1: into the halo, weaving through the interstellar medium in vast
• 51:01
  Speaker 1: patterns we're still learning to map. The field is weak
• 51:05
  Speaker 1: by earthly standards, millions of times weaker than a refrigerator magnet.
• 51:12
  Speaker 1: But across galactic distances, it has real and measurable effects.
• 51:17
  Speaker 1: It shapes how cosmic rays travel through the galaxy, bending
• 51:21
  Speaker 1: their paths and confining them. It influences how dust grains
• 51:26
  Speaker 1: align in space, causing them to polarize the starlight that
• 51:30
  Speaker 1: passes through them. It can affect the collapse of gas clouds,
• 51:35
  Speaker 1: adding pressure that resists gravity, potentially slowing or directing star formation.
• 51:41
  Speaker 1: And it forms loops and arcs above and below the
• 51:46
  Speaker 1: galactic plane, towering magnetic structures that we're only beginning to understand.
• 51:52
  Speaker 1: The galaxy is not just a gravitational structure. It is
• 51:56
  Speaker 1: an electromagnetic one. It hums with a magnetism that is ancient, distributed,
• 52:02
  Speaker 1: and quietly powerful. Now, before we slow towards sleep completely,
• 52:07
  Speaker 1: let me ask you one final lingering question. Do you
• 52:11
  Speaker 1: think the Milky Way is typical? Of the hundreds of
• 52:14
  Speaker 1: billions of galaxies we can observe in the universe, is
• 52:18
  Speaker 1: ours unusual in any way? The honest answer is, we're
• 52:22
  Speaker 1: not sure. The Milky Way is a barred spiral galaxy,
• 52:26
  Speaker 1: which is a common type. It's not particularly large by
• 52:30
  Speaker 1: galactic standards, but not small either. Its central black hole
• 52:35
  Speaker 1: is massive, but not the most massive known. It sits
• 52:39
  Speaker 1: in a relatively modest galaxy group, the Local Group, which
• 52:43
  Speaker 1: is itself a member of the much larger Virgo supercluster.
• 52:48
  Speaker 1: In some ways we might be average, but in others,
• 52:51
  Speaker 1: we might be unusual. Our position in the Galactic Habitable
• 52:55
  Speaker 1: Zone is not guaranteed for every spiral galaxy. our relatively
• 53:00
  Speaker 1: quiet central black hole, compared to galaxies with active quasar nuclei,
• 53:06
  Speaker 1: may have allowed complex structures to form without constant radiation disruption.
• 53:12
  Speaker 1: The Magellanic Clouds, our largest satellite galaxies, may be newcomers
• 53:19
  Speaker 1: rather than long-standing companions, which means we're currently experiencing a
• 53:24
  Speaker 1: gravitational perturbation that other spiral galaxies with stable satellites don't share.
• 53:31
  Speaker 1: Whether the Milky Way is typical or unusual in ways
• 53:34
  Speaker 1: that matter for the emergence of life is a question
• 53:38
  Speaker 1: we genuinely cannot yet answer. But the question is worth holding, quietly,
• 53:44
  Speaker 1: as you drift. Let me spend a little time with
• 53:47
  Speaker 1: something I find endlessly calming. Stellar Streams. We've mentioned them before,
• 53:54
  Speaker 1: but let's sit with the image. Across the halo of
• 53:57
  Speaker 1: the Milky Way, stretched out in long, thin arcs, some
• 54:02
  Speaker 1: of them tens of thousands of light-years long, are ribbons
• 54:06
  Speaker 1: of stars that were once part of other galaxies or
• 54:09
  Speaker 1: the outskirts of globular clusters. Tidal forces from the Milky
• 54:14
  Speaker 1: Way's mass have stretched and shredded these structures, smearing their
• 54:19
  Speaker 1: stars across the sky in delicate, curved threads. Most are
• 54:24
  Speaker 1: invisible to the naked eye, many were invisible to any telescope,
• 54:29
  Speaker 1: until surveys like Sloan and Gaia mapped the positions and
• 54:33
  Speaker 1: velocities of millions of stars in detail. Now we see them,
• 54:37
  Speaker 1: ghostly rivers of ancient stars, tracing the paths of their
• 54:42
  Speaker 1: former homes, orbiting the Milky Way in slow, majestic loops
• 54:47
  Speaker 1: that take hundreds of millions of years to complete. Some
• 54:51
  Speaker 1: of these streams are so faint, so dispersed, that distinguishing
• 54:55
  Speaker 1: their member stars from the background of the halo requires
• 54:59
  Speaker 1: careful statistical analysis. But they're there. The Sagittarius Stream. The
• 55:06
  Speaker 1: Orphan Stream. The Styx Stream. The GD1 Stream. Each one
• 55:11
  Speaker 1: a different destroyed structure, each one a record of a
• 55:14
  Speaker 1: gravitational encounter. The halo of the Milky Way is a
• 55:18
  Speaker 1: museum of destructions, a gallery of past encounters. The sky
• 55:24
  Speaker 1: is full of the remnants of things that no longer
• 55:27
  Speaker 1: exist in the form they once had. Now, while you rest,
• 55:31
  Speaker 1: I want to take you through something that the Milky
• 55:34
  Speaker 1: Way does over vast timescales that I find almost meditative
• 55:39
  Speaker 1: to think about. The Galactic Fountains. Above and below the
• 55:44
  Speaker 1: flat disk of the Milky Way, gas is constantly flowing.
• 55:49
  Speaker 1: Supernova explosions and the intense stellar winds of massive stars
• 55:54
  Speaker 1: heat the interstellar medium, creating enormous bubbles of hot gas
• 55:58
  Speaker 1: that expand and push upward, punching through the disk and
• 56:02
  Speaker 1: venting into the halo. This gas rises, slowly by cosmic standards,
• 56:08
  Speaker 1: but rising nonetheless. carrying with it heavy elements forged in
• 56:12
  Speaker 1: the stellar explosions that drove it out. As it rises
• 56:16
  Speaker 1: into the cooler halo, it begins to slow and cool. Eventually,
• 56:21
  Speaker 1: it condenses, and it falls back down, a slow cosmic
• 56:25
  Speaker 1: rain of enriched gas returning to the disk where it
• 56:29
  Speaker 1: will eventually join a molecular cloud and collapse into new stars.
• 56:34
  Speaker 1: The galaxy breathes. It exhales superheated gas through chimneys above
• 56:39
  Speaker 1: the disk and inhales it back as cool, enriched clouds
• 56:43
  Speaker 1: returning to the plane. The cycle repeats over hundreds of
• 56:48
  Speaker 1: millions of years. The Milky Way is not a static structure.
• 56:52
  Speaker 1: It circulates, and similarly the galaxy is not a closed system.
• 56:57
  Speaker 1: It doesn't just recycle its own material. It draws in
• 57:01
  Speaker 1: fresh gas from outside itself, the cosmic web, The enormous,
• 57:06
  Speaker 1: large-scale structure of the universe, made of filaments of dark
• 57:10
  Speaker 1: matter and gas connecting galaxy clusters like threads in a
• 57:14
  Speaker 1: cosmic spider's web, delivers material to galaxies continuously. Long, tenuous
• 57:21
  Speaker 1: streams of gas fall inward from the intergalactic medium, slipping
• 57:26
  Speaker 1: into the halo, eventually reaching the disk, eventually joining the
• 57:31
  Speaker 1: cycle of star formation. The Milky Way drinks from the
• 57:35
  Speaker 1: cosmic web. It is always receiving new material, always incorporating
• 57:40
  Speaker 1: outside mass into its own ongoing story. Let me tell
• 57:45
  Speaker 1: you about the Corotation Radius one more time, but more
• 57:49
  Speaker 1: gently this time, because I think it's one of the
• 57:51
  Speaker 1: most quietly important facts about our place in the galaxy.
• 57:57
  Speaker 1: The spiral arms of the Milky Way rotate at a
• 58:00
  Speaker 1: certain speed. Individual stars orbit at their own speeds, depending
• 58:05
  Speaker 1: on their distance from the centre. At most distances, these
• 58:09
  Speaker 1: speeds are different. Stars move faster or slower than the
• 58:14
  Speaker 1: pattern of the arms, but at one specific distance from
• 58:18
  Speaker 1: the centre, the speeds match. A star at this distance
• 58:22
  Speaker 1: orbits the galaxy at almost exactly the same speed as
• 58:26
  Speaker 1: the spiral arm pattern. This means the star doesn't frequently
• 58:30
  Speaker 1: pass through the arm. It doesn't repeatedly encounter the dense, radiation-intense,
• 58:37
  Speaker 1: supernova-heavy environments inside the arms. It drifts in relative peace
• 58:43
  Speaker 1: in the quiet space between the arms' passages. Our sun
• 58:47
  Speaker 1: orbits near this corrotation radius. We have been for billions
• 58:51
  Speaker 1: of years, riding roughly in step with the galaxy's spiral arms,
• 58:57
  Speaker 1: not swept repeatedly through their dangerous hearts, but hovering at
• 59:02
  Speaker 1: the edge of the pattern, in a relatively calm corridor.
• 59:06
  Speaker 1: Some astronomers think this may be one of the reasons
• 59:09
  Speaker 1: complex life had the time to evolve on Earth. The galaxy,
• 59:14
  Speaker 1: in its structure and its dynamics, may have provided us
• 59:18
  Speaker 1: a protected lane. Not because it meant to. Simply because
• 59:23
  Speaker 1: the mathematics of its rotation happened to place a quiet
• 59:26
  Speaker 1: zone at exactly the distance where our sun orbits. The
• 59:31
  Speaker 1: universe didn't choose us, but we may have found ourselves,
• 59:35
  Speaker 1: by sheer orbital coincidence, in exactly the right place. As
• 59:40
  Speaker 1: we move toward the close of our journey, I want
• 59:44
  Speaker 1: to give you something for the very edge of sleep,
• 59:47
  Speaker 1: something to carry you across, the oldest light that reaches
• 59:51
  Speaker 1: your eyes on a clear night left its source years ago.
• 59:55
  Speaker 1: in some cases, thousands of years ago. You are not
• 59:59
  Speaker 1: seeing the star as it is. You are seeing the
• 1:00:02
  Speaker 1: star as it was, sometimes before your earliest ancestors built
• 1:00:06
  Speaker 1: the first cities. The galaxy you see is not the
• 1:00:10
  Speaker 1: galaxy that currently exists. It is a mosaic of different moments,
• 1:00:16
  Speaker 1: different pasts, layered together in one simultaneous sky. The light
• 1:00:21
  Speaker 1: from nearby stars left them years or decades ago, The
• 1:00:25
  Speaker 1: light from the galactic centre, if it could reach your
• 1:00:29
  Speaker 1: eyes without dust in the way, would have left its
• 1:00:32
  Speaker 1: source 26,000 years ago, when our ancestors were drawing animals
• 1:00:38
  Speaker 1: on cave walls in France. The galaxy is not just
• 1:00:41
  Speaker 1: a place. It is a time machine. Every direction you look,
• 1:00:46
  Speaker 1: you're looking backward. Every point of light is a message
• 1:00:50
  Speaker 1: from the past, crossing the void to reach your retina
• 1:00:53
  Speaker 1: in this exact moment. on this exact night, as you
• 1:00:57
  Speaker 1: lie there and breathe slowly in the dark. And the
• 1:01:01
  Speaker 1: Milky Way, for all its scale, for all its hundred
• 1:01:05
  Speaker 1: billion stars and its dark matter and its ancient mergers
• 1:01:09
  Speaker 1: and its orbiting black hole, is still only one galaxy
• 1:01:13
  Speaker 1: among two hundred billion in the observable universe. And the
• 1:01:18
  Speaker 1: observable universe is only the portion we can see, The
• 1:01:22
  Speaker 1: sphere of space from which light has had time to
• 1:01:25
  Speaker 1: reach us since the Big Bang. Beyond that horizon, the
• 1:01:29
  Speaker 1: universe continues. Stars burning in galaxies we will never see.
• 1:01:35
  Speaker 1: Mergers happening in silence across distances that light will never
• 1:01:39
  Speaker 1: bridge for us. A cosmos so vast that our most
• 1:01:43
  Speaker 1: powerful telescopes are like pinpricks in a wall. letting through
• 1:01:49
  Speaker 1: just enough light to tell us that the darkness on
• 1:01:51
  Speaker 1: the other side is not empty. And within all of that,
• 1:01:55
  Speaker 1: all of that vastness, all of that ancient, ongoing, violent, creative, patient,
• 1:02:01
  Speaker 1: indifferent cosmos, here you are, a warm creature on a
• 1:02:05
  Speaker 1: wet rock, orbiting a middle-aged star in the outer arm
• 1:02:09
  Speaker 1: of a spiral galaxy, in a small cluster of galaxies,
• 1:02:13
  Speaker 1: in a larger supercluster, In a universe 13 and a
• 1:02:17
  Speaker 1: half billion years old, you are cosmically tiny, and you
• 1:02:21
  Speaker 1: are cosmically precious. Because as far as we know, you
• 1:02:26
  Speaker 1: are one of the places the universe has learned to
• 1:02:29
  Speaker 1: look at itself, to wonder, to ask where it came from,
• 1:02:33
  Speaker 1: where it's going, and whether there is anyone else out
• 1:02:37
  Speaker 1: there asking the same question. The Milky Way doesn't care
• 1:02:41
  Speaker 1: whether you sleep or stay awake. It doesn't know you exist.
• 1:02:46
  Speaker 1: It will keep turning for billions of years after you,
• 1:02:50
  Speaker 1: and everything you've ever known is gone. And somehow, somehow,
• 1:02:55
  Speaker 1: that is the most comforting thought I know. Because it
• 1:02:59
  Speaker 1: means the universe was beautiful before you, and it will
• 1:03:02
  Speaker 1: be beautiful after you, and right now, in this quiet moment,
• 1:03:07
  Speaker 1: you are the part of it that is aware of
• 1:03:09
  Speaker 1: that beauty. Let your breath slow. Let the thoughts untangle.
• 1:03:14
  Speaker 1: The galaxy is turning. Stars are forming in cold clouds
• 1:03:19
  Speaker 1: hundreds of light years away. A rogue star is crossing
• 1:03:23
  Speaker 1: the void between galaxies. A pulsar is blinking steadily in
• 1:03:27
  Speaker 1: the dark. Neutrinos are passing through you right now, gentle
• 1:03:32
  Speaker 1: and invisible and ancient. The Magellanic clouds are completing their
• 1:03:36
  Speaker 1: slow first orbit of our home. Somewhere in the halo,
• 1:03:41
  Speaker 1: a star older than the galaxy's disk is burning softly,
• 1:03:45
  Speaker 1: having waited 13 billion years to be noticed. You don't
• 1:03:49
  Speaker 1: have to understand it all. You don't have to hold
• 1:03:52
  Speaker 1: it in your mind. You can simply be in it.
• 1:03:56
  Speaker 1: Floating in the spiral arm of a quiet galaxy, on
• 1:04:00
  Speaker 1: an ordinary night, as the universe goes about its business,
• 1:04:03
  Speaker 1: sleep well. Dream softly. The Milky Way will still be
• 1:04:08
  Speaker 1: there when you wake up. It always is. Good night.
• 1:04:12
  Speaker 1: But wait. Don't drift away just yet. There are still
• 1:04:16
  Speaker 1: so many quiet corners of the Milky Way we haven't visited.
• 1:04:20
  Speaker 1: And the strangest ones, perhaps, are saved for last. So
• 1:04:24
  Speaker 1: let your body stay heavy where it is. Let your
• 1:04:27
  Speaker 1: breathing stay slow. And let me carry you just a
• 1:04:31
  Speaker 1: little further into the dark. Have you ever thought about
• 1:04:35
  Speaker 1: what it means that we don't know how many arms
• 1:04:38
  Speaker 1: the Milky Way has? It's such a fundamental question, like
• 1:04:43
  Speaker 1: not knowing how many floors are in the building you
• 1:04:46
  Speaker 1: live in. And yet, despite decades of observation, astronomers still
• 1:04:52
  Speaker 1: debate the exact number and extent of the Milky Way's
• 1:04:55
  Speaker 1: spiral arms. The problem is our location. We sit inside
• 1:05:00
  Speaker 1: the disk, peering outward through layers of gas and dust,
• 1:05:04
  Speaker 1: trying to map structures that span tens of thousands of
• 1:05:08
  Speaker 1: light years, when our instruments can only see a few
• 1:05:12
  Speaker 1: thousand light-years clearly in some directions before the dust becomes impenetrable.
• 1:05:19
  Speaker 1: It's like trying to map the coastline of a continent
• 1:05:22
  Speaker 1: while standing in the middle of a forest on that coast.
• 1:05:25
  Speaker 1: For a long time, textbooks said four major arms, the
• 1:05:30
  Speaker 1: Norma arm, the Scutum Centaurus arm, the Sagittarius arm, and
• 1:05:35
  Speaker 1: the Perseus arm, and a smaller spur, the Orion Spur,
• 1:05:40
  Speaker 1: where our Sun resides. But more recent surveys, using radio
• 1:05:44
  Speaker 1: telescopes that can pierce through the dust, and the extraordinary
• 1:05:48
  Speaker 1: data from the Gaia spacecraft, suggest the picture is more complex.
• 1:05:53
  Speaker 1: Some arms appear to fragment or bifurcate. Some seem to
• 1:05:58
  Speaker 1: fade and strengthen at different points along their length. The
• 1:06:02
  Speaker 1: galaxy may not be a clean, symmetric, four-armed spiral at all,
• 1:06:07
  Speaker 1: but something more ragged and irregular. A pattern imposed on
• 1:06:12
  Speaker 1: underlying randomness, beautiful from a distance but full of irregularities
• 1:06:17
  Speaker 1: up close. Like most beautiful things. And do you know
• 1:06:22
  Speaker 1: what the Gaia spacecraft has done for our understanding of
• 1:06:26
  Speaker 1: the Milky Way? It has been quietly revolutionary. Launched by
• 1:06:31
  Speaker 1: the European Space Agency in 2013, Gaia has spent years
• 1:06:36
  Speaker 1: measuring the positions, distances, and motions of more than a
• 1:06:41
  Speaker 1: billion stars with extraordinary precision. Precision that allows astronomers to
• 1:06:47
  Speaker 1: reconstruct three-dimensional maps of the galaxy and trace the motions
• 1:06:52
  Speaker 1: of stars backward and forward through time. From Gaia's data,
• 1:06:58
  Speaker 1: scientists have discovered stellar streams that were previously invisible. They've
• 1:07:03
  Speaker 1: mapped the warp of the galactic disk in greater detail
• 1:07:07
  Speaker 1: than ever before. They've identified the signatures of ancient mergers
• 1:07:13
  Speaker 1: in the orbital patterns of halo stars. They've confirmed that
• 1:07:17
  Speaker 1: the sun bobs gently above and below the galactic plane
• 1:07:21
  Speaker 1: as it orbits. And they've given us, for the first time,
• 1:07:25
  Speaker 1: a dynamic picture of the galaxy. Not a snapshot, but
• 1:07:29
  Speaker 1: a movie, showing how stars have moved and will move.
• 1:07:34
  Speaker 1: Gaia is the closest thing we have to truly seeing
• 1:07:37
  Speaker 1: the Milky Way from outside. Not from above, not from
• 1:07:41
  Speaker 1: a distance. We're still trapped inside, but with enough precision
• 1:07:46
  Speaker 1: that we can reconstruct what that external view would look like.
• 1:07:51
  Speaker 1: It is a remarkable technological achievement, sitting in the quiet
• 1:07:55
  Speaker 1: routine of data transmission. A small spacecraft a million miles
• 1:08:00
  Speaker 1: from Earth, counting stars one by one. Now let me
• 1:08:04
  Speaker 1: ask you something that might seem strange at first. What
• 1:08:08
  Speaker 1: do you think it would be like to live on
• 1:08:10
  Speaker 1: a planet in a globular cluster? One of those ancient,
• 1:08:15
  Speaker 1: dense spheres of hundreds of thousands of stars orbiting the
• 1:08:19
  Speaker 1: Milky Way's halo. We touched on this briefly, but let's
• 1:08:23
  Speaker 1: go deeper. In the densest globular clusters, stars are packed
• 1:08:28
  Speaker 1: together so tightly that they're separated by distances of light weeks.
• 1:08:33
  Speaker 1: not light years. Our nearest stellar neighbour, Proxima Centauri, is
• 1:08:39
  Speaker 1: about 4.2 light years away. In the core of a
• 1:08:43
  Speaker 1: globular cluster, that distance might be reduced to a fraction
• 1:08:47
  Speaker 1: of a light year. On the surface of a planet
• 1:08:50
  Speaker 1: in such a cluster, if stable planets can exist there
• 1:08:54
  Speaker 1: at all, given the constant gravitational nudges from nearby stars,
• 1:08:59
  Speaker 1: the night sky would be unimaginable. Not dark, never fully dark.
• 1:09:05
  Speaker 1: Thousands of stars close enough to cast shadows. A sky
• 1:09:09
  Speaker 1: so crowded with light that night would simply be a dimming,
• 1:09:13
  Speaker 1: not a darkening. a sky where multiple stars burned bright
• 1:09:17
  Speaker 1: enough to be visible at noon, visible alongside the sun itself.
• 1:09:22
  Speaker 1: For early astronomers on such a world, the challenge would
• 1:09:26
  Speaker 1: not be seeing stars, it would be making sense of
• 1:09:30
  Speaker 1: the overwhelming number of them. Navigation by stars would be
• 1:09:34
  Speaker 1: an incomprehensible task, and the recognition that space was mostly empty,
• 1:09:40
  Speaker 1: that stars were enormously distant from each other, would come
• 1:09:44
  Speaker 1: much later. Because from inside a globular cluster, they are not.
• 1:09:50
  Speaker 1: The universe would feel smaller, more intimate, more crowded, and
• 1:09:55
  Speaker 1: yet perhaps no more familiar, no more knowable. We are lucky,
• 1:09:59
  Speaker 1: in a way, to live somewhere quieter, where the night
• 1:10:03
  Speaker 1: sky is dark enough to see, but not so crowded
• 1:10:06
  Speaker 1: that it overwhelms, where the stars are few enough to name,
• 1:10:10
  Speaker 1: to group into constellations, to use as a map, Our sky,
• 1:10:15
  Speaker 1: sparse as it sometimes feels, is perfectly scaled for wonder
• 1:10:20
  Speaker 1: without chaos. Let me tell you something about the interstellar
• 1:10:24
  Speaker 1: medium that I find almost meditative to sit with. The
• 1:10:29
  Speaker 1: gas between the stars exists in several distinct phases, and
• 1:10:33
  Speaker 1: they coexist across the galaxy in a kind of thermal
• 1:10:37
  Speaker 1: equilibrium maintained by the energy input of stars and supernovae.
• 1:10:42
  Speaker 1: There is the cold neutral medium, dense clouds of hydrogen,
• 1:10:46
  Speaker 1: just a few tens of degrees above absolute zero, dark
• 1:10:51
  Speaker 1: and dormant, waiting for a gravitational nudge to collapse. There
• 1:10:55
  Speaker 1: is the warm neutral medium, more diffuse hydrogen at temperatures
• 1:11:00
  Speaker 1: of a few thousand degrees, filling much of the galactic disk.
• 1:11:05
  Speaker 1: There is the warm ionized medium, Hydrogen heated and ionized
• 1:11:10
  Speaker 1: by ultraviolet light from hot young stars, glowing in faint
• 1:11:14
  Speaker 1: emission lines detectable by radio telescopes. And there is the
• 1:11:18
  Speaker 1: hot ionized medium, gas heated to millions of degrees by
• 1:11:23
  Speaker 1: supernova shock waves forming vast, tenuous bubbles and the hot
• 1:11:28
  Speaker 1: plasma that fills much of the volume between the galaxy's
• 1:11:32
  Speaker 1: denser structures. These four phases are in rough balance. constantly
• 1:11:37
  Speaker 1: exchanging mass and energy supernova heat the hot phase the
• 1:11:42
  Speaker 1: hot phase slowly cools and condenses condensed gas forms cold
• 1:11:47
  Speaker 1: clouds cold clouds collapse into stars stars heat their surroundings
• 1:11:52
  Speaker 1: and eventually explode into supernova the cycle continues as it
• 1:11:58
  Speaker 1: has for billions of years as it will continue for
• 1:12:01
  Speaker 1: billions more the interstellar medium is not background it is
• 1:12:06
  Speaker 1: the engine And within that engine, something extraordinary happens with
• 1:12:11
  Speaker 1: magnetic fields. The galaxy's magnetic field doesn't just thread passively
• 1:12:17
  Speaker 1: through the interstellar medium, it actively shapes it. When a
• 1:12:22
  Speaker 1: molecular cloud begins to collapse towards star formation, the magnetic
• 1:12:27
  Speaker 1: field threading through it provides a pressure that opposes the collapse.
• 1:12:33
  Speaker 1: Gas can flow along field lines relatively freely, but moving
• 1:12:37
  Speaker 1: across field lines requires more energy. This means that the
• 1:12:42
  Speaker 1: magnetic field can slow or channel the collapse of a cloud,
• 1:12:46
  Speaker 1: influencing how many stars form, how quickly, and in what configuration.
• 1:12:52
  Speaker 1: In the densest clouds, the magnetic field is eventually overcome.
• 1:12:57
  Speaker 1: Gravity wins. The collapse proceeds. But in more diffuse regions,
• 1:13:02
  Speaker 1: the magnetic field can hold clouds in suspension. preventing collapse
• 1:13:07
  Speaker 1: for millions of years longer than gravity alone would allow.
• 1:13:11
  Speaker 1: The Milky Way's magnetic field is not just a curiosity.
• 1:13:17
  Speaker 1: It is a governor of star formation, an invisible hand
• 1:13:20
  Speaker 1: shaping the rate at which the galaxy produces new suns. Now,
• 1:13:25
  Speaker 1: and here is a question worth asking very slowly, in
• 1:13:29
  Speaker 1: the quiet just before sleep, do you think there are
• 1:13:32
  Speaker 1: other civilizations in the Milky Way? Not in other galaxies,
• 1:13:37
  Speaker 1: just in ours. Just among the 100 billion to 400
• 1:13:41
  Speaker 1: billion stars of the galaxy you call home. We have
• 1:13:45
  Speaker 1: been searching, formerly, for extraterrestrial intelligence since the early 1960s.
• 1:13:53
  Speaker 1: Radio telescopes have swept the sky for artificial signals, narrowband transmissions,
• 1:13:59
  Speaker 1: laser pulses, techno signatures. So far, silence. But the silence
• 1:14:04
  Speaker 1: is not necessarily an answer. We have searched a tiny
• 1:14:08
  Speaker 1: fraction of the galaxy with a tiny fraction of the
• 1:14:11
  Speaker 1: techniques available to us. The Milky Way is vast. Radio
• 1:14:16
  Speaker 1: signals travel at the speed of light are, and the
• 1:14:19
  Speaker 1: galaxy is a hundred thousand light years across. A civilization
• 1:14:25
  Speaker 1: on the far side of the galaxy that began transmitting
• 1:14:29
  Speaker 1: when the pyramids were being built is still more than 95,000
• 1:14:34
  Speaker 1: light-years away from us. Their signals haven't reached us yet.
• 1:14:38
  Speaker 1: The question of life in the galaxy touches everything we've
• 1:14:42
  Speaker 1: discussed tonight. The right elements, forged in dying stars. The
• 1:14:48
  Speaker 1: right chemistry, assembling in cold clouds. Stable planetary orbits in
• 1:14:54
  Speaker 1: galactic habitable zones. Enough time, billions of years, for complexity
• 1:14:59
  Speaker 1: to emerge. All of these conditions are met, repeatedly, throughout
• 1:15:04
  Speaker 1: the galaxy. Whether the final step, the emergence of something
• 1:15:09
  Speaker 1: that looks at the sky and wonders, happens commonly or rarely,
• 1:15:13
  Speaker 1: we genuinely don't know. But the galaxy gives it every
• 1:15:17
  Speaker 1: reasonable chance. Let me slow further. Let me walk you
• 1:15:22
  Speaker 1: through something at a quieter pace. The sun will die
• 1:15:25
  Speaker 1: one day. Not for another five billion years or so,
• 1:15:30
  Speaker 1: which is an incomprehensible span of time, roughly equal to
• 1:15:34
  Speaker 1: the sun's current age again. But eventually, the sun will
• 1:15:38
  Speaker 1: exhaust the hydrogen in its core. Fusion will shift to
• 1:15:42
  Speaker 1: its outer layers. The sun will expand, slowly at first,
• 1:15:46
  Speaker 1: then more rapidly, swelling into a red giant, engulfing Mercury, Venus,
• 1:15:52
  Speaker 1: and possibly Earth in the process. The outer layers will
• 1:15:56
  Speaker 1: puff outward in a glowing shell called a planetary nebula,
• 1:16:00
  Speaker 1: a beautiful ring-shaped cloud of ionized gas lit from within
• 1:16:04
  Speaker 1: by the hot core left behind. And that core, a dense,
• 1:16:09
  Speaker 1: cooling object about the size of the Earth, but with
• 1:16:12
  Speaker 1: half the Sun's original mass, will become a white dwarf.
• 1:16:17
  Speaker 1: White dwarfs are the most common final state for stars
• 1:16:21
  Speaker 1: like the Sun. They don't undergo fusion. They simply radiate
• 1:16:25
  Speaker 1: stored heat, gradually cooling over trillions of years. in a
• 1:16:30
  Speaker 1: hundred trillion years. Far longer than the current age of
• 1:16:35
  Speaker 1: the universe, the sun's white dwarf will have cooled to
• 1:16:38
  Speaker 1: match the temperature of space itself, becoming a cold, dark
• 1:16:43
  Speaker 1: cinder orbiting in the dark. A black dwarf, but the
• 1:16:47
  Speaker 1: material of the sun, the hydrogen, the helium, the carbon,
• 1:16:51
  Speaker 1: the oxygen built up over its lifetime, will not be lost.
• 1:16:55
  Speaker 1: Some of it will be scattered by the planetary nebula,
• 1:16:59
  Speaker 1: into the interstellar medium. It will cool, condense, and eventually
• 1:17:04
  Speaker 1: join a molecular cloud. And from that cloud, perhaps billions
• 1:17:09
  Speaker 1: of years from now, new stars will form, new planets,
• 1:17:12
  Speaker 1: new worlds. The Sun is not eternal, but its matter is.
• 1:17:17
  Speaker 1: It will continue in different forms as long as the
• 1:17:20
  Speaker 1: galaxy persists. Now I want to tell you about something
• 1:17:24
  Speaker 1: called blue straggler stars. because they are among the universe's
• 1:17:29
  Speaker 1: more peculiar characters. In a globular cluster, where all the
• 1:17:34
  Speaker 1: stars formed at roughly the same time from the same cloud,
• 1:17:38
  Speaker 1: you'd expect a uniform distribution of ages. Old stars should
• 1:17:43
  Speaker 1: look old, reddish, dimmer, beginning to expand. But scattered among
• 1:17:48
  Speaker 1: the cluster's population, astronomers find a handful of stars that
• 1:17:52
  Speaker 1: appear younger, bluer, hotter, more luminous. Stars that look like
• 1:17:59
  Speaker 1: they shouldn't be there, based on the cluster's age. These
• 1:18:03
  Speaker 1: are blue stragglers, and the most widely accepted explanation is
• 1:18:08
  Speaker 1: that they are the products of stellar mergers. Two stars
• 1:18:12
  Speaker 1: in a tight binary system have spiraled together, perhaps nudged
• 1:18:17
  Speaker 1: by a close gravitational encounter with another star, and merged.
• 1:18:22
  Speaker 1: The merger has combined their hydrogen reserves essentially giving the
• 1:18:27
  Speaker 1: resulting star a fresh supply of fuel and making it
• 1:18:32
  Speaker 1: appear younger than its surroundings. There's something deeply interesting about this.
• 1:18:38
  Speaker 1: In the crowded environment of a globular cluster, stars occasionally touch,
• 1:18:43
  Speaker 1: occasionally merge, two separate stellar lives become one, and the
• 1:18:48
  Speaker 1: result appears reborn. a blue, hot, luminous star burning in
• 1:18:53
  Speaker 1: a cluster of ancients, conspicuous precisely because it defies the
• 1:18:58
  Speaker 1: expected pattern of aging. The universe, it seems, occasionally arranges
• 1:19:03
  Speaker 1: for second acts. Are you still here? Good. Then let
• 1:19:08
  Speaker 1: me offer you something truly enormous. The question of the
• 1:19:13
  Speaker 1: Milky Way's mass. You would think that measuring the mass
• 1:19:17
  Speaker 1: of our own galaxy would be something we could do
• 1:19:20
  Speaker 1: with confidence. But it remains genuinely uncertain, and the uncertainty
• 1:19:25
  Speaker 1: is large. Current estimates suggest the Milky Way's total mass,
• 1:19:31
  Speaker 1: including its dark matter halo, is somewhere between 700 billion...
• 1:19:36
  Speaker 1: and two trillion times the mass of the sun. That
• 1:19:39
  Speaker 1: factor of roughly three between the lower and upper estimates
• 1:19:44
  Speaker 1: is enormous. We're uncertain about the mass of our own
• 1:19:48
  Speaker 1: home by a factor of three. The reason is dark matter.
• 1:19:52
  Speaker 1: We can measure the mass of the visible components, the stars,
• 1:19:56
  Speaker 1: the gas, the dust, with reasonable accuracy. But dark matter
• 1:20:01
  Speaker 1: is invisible. and its distribution beyond the visible disk of
• 1:20:05
  Speaker 1: the galaxy is difficult to constrain. We infer its presence
• 1:20:11
  Speaker 1: from the orbits of distant stars and globular clusters, but
• 1:20:15
  Speaker 1: our sampling of those distant orbits is incomplete. Better constraints
• 1:20:20
  Speaker 1: are coming, from Gaia's ongoing measurements, from pulsar timing arrays,
• 1:20:25
  Speaker 1: from future telescopes, but for now, the Milky Way's true
• 1:20:30
  Speaker 1: weight remains one of its more quietly humbling secrets. And interestingly,
• 1:20:36
  Speaker 1: this uncertainty matters for the future. When the Milky Way
• 1:20:40
  Speaker 1: and Andromeda eventually collide and merge, the relative masses of
• 1:20:45
  Speaker 1: the two galaxies will determine the dynamics of the merger,
• 1:20:50
  Speaker 1: which one's black hole ends up at the center of
• 1:20:53
  Speaker 1: the merged system. How the stars are redistributed what shape
• 1:20:58
  Speaker 1: the final galaxy takes. For decades, astronomers assumed Andromeda was
• 1:21:04
  Speaker 1: more massive based on its slightly larger apparent size. But
• 1:21:09
  Speaker 1: some recent measurements suggest the Milky Way's dark matter halo
• 1:21:13
  Speaker 1: may be more massive than previously thought, possibly even more
• 1:21:18
  Speaker 1: massive than Andromeda's. If so, Our galaxy may be the
• 1:21:22
  Speaker 1: more dominant partner in the merger that lies four and
• 1:21:26
  Speaker 1: a half billion years in the future. The quiet one,
• 1:21:30
  Speaker 1: it turns out, may be carrying the greater weight. Let
• 1:21:34
  Speaker 1: me take you to one more place before we truly
• 1:21:37
  Speaker 1: let go for the night, somewhere in the halo of
• 1:21:39
  Speaker 1: the Milky Way, high above or below the flat disk,
• 1:21:44
  Speaker 1: drifting in a long, tilted orbit that takes hundreds of
• 1:21:48
  Speaker 1: millions of years to complete. There is a star. It
• 1:21:51
  Speaker 1: is faint, small. Its surface is cooler than the sun's,
• 1:21:56
  Speaker 1: burning with a dim orange-red glow. Most people who've ever
• 1:22:00
  Speaker 1: looked at the night sky have never noticed it. But
• 1:22:04
  Speaker 1: this star is extraordinary for one reason. It is very
• 1:22:08
  Speaker 1: possibly the oldest star in the galaxy. It was given
• 1:22:12
  Speaker 1: the name SMSS J03130. 36670839. 3. Not a romantic name, but astronomy
• 1:22:21
  Speaker 1: often isn't. Its atmosphere contains almost no iron, no calcium,
• 1:22:26
  Speaker 1: almost nothing beyond hydrogen and helium. Its chemical composition is
• 1:22:30
  Speaker 1: the signature of a star born before generations of supernova
• 1:22:35
  Speaker 1: had enriched the universe with heavy elements. Astronomers estimate it
• 1:22:40
  Speaker 1: formed more than 13, 6 billion years ago, within 200
• 1:22:43
  Speaker 1: million years of the Big Bang itself. It is as
• 1:22:49
  Speaker 1: close to a first-born star as we've ever found, a
• 1:22:53
  Speaker 1: star that was already ancient when our galaxy was taking shape,
• 1:22:58
  Speaker 1: a star that has been burning, quietly, faithfully, without much fanfare,
• 1:23:03
  Speaker 1: for almost as long as time itself. And it is
• 1:23:06
  Speaker 1: still there, drifting in the halo, completing its slow orbit,
• 1:23:11
  Speaker 1: shining its dim orange light into the dark. It has
• 1:23:15
  Speaker 1: watched galaxies form around it, It has watched the Milky
• 1:23:19
  Speaker 1: Way assemble from a million smaller structures. It has watched
• 1:23:23
  Speaker 1: stars form and die across hundreds of generations while it continues, unchanged,
• 1:23:30
  Speaker 1: at the very edge of the galaxy's ancient memory. If
• 1:23:34
  Speaker 1: you could stand on a world orbiting that star, and
• 1:23:37
  Speaker 1: we don't know if any world exists there, and look
• 1:23:41
  Speaker 1: out at the galaxy below you, you would see a
• 1:23:44
  Speaker 1: different Milky Way than the one we see today. more chaotic,
• 1:23:49
  Speaker 1: still assembling, still violent with mergers and explosions and the early,
• 1:23:54
  Speaker 1: furious business of galaxy formation. But the star itself, where
• 1:23:59
  Speaker 1: you stand, would be quiet, just as it has always been,
• 1:24:03
  Speaker 1: burning its hydrogen slowly, patiently, with the steadiness of something
• 1:24:08
  Speaker 1: that has outlasted everything it was born alongside. That star,
• 1:24:13
  Speaker 1: I think, is a good thing to imagine as you
• 1:24:16
  Speaker 1: fall asleep. not because it's comforting in any obvious way,
• 1:24:20
  Speaker 1: but because it puts things in perspective. In 13.6 billion
• 1:24:25
  Speaker 1: years of cosmic history, wars, extinctions, collisions of galaxies, the
• 1:24:31
  Speaker 1: deaths of trillions of stars, that small, dim, ancient sun
• 1:24:35
  Speaker 1: has simply continued, turning hydrogen to helium, one quiet nuclear
• 1:24:41
  Speaker 1: reaction at a time. You don't have to hold the
• 1:24:43
  Speaker 1: whole galaxy. You don't have to understand its full history,
• 1:24:48
  Speaker 1: or map its spiral arms, or know the mass of
• 1:24:51
  Speaker 1: its dark matter halo. You just have to continue. One
• 1:24:55
  Speaker 1: breath at a time. One heartbeat at a time. In
• 1:24:59
  Speaker 1: the halo of something ancient and enormous and beautiful, the
• 1:25:04
  Speaker 1: Milky Way will keep turning tonight while you sleep. Sagittarius
• 1:25:09
  Speaker 1: A-star will pulse faintly at the centre. Somewhere, A molecular
• 1:25:13
  Speaker 1: cloud will begin its slow million-year collapse towards starhood. A
• 1:25:18
  Speaker 1: stellar stream will continue its quiet, invisible arc across the halo.
• 1:25:24
  Speaker 1: A neutron star will tick its faithful pulsar rhythm into
• 1:25:28
  Speaker 1: the void. The Magellanic clouds will move a tiny increment
• 1:25:32
  Speaker 1: closer on their first orbit around our home. And you,
• 1:25:36
  Speaker 1: in your warm room, in your breathing body on your
• 1:25:39
  Speaker 1: quiet planet, Orbiting your ordinary star in the outer arm
• 1:25:44
  Speaker 1: of a barred spiral galaxy in a small corner of
• 1:25:47
  Speaker 1: an unimaginably large universe. You will sleep. That is enough.
• 1:25:53
  Speaker 1: More than enough. It is, in fact, everything good night.
• 1:25:57
  Speaker 1: But actually, one more thing. Just one more. I promise
• 1:26:01
  Speaker 1: it's worth staying awake for. Have you thought yet about
• 1:26:04
  Speaker 1: what it means that gravity travels at the speed of light?
• 1:26:08
  Speaker 1: Not just light. gravity itself. When Sagittarius A-star, the supermassive
• 1:26:15
  Speaker 1: black hole at the center of the Milky Way, tugs
• 1:26:18
  Speaker 1: on the Sun, that gravitational influence doesn't arrive instantly. It
• 1:26:24
  Speaker 1: left the galactic center 26,000 years ago. It has been
• 1:26:29
  Speaker 1: traveling across the galaxy at the speed of light, crossing
• 1:26:33
  Speaker 1: gas clouds and stellar nurseries, and the quiet space between
• 1:26:37
  Speaker 1: stars and it is arriving at the Sun right now.
• 1:26:41
  Speaker 1: The gravity you feel from the galactic center, the force
• 1:26:45
  Speaker 1: that keeps the solar system in its orbit, is 26,000
• 1:26:50
  Speaker 1: years old. You are responding, right now, to a gravitational
• 1:26:55
  Speaker 1: message sent when our ancestors were beginning to develop agriculture.
• 1:27:01
  Speaker 1: When the first cities were still thousands of years in
• 1:27:04
  Speaker 1: the future, When the most complex human technology was a
• 1:27:08
  Speaker 1: flint tool and a fire, this is true for every
• 1:27:12
  Speaker 1: force in the universe. Nothing propagates faster than light, no information,
• 1:27:17
  Speaker 1: no influence, no force, which means that the universe we
• 1:27:22
  Speaker 1: observe and the universe we interact with is always the past.
• 1:27:27
  Speaker 1: We are never in contact with the present state of
• 1:27:31
  Speaker 1: anything beyond our immediate surroundings. The Sun's light takes 8
• 1:27:35
  Speaker 1: minutes to reach us, so the Sun we see is
• 1:27:38
  Speaker 1: 8 minutes old. The nearest star, Proxima Centauri, is 4.2
• 1:27:44
  Speaker 1: light-years away, so the Proxima we see left 4.2 years ago,
• 1:27:49
  Speaker 1: and we don't know what it looks like today. The
• 1:27:51
  Speaker 1: galactic centre is 26,000 light-years away. Andromeda is 2.5 million
• 1:27:58
  Speaker 1: light-years away. The most distant visible galaxy, GNZ 11, is
• 1:28:04
  Speaker 1: so far away that the light we detect left it 13.4
• 1:28:08
  Speaker 1: billion years ago, when the universe was only 400 million
• 1:28:13
  Speaker 1: years old. The galaxy we observe may not even exist
• 1:28:17
  Speaker 1: anymore in the same form. We live surrounded by the past.
• 1:28:21
  Speaker 1: The universe presents itself to us as a fossil record.
• 1:28:25
  Speaker 1: Layers of time compressed into a single, simultaneous sky. Every
• 1:28:30
  Speaker 1: night you look up, you're looking into an archive. Not
• 1:28:34
  Speaker 1: a photograph. A library, with volumes spanning billions of years,
• 1:28:38
  Speaker 1: all open at once. And the Milky Way, your galaxy,
• 1:28:42
  Speaker 1: your gravitational home, is part of that archive. Its oldest
• 1:28:47
  Speaker 1: stars carry the chemistry of the universe's first billion years.
• 1:28:52
  Speaker 1: Its youngest stars are still forming, in clouds that have
• 1:28:56
  Speaker 1: been contracting for only a few million years. the blink
• 1:29:00
  Speaker 1: of an eye on a galactic timescale. Its spiral arms
• 1:29:04
  Speaker 1: carry the compressed echo of density waves that have been
• 1:29:09
  Speaker 1: moving through the disk for billions of revolutions. Its dark
• 1:29:13
  Speaker 1: matter halo contains material that has been gravitationally bound to
• 1:29:18
  Speaker 1: this structure since before the Earth existed. The Milky Way
• 1:29:22
  Speaker 1: is not just a place in space. It is a
• 1:29:26
  Speaker 1: document in time. Now, And here is one of the
• 1:29:29
  Speaker 1: most beautiful, subtle facts we'll visit tonight. The Sun itself
• 1:29:34
  Speaker 1: is not where it was born. No star is. Over
• 1:29:38
  Speaker 1: billions of years, gravitational interactions with other stars, with giant
• 1:29:43
  Speaker 1: molecular clouds, with the pattern of the spiral arms, gradually
• 1:29:47
  Speaker 1: shift a star's orbit. Not dramatically. Not in a single generation.
• 1:29:53
  Speaker 1: But over billions of orbits around the galactic centre, a
• 1:29:57
  Speaker 1: star can migrate inward or outward by significant distances. Our Sun,
• 1:30:02
  Speaker 1: based on its current chemical composition, which is slightly richer
• 1:30:07
  Speaker 1: in heavy elements than you'd expect for a star at
• 1:30:11
  Speaker 1: its distance from the galactic centre, may have formed a
• 1:30:15
  Speaker 1: bit closer to the centre, in a region where star
• 1:30:18
  Speaker 1: formation was more active and heavy element enrichment was higher.
• 1:30:24
  Speaker 1: Over its 4.6 billion years of life, it has slowly
• 1:30:28
  Speaker 1: drifted outward to its current position. The solar system's address
• 1:30:32
  Speaker 1: has changed. Not the house number, but the zip code,
• 1:30:37
  Speaker 1: at least, and the sun's siblings. The stars it formed
• 1:30:41
  Speaker 1: alongside in its birth cluster are scattered across the galaxy.
• 1:30:46
  Speaker 1: Over billions of years, the birth cluster has dissolved. Stars
• 1:30:50
  Speaker 1: peeled away by tidal forces and gravitational encounters drifting to
• 1:30:56
  Speaker 1: different orbits, different positions in the disk or halo. We
• 1:31:00
  Speaker 1: don't know where the Sun's siblings are. They're out there,
• 1:31:04
  Speaker 1: stars born from the same cloud of gas, carrying similar
• 1:31:08
  Speaker 1: chemical fingerprints, now separated by thousands of light years. Astronomers
• 1:31:14
  Speaker 1: using Gaia data are actively searching for them, comparing chemical abundances,
• 1:31:20
  Speaker 1: orbital properties, ages. A few candidate solar siblings have been identified.
• 1:31:27
  Speaker 1: Stars that might have grown up alongside our sun in
• 1:31:31
  Speaker 1: the same stellar nursery before time and gravity carried them away.
• 1:31:36
  Speaker 1: It's one of the lonelier thoughts in astronomy when you
• 1:31:40
  Speaker 1: sit with it. A family scattered across 100,000 light years.
• 1:31:45
  Speaker 1: Each star burning quietly in its own corner of the galaxy,
• 1:31:50
  Speaker 1: unaware of its siblings, carrying the same chemical memory of
• 1:31:54
  Speaker 1: a birth long forgotten. Let me spend a moment with
• 1:31:58
  Speaker 1: something that operates at the very opposite extreme of scale.
• 1:32:03
  Speaker 1: Not the whole galaxy, but a single phenomenon within it.
• 1:32:07
  Speaker 1: Variable stars. These are stars that change in brightness over
• 1:32:11
  Speaker 1: regular intervals, pulsing in and out over hours, days or
• 1:32:16
  Speaker 1: weeks as their outer layers expand and contract. The most
• 1:32:21
  Speaker 1: useful for astronomy are called Cepheid variables, named after the
• 1:32:26
  Speaker 1: first well-studied example in the constellation Cepheus. Cepheids pulse in
• 1:32:31
  Speaker 1: a remarkably consistent way. The longer the period of the pulse,
• 1:32:36
  Speaker 1: the more luminous the star. This period-luminosity relationship means that
• 1:32:42
  Speaker 1: once you measure how long a Cepheid takes to pulse,
• 1:32:46
  Speaker 1: you know its true brightness. And by comparing its true
• 1:32:50
  Speaker 1: brightness to how bright it appears in the sky, you
• 1:32:54
  Speaker 1: can calculate its distance. It's a form of cosmic distance
• 1:32:58
  Speaker 1: measurement so reliable that it's called the standard candle. In
• 1:33:03
  Speaker 1: the early 20th century, an astronomer named Henrietta Swan Leavitt,
• 1:33:09
  Speaker 1: working at the Harvard College Observatory, cataloging stars rather than
• 1:33:14
  Speaker 1: conducting research, as she would have liked, Noticed this relationship
• 1:33:20
  Speaker 1: in Cepheid variables in the Magellanic clouds, her discovery of
• 1:33:25
  Speaker 1: the period-luminosity relationship became one of the most important in
• 1:33:29
  Speaker 1: the history of astronomy. It allowed Edwin Hubble, a decade later,
• 1:33:35
  Speaker 1: to measure the distance to Andromeda and demonstrate definitively that
• 1:33:40
  Speaker 1: it lay far outside our own galaxy. that the universe
• 1:33:45
  Speaker 1: contained billions of galaxies beyond the Milky Way, each containing
• 1:33:50
  Speaker 1: billions of stars. The modern cosmological picture of the universe begins,
• 1:33:56
  Speaker 1: in a very real sense, with Leavitt's careful cataloguing of
• 1:34:00
  Speaker 1: pulsing stars in the Magellanic clouds. Variable stars still blink
• 1:34:05
  Speaker 1: quietly across the galaxy today, measuring themselves for anyone patient
• 1:34:11
  Speaker 1: enough to watch. Now here is something that might help
• 1:34:14
  Speaker 1: you understand the Milky Way's size at the most human
• 1:34:18
  Speaker 1: level possible. Light travels at approximately 300 zero-zoned kilometers per second,
• 1:34:26
  Speaker 1: fast enough to circle the Earth more than seven times
• 1:34:30
  Speaker 1: in a single second. And the Milky Way is approximately 100,000
• 1:34:36
  Speaker 1: light-years in diameter. That means that to cross the galaxy,
• 1:34:41
  Speaker 1: from one edge of the disk to the other, traveling
• 1:34:44
  Speaker 1: at the speed of light, would take 100,000 years. 100,000 years.
• 1:34:47
  Speaker 1: Not 100,000 hours. Not 100 zoo zone days. Years. At
• 1:34:49
  Speaker 1: the speed of light, our fastest spacecraft, the Voyager probes,
• 1:34:53
  Speaker 1: launched in 1977, now nearly 24 billion kilometers from the sun,
• 1:35:09
  Speaker 1: are traveling at roughly 17 kilometers per second. At that speed,
• 1:35:13
  Speaker 1: to cross the Milky Way would take approximately 1.7 billion years,
• 1:35:19
  Speaker 1: almost half the current age of the galaxy. We have
• 1:35:23
  Speaker 1: built nothing that could traverse it in a human lifetime.
• 1:35:27
  Speaker 1: We have built nothing that could traverse it in the
• 1:35:30
  Speaker 1: lifetime of our civilization. The galaxy is not just large
• 1:35:35
  Speaker 1: in the abstract. It is unreachably large by any measure
• 1:35:39
  Speaker 1: we have ever applied to speed and distance, and yet
• 1:35:42
  Speaker 1: every point of light you see in the sky is
• 1:35:46
  Speaker 1: within it. Every star visible to the naked eye, every constellation,
• 1:35:51
  Speaker 1: every smear of the Milky Way across the summer sky,
• 1:35:55
  Speaker 1: all of it is within 100,000 light-years of us. All
• 1:36:00
  Speaker 1: of it is local in galactic terms. The nearest large
• 1:36:03
  Speaker 1: galaxy beyond our own is Andromeda, at 2 to 5
• 1:36:08
  Speaker 1: million light-years, 25 times the diameter of the Milky Way,
• 1:36:13
  Speaker 1: and there are galaxies far beyond that, billions of light-years
• 1:36:17
  Speaker 1: in every direction. The scale increases at every level. The
• 1:36:22
  Speaker 1: universe is not just larger than we imagine, it is
• 1:36:26
  Speaker 1: larger than we are capable of imagining, even when we try.
• 1:36:30
  Speaker 1: And there's something else about scale worth sitting with as
• 1:36:34
  Speaker 1: you drift. The supermassive black hole at the center of
• 1:36:37
  Speaker 1: the Milky Way, Sagittarius A-star, four million solar masses, event
• 1:36:44
  Speaker 1: horizon spanning tens of millions of kilometers, is, relative to
• 1:36:49
  Speaker 1: the galaxy, impossibly small. The galaxy is 100,000 light years across.
• 1:36:55
  Speaker 1: The black hole's event horizon is roughly 24 million kilometers
• 1:37:00
  Speaker 1: in diameter, which sounds enormous, but is in fact far
• 1:37:05
  Speaker 1: smaller than the distance from the Earth to the Sun.
• 1:37:08
  Speaker 1: On a galactic scale, the black hole is a microscopic point,
• 1:37:13
  Speaker 1: a marble in the center of a city, and yet
• 1:37:16
  Speaker 1: its gravity organizes everything. Its mass shapes the orbits of
• 1:37:21
  Speaker 1: stars for thousands of light years in every direction. Its
• 1:37:25
  Speaker 1: past activity echoes across tens of thousands of light years
• 1:37:29
  Speaker 1: in the Fermi bubbles. Its appetite for gas and stars
• 1:37:33
  Speaker 1: determines how much material flows through the galactic center and
• 1:37:38
  Speaker 1: out into the disk. The smallest significant object in the galaxy,
• 1:37:43
  Speaker 1: by size, is also its most powerful, by gravity. Scale
• 1:37:48
  Speaker 1: and power are not always proportional. Sometimes a small, Dense
• 1:37:53
  Speaker 1: thing is the axis around which everything else turns before
• 1:37:57
  Speaker 1: I let you go completely. And I will let you go.
• 1:38:00
  Speaker 1: Very soon. I want to give you one more image.
• 1:38:04
  Speaker 1: One more place to take with you into the dark
• 1:38:07
  Speaker 1: behind your eyes. Imagine a night, far in the future.
• 1:38:11
  Speaker 1: Four and a half billion years from now. A night
• 1:38:14
  Speaker 1: on a world orbiting some surviving star. Somewhere in what
• 1:38:19
  Speaker 1: was once the outer disk of the Milky Way. The
• 1:38:22
  Speaker 1: sky above this world looks nothing like the sky above
• 1:38:26
  Speaker 1: the earth tonight. The two great spirals. the Milky Way
• 1:38:30
  Speaker 1: and Andromeda, have been merging for hundreds of millions of years.
• 1:38:35
  Speaker 1: Their spiral arms have been pulled into vast tidal streams.
• 1:38:40
  Speaker 1: Their central bars have been disrupted. The gas clouds that
• 1:38:44
  Speaker 1: once wound through the arms have been compressed and agitated.
• 1:38:49
  Speaker 1: triggering enormous bursts of star formation that lit the merging
• 1:38:54
  Speaker 1: galaxies from within. Seen from a distance, during the peak
• 1:38:58
  Speaker 1: of the merger, the combined system would have looked like
• 1:39:02
  Speaker 1: two fireworks displays happening simultaneously, arcing light across millions of
• 1:39:08
  Speaker 1: light years. But now, a long time later, it has quieted.
• 1:39:13
  Speaker 1: The two galactic cores have merged. The supermassive black holes
• 1:39:18
  Speaker 1: at the centre of each galaxy have found each other,
• 1:39:21
  Speaker 1: spiralled together and merged into one. A single, enormous black
• 1:39:27
  Speaker 1: hole at the centre of the new combined structure. The
• 1:39:30
  Speaker 1: spirals are gone. What remains is a large elliptical galaxy.
• 1:39:35
  Speaker 1: A smooth, featureless oval of ancient stars, their orbits now
• 1:39:40
  Speaker 1: randomly distributed in all directions. rather than the orderly disk
• 1:39:44
  Speaker 1: rotation of the old spirals. Star formation has nearly stopped.
• 1:39:50
  Speaker 1: The gas has been used up or expelled. What remains
• 1:39:54
  Speaker 1: is a vast, quiet congregation of old stars, slowly cooling
• 1:39:58
  Speaker 1: in the dark. Somewhere in that elliptical galaxy, drifting in
• 1:40:02
  Speaker 1: whatever orbit the merger left it in, is the descendant
• 1:40:06
  Speaker 1: of the solar system. Or at least, the white dwarf
• 1:40:10
  Speaker 1: remnant of the sun, cool and dark, with no planet
• 1:40:14
  Speaker 1: left to orbit it. The Earth will have been long
• 1:40:17
  Speaker 1: since consumed by the expanding Sun, billions of years before
• 1:40:22
  Speaker 1: the merger even began. But the Sun's matter persists. It
• 1:40:26
  Speaker 1: is part of the new merged galaxy, incorporated into its history,
• 1:40:31
  Speaker 1: its chemical makeup, its halo. Nothing is lost. Matter is rearranged, recycled, redistributed.
• 1:40:38
  Speaker 1: The Milky Way will not die. It will become something else.
• 1:40:43
  Speaker 1: as it always has, as it always will. You are
• 1:40:46
  Speaker 1: part of a story that has no ending, only transformation.
• 1:40:51
  Speaker 1: The atoms of your body will outlast everything you've ever known.
• 1:40:55
  Speaker 1: They will be part of stars not yet born, planets
• 1:40:58
  Speaker 1: not yet formed, perhaps life not yet imagined. The universe
• 1:41:03
  Speaker 1: doesn't have a recycling bin. It has a recycling engine,
• 1:41:07
  Speaker 1: and it never stops running. So when you look at
• 1:41:10
  Speaker 1: the night sky, When you look at that quiet band
• 1:41:14
  Speaker 1: of light crossing the dark above you, know that you
• 1:41:17
  Speaker 1: are looking at home. Not just the planet beneath you,
• 1:41:22
  Speaker 1: not just the solar system around you, but the whole enormous, ancient,
• 1:41:27
  Speaker 1: slowly turning, dark matter-laced, spiral-armed, black hole-centered, globular cluster-orbiting, neutron star-blinking, supernova-stirring,
• 1:41:38
  Speaker 1: stellar stream-trailing, magnetically threaded, gravitationally warped, chemically rich, cosmically ordinary,
• 1:41:46
  Speaker 1: and yet absolutely extraordinary Milky Way, your galaxy, yours. Now
• 1:41:52
  Speaker 1: let it carry you. Let the turning of the disc
• 1:41:55
  Speaker 1: be the thing that rocks you gently into sleep. Let
• 1:41:59
  Speaker 1: the slow, patient orbit of the solar system be the cradle.
• 1:42:03
  Speaker 1: Let the fact that everything, everything, is in motion, but
• 1:42:08
  Speaker 1: nothing feels like it, From here be the reason you
• 1:42:10
  Speaker 1: can rest. You don't have to move. The galaxy is
• 1:42:14
  Speaker 1: moving for you. It always is. Slim, now. The stars
• 1:42:19
  Speaker 1: will still be there in the morning, a little further
• 1:42:22
  Speaker 1: along in their orbits. The galaxy, a little further along
• 1:42:26
  Speaker 1: in its year. And you will wake into the same ancient, beautiful,
• 1:42:31
  Speaker 1: incomprehensible home. Good night. Truly, good night. Still here? Then
• 1:42:37
  Speaker 1: you must be the kind of person who finds the
• 1:42:39
  Speaker 1: darkness too rich to leave just yet. And that's fine.
• 1:42:44
  Speaker 1: That's more than fine. Because there is one more layer
• 1:42:47
  Speaker 1: to the Milky Way that we haven't fully visited. The
• 1:42:51
  Speaker 1: layer that exists not in infrared or radio or gamma rays,
• 1:42:56
  Speaker 1: but in pure mathematics. The layer of the galaxy that
• 1:42:59
  Speaker 1: we know not through any telescope, but through equations. The
• 1:43:04
  Speaker 1: Milky Way obeys the laws of physics. This sounds obvious,
• 1:43:09
  Speaker 1: but it's actually one of the most profound things we
• 1:43:12
  Speaker 1: can say about it. Gravity, electromagnetism, the strong and weak
• 1:43:18
  Speaker 1: nuclear forces, the same fundamental rules that govern a falling
• 1:43:23
  Speaker 1: apple or an electron in a laboratory, govern the orbit
• 1:43:26
  Speaker 1: of a star around a black hole four million times
• 1:43:30
  Speaker 1: the mass of the sun. The universe does not make exceptions.
• 1:43:35
  Speaker 1: The same equations that Isaac Newton wrote down in the
• 1:43:39
  Speaker 1: 17th century, that Einstein refined in the 20th, that quantum
• 1:43:44
  Speaker 1: mechanics extended to the subatomic level. These equations describe the
• 1:43:49
  Speaker 1: Milky Way with the same reliability. They describe an experiment
• 1:43:54
  Speaker 1: on a tabletop. The galaxy is enormous and ancient and
• 1:43:58
  Speaker 1: mostly invisible, but it is knowable. It follows rules. And
• 1:44:03
  Speaker 1: because it follows rules, we can understand it, even from
• 1:44:07
  Speaker 1: inside it, even without ever seeing it from outside. This is,
• 1:44:11
  Speaker 1: when you think about it, one of the most remarkable
• 1:44:14
  Speaker 1: facts of all. The universe is comprehensible. Not fully. Not yet.
• 1:44:21
  Speaker 1: But the fact that human minds evolved to navigate a savannah,
• 1:44:25
  Speaker 1: to hunt to find water, to raise children, can formulate
• 1:44:29
  Speaker 1: equations that accurately describe the behavior of objects billions of
• 1:44:34
  Speaker 1: light years away, objects that existed before the Earth was born.
• 1:44:40
  Speaker 1: This is astonishing. There is no reason, from an evolutionary standpoint,
• 1:44:45
  Speaker 1: why the human brain should be capable of understanding quantum
• 1:44:50
  Speaker 1: mechanics or general relativity or the large-scale structure of the universe.
• 1:44:56
  Speaker 1: These things did not matter for survival on the African plains.
• 1:45:01
  Speaker 1: And yet our minds, with effort and creativity and the
• 1:45:05
  Speaker 1: accumulated knowledge of generations, can reach them. The Milky Way
• 1:45:10
  Speaker 1: is not just real, it is understandable. And that may
• 1:45:14
  Speaker 1: be the most hopeful fact in science. Now let me
• 1:45:18
  Speaker 1: ask you something about time. How long is 13 billion years, really?
• 1:45:23
  Speaker 1: Not in numbers, in feeling. You've been alive for... At most,
• 1:45:28
  Speaker 1: a few decades, your grandparents might span another few decades
• 1:45:32
  Speaker 1: behind you. Recorded human history stretches back perhaps 5,000 to 6,000 years.
• 1:45:39
  Speaker 1: The existence of our species, Homo sapiens, goes back perhaps 300,000 years.
• 1:45:46
  Speaker 1: Life on Earth has existed for approximately 3.8 billion years.
• 1:45:48
  Speaker 1: The Earth itself formed 4.5 billion years ago. and the
• 1:45:55
  Speaker 1: Milky Way began forming about 13.5 billion years ago. On
• 1:46:00
  Speaker 1: a timeline where the Milky Way's history is compressed into
• 1:46:04
  Speaker 1: a single year, the Earth doesn't form until October. Multicellular
• 1:46:09
  Speaker 1: life doesn't appear until mid-November. The age of the dinosaurs
• 1:46:14
  Speaker 1: spans roughly two weeks in December. Homo sapiens appears at
• 1:46:18
  Speaker 1: approximately 11.54pm on December 31st. All of recorded human history.
• 1:46:25
  Speaker 1: Every war, every empire, every invention, every book ever written,
• 1:46:30
  Speaker 1: occupies the last 13 seconds of the year. The entire
• 1:46:34
  Speaker 1: industrial age, the age of science, the age of space exploration,
• 1:46:39
  Speaker 1: the last fraction of a second. We are very, very
• 1:46:43
  Speaker 1: new here. The galaxy was already incomprehensibly old before the
• 1:46:48
  Speaker 1: first human being looked up and wondered what the light
• 1:46:52
  Speaker 1: in the sky was. And it will continue, long after
• 1:46:56
  Speaker 1: the last human being does the same. This is not
• 1:46:59
  Speaker 1: a sad thought, or at least it doesn't have to be.
• 1:47:02
  Speaker 1: It can be a freeing one. We are young. We
• 1:47:05
  Speaker 1: are new. We are at the very beginning of whatever
• 1:47:08
  Speaker 1: this species might become, in whatever time remains. The galaxy
• 1:47:14
  Speaker 1: has time for us. More time than we can use.
• 1:47:18
  Speaker 1: Something I find beautiful about neutron stars, specifically, is how
• 1:47:22
  Speaker 1: they transform the violence of death into the precision of
• 1:47:26
  Speaker 1: a clock. A massive star lives a brief, brilliant life,
• 1:47:31
  Speaker 1: burning hot and fast, exhausting its fuel in millions rather
• 1:47:35
  Speaker 1: than billions of years. Its death is catastrophic. The supernova
• 1:47:41
  Speaker 1: is one of the most energetic events in the universe.
• 1:47:44
  Speaker 1: The shockwave devastates the surrounding space. And yet what remains,
• 1:47:49
  Speaker 1: the neutron star, is the most orderly, most precisely measurable
• 1:47:54
  Speaker 1: object we know. A city-sized ball of neutrons, spinning hundreds
• 1:47:59
  Speaker 1: of times per second, emitting beams of radiation with the
• 1:48:03
  Speaker 1: regularity of an atomic clock. The violence has been converted,
• 1:48:08
  Speaker 1: through physics, into order. The explosion has left behind something
• 1:48:13
  Speaker 1: that ticks. There's a metaphor there, if you want one.
• 1:48:18
  Speaker 1: But you don't have to take it. You can simply
• 1:48:20
  Speaker 1: appreciate the neutron star for what it is. An object
• 1:48:25
  Speaker 1: that should not exist, that arises from destruction, that produces
• 1:48:30
  Speaker 1: something useful and beautiful and precise. And that continues to
• 1:48:35
  Speaker 1: tick faithfully in the dark long after everything that made
• 1:48:38
  Speaker 1: it has been forgotten. The Milky Way contains hundreds of
• 1:48:43
  Speaker 1: millions of stellar remnants, white dwarfs, neutron stars, and black
• 1:48:49
  Speaker 1: holes scattered throughout the disk and halo. These are the
• 1:48:53
  Speaker 1: ghosts of past stars. They do not shine the way
• 1:48:57
  Speaker 1: a living star shines. They do not produce new energy
• 1:49:01
  Speaker 1: through fusion. They simply persist. Cooling white dwarfs radiate their
• 1:49:06
  Speaker 1: stored heat into space. Neutron stars spin and pulse, and
• 1:49:11
  Speaker 1: in some binary systems, accumulate material from a companion star,
• 1:49:16
  Speaker 1: occasionally flaring in X-ray bursts. Stellar mass black holes, black
• 1:49:22
  Speaker 1: holes formed from the collapse of massive stars, much smaller
• 1:49:26
  Speaker 1: than the supermassive central black hole, typically a few to
• 1:49:30
  Speaker 1: a few dozen times the mass of the Sun, drift
• 1:49:34
  Speaker 1: quietly through space, invisible unless they happen to be near
• 1:49:38
  Speaker 1: enough to a companion star to draw in material and
• 1:49:41
  Speaker 1: shine in X-rays. These remnants are part of the galaxy's
• 1:49:46
  Speaker 1: population in enormous numbers. For every star currently burning in
• 1:49:51
  Speaker 1: the Milky Way, many more have already died and left
• 1:49:55
  Speaker 1: a remnant behind. We live in a galaxy whose history
• 1:49:59
  Speaker 1: is written in its dead as much as in its living.
• 1:50:03
  Speaker 1: Here is a fact that lives at the boundary of
• 1:50:05
  Speaker 1: what we know and what we suspect. The Milky Way
• 1:50:09
  Speaker 1: may have a second, hidden reservoir of baryonic matter, ordinary,
• 1:50:14
  Speaker 1: visible matter, in the form of a diffuse hot gas
• 1:50:17
  Speaker 1: halo extending far beyond the galactic disk. We know about
• 1:50:22
  Speaker 1: the dark matter halo, which extends even further. But recent
• 1:50:27
  Speaker 1: observations suggest that there may also be an enormous quantity
• 1:50:31
  Speaker 1: of hot, ionized gas, actual ordinary matter, made of protons
• 1:50:36
  Speaker 1: and electrons, surrounding the galaxy in a spherical halo far
• 1:50:40
  Speaker 1: larger than the stellar disk. This gas is too hot
• 1:50:44
  Speaker 1: and too diffuse to glow in visible light, but in X-rays,
• 1:50:49
  Speaker 1: it emits a faint detectable signal. If confirmed, this hot
• 1:50:53
  Speaker 1: gas halo could contain as much mass as all the
• 1:50:56
  Speaker 1: stars in the galaxy combined. It could be the Milky
• 1:51:00
  Speaker 1: Way's missing baryons. the ordinary matter that should be present
• 1:51:05
  Speaker 1: based on cosmological models, but that hasn't been accounted for
• 1:51:09
  Speaker 1: in stars and gas in the disk, the galaxy, in
• 1:51:13
  Speaker 1: other words, might be hiding an enormous amount of itself
• 1:51:17
  Speaker 1: in a hot, invisible envelope that we are only just
• 1:51:21
  Speaker 1: beginning to detect. What we see, the beautiful spiral of
• 1:51:25
  Speaker 1: stars and nebulae and star clusters, might be only a
• 1:51:29
  Speaker 1: fraction of the galaxy's true extent. Is there anything left? Yes, always.
• 1:51:35
  Speaker 1: The universe is not a topic you can exhaust. But
• 1:51:39
  Speaker 1: let me give you the last few things gently, as
• 1:51:42
  Speaker 1: your breathing has probably slowed and your thoughts have probably
• 1:51:47
  Speaker 1: begun to blur into the pleasant, soft noise of pre-sleep.
• 1:51:52
  Speaker 1: The Milky Way is part of the Laniakea Supercluster, a
• 1:51:56
  Speaker 1: vast gravitational structure containing more than a hundred thousand galaxies.
• 1:52:02
  Speaker 1: spanning approximately 520 million light years. Laniakea is not a
• 1:52:08
  Speaker 1: cluster in the traditional sense. Its galaxies are not bound
• 1:52:12
  Speaker 1: by gravity the way the local group is, but they
• 1:52:15
  Speaker 1: share a common region of the universe from which they
• 1:52:18
  Speaker 1: are all flowing inward, pulled by the gravity of an
• 1:52:22
  Speaker 1: enormous concentration of mass called the Great Attractor. The Great
• 1:52:28
  Speaker 1: Attractor is itself a gravitational anomaly a region of enhanced
• 1:52:33
  Speaker 1: density pulling thousands of galaxies across hundreds of millions of
• 1:52:38
  Speaker 1: light years. And the Milky Way, the Local Group, the
• 1:52:42
  Speaker 1: Virgo Cluster, all of us, are streaming toward it very
• 1:52:46
  Speaker 1: slowly at hundreds of kilometers per second. We won't arrive
• 1:52:51
  Speaker 1: for tens of billions of years if we ever do.
• 1:52:54
  Speaker 1: The universe's expansion is pushing everything apart faster than the
• 1:52:59
  Speaker 1: greater tractor can pull us in. But the pull is there.
• 1:53:04
  Speaker 1: The Milky Way is in motion, not just on the
• 1:53:06
  Speaker 1: scale of the galaxy, but on the scale of the supercluster.
• 1:53:10
  Speaker 1: Everything is moving. Everything is being pulled by everything else,
• 1:53:15
  Speaker 1: across every scale, from the subatomic to the cosmic. And yet,
• 1:53:20
  Speaker 1: and yet, here you are, in the middle of all
• 1:53:23
  Speaker 1: that motion, all that scale, all that ancient light and
• 1:53:28
  Speaker 1: invisible matter. and spinning remnants and gravitational whispers from a
• 1:53:33
  Speaker 1: hundred million light years away. Here you are. Still. Warm. Breathing.
• 1:53:40
  Speaker 1: In a bed, on a planet, in a solar system,
• 1:53:43
  Speaker 1: in a galaxy that has been turning for 13 billion
• 1:53:46
  Speaker 1: years and will turn for billions more. You don't have
• 1:53:50
  Speaker 1: to hold all of this. You don't have to understand it.
• 1:53:53
  Speaker 1: You just have to be in it. And you are.
• 1:53:56
  Speaker 1: You have been. Every moment of your life. whether you
• 1:54:00
  Speaker 1: thought about it or not. You are swimming in the
• 1:54:03
  Speaker 1: Milky Way. You are made of it. And tonight, knowing
• 1:54:08
  Speaker 1: just a little more of its story, you can close
• 1:54:11
  Speaker 1: your eyes with the quiet certainty that you are not
• 1:54:14
  Speaker 1: alone in the universe. Not just in the sense that
• 1:54:18
  Speaker 1: others might be out there somewhere, but in the deeper
• 1:54:22
  Speaker 1: sense that you are continuous with the universe. That the
• 1:54:26
  Speaker 1: boundary between you and the cosmos is less a wall
• 1:54:30
  Speaker 1: and more a membrane. That the iron in your blood
• 1:54:33
  Speaker 1: and the iron in a star and the iron in
• 1:54:36
  Speaker 1: the rocky skin of Mars were all made in the
• 1:54:39
  Speaker 1: same process, by the same physics, in the same galaxy.
• 1:54:43
  Speaker 1: You are the Milky Way, looking at itself, ret now.
• 1:54:48
  Speaker 1: The galaxy will still be here when you open your eyes.
• 1:54:52
  Speaker 1: The stars will still be burning. The spiral arms will
• 1:54:55
  Speaker 1: still be turning. Sagittarius A-star will still be waiting at
• 1:54:59
  Speaker 1: the center, patient and dark and four million times the
• 1:55:03
  Speaker 1: mass of the sun. And the oldest star in the halo,
• 1:55:07
  Speaker 1: the dim, orange, 13 billion year old survivor, will still
• 1:55:13
  Speaker 1: be drifting on its long, slow orbit, carrying its ancient
• 1:55:17
  Speaker 1: light patiently through the dark, as it has since nearly
• 1:55:22
  Speaker 1: the beginning of everything. You are in good company. You
• 1:55:25
  Speaker 1: always have been. Good night, one last thought. Just one.
• 1:55:29
  Speaker 1: A small one that will fit easily behind your eyes.
• 1:55:33
  Speaker 1: Somewhere right now. At this very moment. As you read
• 1:55:37
  Speaker 1: or listen to this, a star is being born in
• 1:55:40
  Speaker 1: the Milky Way. Not in a year from now. Not
• 1:55:43
  Speaker 1: in a century. Right now. In a cold molecular cloud
• 1:55:47
  Speaker 1: somewhere in the disk of our galaxy, A region of
• 1:55:51
  Speaker 1: gas has crossed the threshold of gravitational collapse and begun
• 1:55:55
  Speaker 1: to fall inward. It will take another few million years
• 1:56:00
  Speaker 1: for the process to complete, for the core to heat
• 1:56:03
  Speaker 1: enough to ignite fusion and become a true star. But
• 1:56:07
  Speaker 1: the process has begun. The galaxy is forming a new
• 1:56:11
  Speaker 1: sun while you sleep. You will not live to see
• 1:56:14
  Speaker 1: it shine. Your grandchildren's grandchildren's grandchildren will not live to
• 1:56:19
  Speaker 1: see it shine. But it will shine. A new star,
• 1:56:23
  Speaker 1: in the Milky Way, already in progress. And when it does,
• 1:56:28
  Speaker 1: in several million years, it will add its light to
• 1:56:31
  Speaker 1: the galaxy's total. It will illuminate whatever gas and dust
• 1:56:35
  Speaker 1: surrounds it. It may gather planets from the remaining disk
• 1:56:40
  Speaker 1: of its birth cloud. And those planets... if they form
• 1:56:44
  Speaker 1: in the right positions, might one day shelter something that
• 1:56:48
  Speaker 1: looks up at their night sky and sees the band
• 1:56:51
  Speaker 1: of the Milky Way and wonders what it is, what
• 1:56:55
  Speaker 1: it means, whether anything is out there. The galaxy will
• 1:56:59
  Speaker 1: still be answering that question. It always is good night.
• 1:57:04
  Speaker 1: And since we're here, still together in the quiet, let
• 1:57:08
  Speaker 1: me give you a handful of final truths to carry
• 1:57:11
  Speaker 1: into sleep like small smooth stones in your pocket. The
• 1:57:15
  Speaker 1: Milky Way has a defined edge, but it isn't sharp.
• 1:57:19
  Speaker 1: There's no wall, no boundary marker, no sudden end. Instead,
• 1:57:24
  Speaker 1: the density of stars simply diminishes with distance from the centre.
• 1:57:29
  Speaker 1: Thinning from thousands per cubic light-year near the core, to
• 1:57:33
  Speaker 1: dozens in the disk where we live, To single isolated
• 1:57:37
  Speaker 1: stars separated by tens of light years in the outer halo.
• 1:57:41
  Speaker 1: To nothing. The galaxy fades rather than ends. Its stars
• 1:57:46
  Speaker 1: become sparser and its influence becomes weaker until the gravity
• 1:57:51
  Speaker 1: of the intergalactic medium takes over. There are stars that
• 1:57:55
  Speaker 1: may be gravitationally bound to the Milky Way hundreds of
• 1:58:00
  Speaker 1: thousands of light years from the center. far beyond what
• 1:58:04
  Speaker 1: we normally consider the galaxy's disk. The edge of the
• 1:58:08
  Speaker 1: Milky Way is wherever its gravity last has a claim,
• 1:58:13
  Speaker 1: and that turns out to be very, very far indeed.
• 1:58:16
  Speaker 1: The Milky Way travels through space as part of the
• 1:58:19
  Speaker 1: Local Group, which itself moves through space relative to the
• 1:58:24
  Speaker 1: cosmic microwave background radiation. the afterglow of the Big Bang
• 1:58:29
  Speaker 1: that fills the universe uniformly. We are moving at roughly
• 1:58:33
  Speaker 1: 600 kilometers per second relative to this background. Not the
• 1:58:39
  Speaker 1: solar system's orbit around the galactic center, not the galaxy's
• 1:58:44
  Speaker 1: orbit within the local group. The entire local group, the
• 1:58:48
  Speaker 1: Milky Way, Andromeda, all 50-plus member galaxies, hurtling together through
• 1:58:54
  Speaker 1: the cosmic background at 600 kilometers per second, in the
• 1:58:59
  Speaker 1: direction of the Centaurus constellation, toward the great attractor, pulled
• 1:59:06
  Speaker 1: by gravity we can feel but not fully see. We
• 1:59:10
  Speaker 1: are travelers at every scale. There is no frame of
• 1:59:13
  Speaker 1: reference in the universe that is truly, absolutely still. Motion
• 1:59:18
  Speaker 1: is the default state of everything. Some of the hydrogen
• 1:59:22
  Speaker 1: atoms in your body were formed in the first three
• 1:59:25
  Speaker 1: minutes after the Big Bang. Not the carbon, not the iron,
• 1:59:30
  Speaker 1: not the calcium, those came later, forged in stars. But
• 1:59:34
  Speaker 1: the hydrogen, the most common element in your body, in water,
• 1:59:38
  Speaker 1: in the organic molecules of your cells, that hydrogen is original, primordial.
• 1:59:45
  Speaker 1: It formed in the first few minutes of the universe's existence,
• 1:59:48
  Speaker 1: when conditions were hot, and dense enough for protons to
• 1:59:52
  Speaker 1: stick to each other and form atomic nuclei. That hydrogen
• 1:59:58
  Speaker 1: eventually became part of gas clouds, then stars, then stellar remnants,
• 2:00:03
  Speaker 1: then new stars, then the solar nebula, then the ocean,
• 2:00:06
  Speaker 1: then rain, then rivers, then food, then you. The hydrogen
• 2:00:11
  Speaker 1: in you has been on a 13 billion year journey,
• 2:00:15
  Speaker 1: and tonight it is here, resting in a bed, at
• 2:00:18
  Speaker 1: the end of a long day, waiting to continue the
• 2:00:21
  Speaker 1: Milky Way hums. Not audibly. Sound does not travel through
• 2:00:26
  Speaker 1: the near vacuum of space. But in the radio frequencies
• 2:00:30
  Speaker 1: that radio telescopes detect, and in the low-frequency gravitational waves
• 2:00:35
  Speaker 1: that wash through the universe from the mergers of supermassive
• 2:00:39
  Speaker 1: black holes, the galaxy ends a resonate. Pulsars tick. Hydrogen
• 2:00:45
  Speaker 1: clouds emit their characteristic 21-centimeter radio-wavelength signal. Supernova remnants glow
• 2:00:52
  Speaker 1: in synchrotron radiation from electrons spiraling in magnetic fields. The
• 2:00:58
  Speaker 1: hot coronal gas emits X-rays. The cold dust emits infrared.
• 2:01:04
  Speaker 1: The galaxy is broadcasting, constantly, in a dozen frequencies simultaneously.
• 2:01:09
  Speaker 1: We live in the middle of a vast, complex, multispectral signal.
• 2:01:14
  Speaker 1: We are bathed in it. We just can't hear it.
• 2:01:18
  Speaker 1: There are stars in the Milky Way that will still
• 2:01:20
  Speaker 1: be burning long after the sun has died. The smallest stars,
• 2:01:25
  Speaker 1: red dwarfs, burn their hydrogen so slowly and so efficiently
• 2:01:30
  Speaker 1: that they can live for trillions of years. Some red
• 2:01:33
  Speaker 1: dwarfs formed early in the galaxy's history will still be
• 2:01:38
  Speaker 1: fusing hydrogen when the universe is a hundred times older
• 2:01:42
  Speaker 1: than it is today. When every star like the sun
• 2:01:45
  Speaker 1: has long since become a white dwarf. When star formation
• 2:01:50
  Speaker 1: has stopped across most of the galaxy. When the night sky,
• 2:01:54
  Speaker 1: from any hypothetical surviving planet, is far darker and emptier
• 2:01:59
  Speaker 1: than the one you'd see tonight. Those small, patient red
• 2:02:03
  Speaker 1: stars will still be burning, outlasting everything, carrying the Milky
• 2:02:08
  Speaker 1: Way's light far into a future we cannot imagine. You
• 2:02:12
  Speaker 1: are alive in the galaxy's middle age, when it still
• 2:02:16
  Speaker 1: has stars like the sun, still has molecular clouds, still
• 2:02:21
  Speaker 1: has star-forming regions lighting up the spiral arms. When the
• 2:02:25
  Speaker 1: sky is rich enough to be beautiful, but orderly enough
• 2:02:29
  Speaker 1: to be readable. When there are enough stars to inspire wonder,
• 2:02:34
  Speaker 1: and enough dark between them to inspire humility. You are
• 2:02:38
  Speaker 1: alive at the right time, in the right galaxy, in
• 2:02:42
  Speaker 1: the right arm, in the right quiet spur, at the
• 2:02:45
  Speaker 1: right distance from the center, on the right kind of planet,
• 2:02:48
  Speaker 1: orbiting the right kind of star. And you are here, alive, aware, wondering,
• 2:02:55
  Speaker 1: against odds so enormous that the universe itself would be
• 2:02:59
  Speaker 1: forgiven for not expecting its sleeps now. The galaxy will
• 2:03:04
  Speaker 1: carry you through the night.