Latest / Boring Science / How Many Stars Exist in the Universe _ Science for Sleep

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How Many Stars Exist in the Universe _ Science for Sleep · Boring Science

• 0:00
  Speaker 1: There is a number that astronomers throw around with a
• 0:02
  Speaker 1: confidence that should make you suspicious. 200 sextillion stars. That
• 0:09
  Speaker 1: is the figure most scientists settle on when someone asks
• 0:13
  Speaker 1: how many stars exist in the observable universe. They say
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  Speaker 1: it calmly, the way you might state a population figure
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  Speaker 1: for a mid-sized city. 200 sextillion. Written out, that is
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  Speaker 1: a 2, followed by 23 zeros. And behind that number,
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  Speaker 1: underneath the calm delivery, is a calculation built on estimates
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  Speaker 1: stacked on top of guesses, stacked on top of assumptions,
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  Speaker 1: each one reasonable, some of them contested, and a few
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  Speaker 1: of them recently discovered to be significantly wrong. Start with
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  Speaker 1: the most basic version of the problem. If you wanted
• 0:51
  Speaker 1: to count the stars in our own galaxy, the Milky Way,
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  Speaker 1: by hand, one per second, No breaks, no sleep. You
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  Speaker 1: would need somewhere between 3,000 and 30,000 years just to
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  Speaker 1: finish the Milky Way alone. That range already tells you something.
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  Speaker 1: Astronomers don't agree on how many stars our own galaxy contains.
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  Speaker 1: The estimates run from 100 billion on the low end
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  Speaker 1: to over a trillion at the high end. And the
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  Speaker 1: honest answer is that nobody knows exactly because nobody has
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  Speaker 1: counted them and nobody ever will. The galaxy is too large,
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  Speaker 1: too dusty in the middle, and too full of stars
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  Speaker 1: that are far too dim to detect at any real distance.
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  Speaker 1: What we have instead is a number derived from measurements
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  Speaker 1: of mass, light, rotation, and inference. It is a good number.
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  Speaker 1: It is the best number available. It is not a count.
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  Speaker 1: Now multiply that uncertainty across two trillion galaxies. That is
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  Speaker 1: the current estimate for how many galaxies exist in the
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  Speaker 1: observable universe alone. Some of those galaxies are enormous, hosting
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  Speaker 1: tens of trillions of stars each. Most of them are small,
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  Speaker 1: dim dwarf galaxies containing a fraction of the Milky Way's
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  Speaker 1: stellar population. The average is hard to calculate because the
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  Speaker 1: distribution is extreme at both ends. Take your uncertain Milky
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  Speaker 1: Way estimate. Multiply it by an uncertain galaxy count. Adjust
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  Speaker 1: for the fact that the Milky Way is not a
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  Speaker 1: typical galaxy. Correct for star types that surveys routinely miss.
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  Speaker 1: Add a factor for stars forming in regions too dusty
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  Speaker 1: for optical telescopes to see. The number that comes out
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  Speaker 1: the other end is 206 dillion, and calling it a
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  Speaker 1: precise answer would be a stretch. Here is what makes
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  Speaker 1: this strange. Astronomers are not being sloppy. The people who
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  Speaker 1: produce these estimates are careful, rigorous scientists using the best
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  Speaker 1: instruments ever built and the most sophisticated mathematical models in
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  Speaker 1: the history of the discipline. The uncertainty is not a failure.
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  Speaker 1: It is a feature of the problem itself. You are
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  Speaker 1: trying to count objects scattered across a volume of space
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  Speaker 1: so vast that the light from the far edge has
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  Speaker 1: been travelling for nearly 14 billion years to reach us.
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  Speaker 1: You're trying to count objects, most of which are too
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  Speaker 1: faint to detect individually, even in principle. You are doing
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  Speaker 1: it from one fixed point, with instruments that have hard limits,
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  Speaker 1: inside a galaxy that blocks your view of a significant
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  Speaker 1: portion of the sky. Under those conditions, getting within an
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  Speaker 1: order of magnitude of the right answer is a genuine achievement.
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  Speaker 1: but an order of magnitude is a factor of 10.
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  Speaker 1: The highest credible estimate for the number of stars in
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  Speaker 1: the observable universe is plausibly 10 times larger than the
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  Speaker 1: lowest credible estimate. That is not a small uncertainty. If
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  Speaker 1: the population of a city were known only to within
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  Speaker 1: a factor of 10, you would not call that known.
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  Speaker 1: You would call it somewhere between a village and a metropolis,
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  Speaker 1: and admit you weren't sure which. So where does the
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  Speaker 1: 200 sextillion figure actually come from? And how much should
• 4:24
  Speaker 1: you trust it? That question leads somewhere unexpected. It pulls
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  Speaker 1: you through the history of how astronomers learned to count
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  Speaker 1: galaxies from a single photograph, how a spacecraft pointed at
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  Speaker 1: a blank patch of sky changed everything in 1995, how
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  Speaker 1: a population of stars barely bright enough to sustain fusion
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  Speaker 1: may have thrown every previous estimate off by a factor
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  Speaker 1: of three, and how a mission launched in 2013 is
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  Speaker 1: still working to resolve a question that should have been
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  Speaker 1: answered decades ago. Every layer of the calculation turns out
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  Speaker 1: to have a story behind it, and every story turns
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  Speaker 1: out to be more complicated than the headline figure suggests.
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  Speaker 1: What follows is an attempt to trace that calculation from
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  Speaker 1: the ground up, Not to arrive at a cleaner answer
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  Speaker 1: than the one astronomers already have, but to understand where
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  Speaker 1: the number comes from, which parts of it are solid,
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  Speaker 1: which parts are educated guesses, and what has forced researchers
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  Speaker 1: to revise the whole thing more than once in the
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  Speaker 1: past 30 years. Along the way, the number itself becomes
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  Speaker 1: less interesting than the machinery behind it. 200 sextillion stars
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  Speaker 1: is a fact the way a blurry photograph is a fact.
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  Speaker 1: Something is definitely there. The exact details are still coming
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  Speaker 1: into focus. The number is real, in the sense that
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  Speaker 1: it came from somewhere, and tracing it back to its
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  Speaker 1: origins turns out to be one of the stranger journeys
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  Speaker 1: in modern science. On a clear night, far from any city,
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  Speaker 1: the human eye can pick out somewhere around 6,000 stars.
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  Speaker 1: That sounds like a lot. Standing under a truly dark sky,
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  Speaker 1: With the Milky Way arching overhead and no moon to
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  Speaker 1: wash things out, it feels like a lot. But 6,000
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  Speaker 1: is nothing. It is not even a rounding error. The
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  Speaker 1: Milky Way alone contains somewhere between 100 billion and a
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  Speaker 1: trillion stars. And on the best night of your life,
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  Speaker 1: from the best dark site on Earth, you are seeing 6,000
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  Speaker 1: of them. You are seeing essentially zero. And the ones
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  Speaker 1: you can see... are not the typical ones. This is
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  Speaker 1: the part that matters. Every star visible to the naked
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  Speaker 1: eye has passed a very specific filter. It is either
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  Speaker 1: unusually close to us, or unusually luminous, or both. The dim,
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  Speaker 1: ordinary stars that make up the overwhelming majority of the
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  Speaker 1: galaxy are simply not there when you look up. They
• 7:02
  Speaker 1: are too faint. A star like our Sun, sitting at
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  Speaker 1: the average distance of stars in the Milky Way, would
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  Speaker 1: be completely invisible without a telescope. The stars you can name,
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  Speaker 1: the ones that defined the constellations for ancient civilizations, are
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  Speaker 1: the exceptions. They are the outliers. Using them to draw
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  Speaker 1: conclusions about the typical star in the universe would be
• 7:23
  Speaker 1: like walking into a professional basketball arena, measuring the height
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  Speaker 1: of everyone on the court, and concluding that human beings
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  Speaker 1: average around six foot six. The sample is not random.
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  Speaker 1: it is aggressively selected for the most extreme cases. Astronomers
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  Speaker 1: call this kind of distortion a selection bias, and in
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  Speaker 1: observational astronomy it shows up constantly. Any survey that detects
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  Speaker 1: objects above some brightness threshold will systematically over-represent the bright ones.
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  Speaker 1: The faint majority stays invisible and gets left out of
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  Speaker 1: the count entirely. When you look at the night sky
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  Speaker 1: with your naked eye, you are experiencing the most extreme
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  Speaker 1: version of this problem possible. The stars you can see
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  Speaker 1: at all are the basketball players, the rest of the galaxy,
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  Speaker 1: the hundreds of billions of dim, low-mass red dwarfs burning
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  Speaker 1: quietly at a fraction of the sun's brightness, and nowhere
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  Speaker 1: to be found. Move one step up in capability and
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  Speaker 1: the picture changes fast. A pair of binoculars immediately reveals
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  Speaker 1: stars that were invisible a moment before, A small backyard
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  Speaker 1: telescope pushes the count into the millions. The band of
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  Speaker 1: the Milky Way, which looks to the naked eye like
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  Speaker 1: a smear of pale light, begins to resolve. You start
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  Speaker 1: to see that it is made of individual points, stacked
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  Speaker 1: so densely that your eye could not separate them without help.
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  Speaker 1: What felt like a glow becomes a crowd. The best
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  Speaker 1: ground-based telescopes push further still. The Sloan Digital Sky Survey
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  Speaker 1: catalogued around 48 million stars across roughly a third of
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  Speaker 1: the sky. That sounds definitive, until you realise it still
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  Speaker 1: represents a tiny fraction of the Milky Way's actual stellar population.
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  Speaker 1: The survey's detection limits meant that anything below a certain
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  Speaker 1: brightness threshold simply didn't register. Stars like our Sun, at
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  Speaker 1: distances of more than a few thousand light years, were
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  Speaker 1: already on the edge of visibility. Fainter stars, closer in
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  Speaker 1: but dimmer, often didn't make it into the catalogue at all.
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  Speaker 1: And there is another problem that no amount of telescope
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  Speaker 1: power fully solves. The centre of the Milky Way is
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  Speaker 1: not just distant, it is hidden. Between us and the
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  Speaker 1: galactic core lies an enormous amount of dust. the same
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  Speaker 1: kind of interstellar material that forms the dark lanes visible
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  Speaker 1: against the bright band of the Milky Way on a
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  Speaker 1: clear night. Optical light, the kind our eyes detect and
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  Speaker 1: the kind traditional telescopes were built for, cannot punch through it.
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  Speaker 1: The stars toward the galactic centre are simply blocked. You
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  Speaker 1: can aim the largest optical telescope on Earth directly at
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  Speaker 1: the centre of our own galaxy and you will not
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  Speaker 1: see through to the stars on the other side. The
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  Speaker 1: dust absorbs the light before it reaches you, and what
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  Speaker 1: should be the most star-dense region of the sky looks
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  Speaker 1: instead like a dark rift running through the middle of
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  Speaker 1: the Milky Way's glow. This is not a solvable problem
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  Speaker 1: with a bigger mirror. Infrared telescopes can see through some
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  Speaker 1: of the dust, and they have revealed an extraordinary density
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  Speaker 1: of stars near the galactic core, but the full picture
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  Speaker 1: remains obscured. Even our own galaxy, which we sit inside,
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  Speaker 1: which surrounds us on every side, cannot be fully inventoried
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  Speaker 1: by direct observation. We are like someone trying to count
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  Speaker 1: the trees in a forest while standing in the middle
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  Speaker 1: of it, with fog filling every gap between the trunks.
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  Speaker 1: So where does this leave any attempt to count stars?
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  Speaker 1: It leaves astronomers in a position where direct observation, even
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  Speaker 1: with the most powerful instruments ever built, can only ever
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  Speaker 1: get them part of the way there. The stars we
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  Speaker 1: can actually see and catalogue are the exceptional ones, the
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  Speaker 1: bright ones, the close ones, the ones not blocked by
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  Speaker 1: intervening dust. The vast majority are inferred rather than detected,
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  Speaker 1: estimated rather than counted, guessed at from the collective properties
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  Speaker 1: of the light they contribute to rather than resolved as
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  Speaker 1: individual points. The number we use for the Milky Way,
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  Speaker 1: the number we then multiply out to reach the totals
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  Speaker 1: people cite for the whole universe, does not come from counting.
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  Speaker 1: It comes from something considerably more indirect and considerably more uncertain.
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  Speaker 1: You can see with your eyes is not a sample
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  Speaker 1: of the universe. It is a very specific, very misleading
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  Speaker 1: advertisement for it. The first step in counting the stars
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  Speaker 1: in the universe is figuring out how many are in
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  Speaker 1: our own galaxy. And that turns out to be hard
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  Speaker 1: enough on its own. Astronomers have two main ways of
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  Speaker 1: approaching this. Neither one gives a clean answer. The first
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  Speaker 1: method is spectroscopic and photometric. You measure starlight directly, analyzing
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  Speaker 1: its color and brightness. And from those measurements, you infer
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  Speaker 1: what kinds of stars are out there and how many
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  Speaker 1: of them exist. The color of starlight tells you the
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  Speaker 1: temperature of the star. The temperature tells you the type.
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  Speaker 1: The type tells you roughly how massive it is and
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  Speaker 1: how much light it puts out. From there, in principle,
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  Speaker 1: you count up the contributions and work backward to a population.
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  Speaker 1: The Milky Way contains roughly 100 billion stars by this method,
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  Speaker 1: according to some estimates. Others push that figure to 400 billion.
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  Speaker 1: The range itself is already telling you something important about
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  Speaker 1: how much confidence to plex. In any single number, The
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  Speaker 1: second method works from the opposite direction entirely. Instead of
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  Speaker 1: counting light, you weigh the galaxy. You calculate the total
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  Speaker 1: mass of the Milky Way, you figure out what fraction
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  Speaker 1: of that mass is locked up in stars, and then
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  Speaker 1: you divide by the mass of an average star. Simple
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  Speaker 1: in concept, deeply complicated in practice. To understand why, you
• 13:32
  Speaker 1: need to understand how astronomers actually weigh a galaxy. You
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  Speaker 1: cannot put the Milky Way on a scale. What you
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  Speaker 1: can do is watch how things move inside it. Stars,
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  Speaker 1: gas clouds and other objects orbit the galactic center. The
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  Speaker 1: speed at which they orbit depends on how much mass
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  Speaker 1: is pulling on them from inside their orbit. This relationship
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  Speaker 1: between orbital speed and enclosed mass is called the rotation
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  Speaker 1: curve of the galaxy. In our solar system, Planets farther
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  Speaker 1: from the Sun move slower because they feel less gravitational pull.
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  Speaker 1: You would expect the same pattern in a galaxy. Stars
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  Speaker 1: farther from the centre, with less mass enclosed inside their orbit,
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  Speaker 1: should orbit more slowly. They don't. The rotation curve of
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  Speaker 1: the Milky Way is flat. Stars at vastly different distances
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  Speaker 1: from the galactic centre all orbit at roughly the same speed.
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  Speaker 1: This does not make sense if all the mass is
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  Speaker 1: concentrated where you can see it. Something invisible is providing
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  Speaker 1: extra gravitational pull. That something is dark matter. It doesn't
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  Speaker 1: emit light, doesn't reflect light, doesn't interact with anything except
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  Speaker 1: through gravity. But it is there, filling a vast halo
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  Speaker 1: that extends well beyond the visible disk of the galaxy,
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  Speaker 1: and it dominates the total mass budget. When astronomers use
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  Speaker 1: the rotation curve to estimate the Milky Way's total mass,
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  Speaker 1: most of what they are measuring is this invisible component.
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  Speaker 1: The total mass of the Milky Way is somewhere in
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  Speaker 1: the range of a trillion solar masses. Ordinary matter, stars, gas, dust,
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  Speaker 1: everything you can see, accounts for only a fraction of that.
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  Speaker 1: Dark matter accounts for the rest. So now you have
• 15:21
  Speaker 1: a problem. you have a total mass figure that is
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  Speaker 1: mostly dark matter. You need to strip that out. Estimates
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  Speaker 1: suggest that the visible, ordinary mass of the galaxy is
• 15:32
  Speaker 1: somewhere between 50 and 100 billion solar masses, depending on
• 15:36
  Speaker 1: the model. Of that, only about 3% is in the
• 15:40
  Speaker 1: form of stars. The rest is gas and dust. If
• 15:43
  Speaker 1: you accept those numbers, you end up with stellar mass
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  Speaker 1: somewhere in the range of a few billion to a
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  Speaker 1: few tens of billions of solar masses. Then you divide
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  Speaker 1: by the mass of a typical star, and you get
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  Speaker 1: your count. But what is a typical star? This is
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  Speaker 1: where everything breaks down. Most discussions of stellar populations use
• 16:04
  Speaker 1: something close to the mass of the Sun as a
• 16:06
  Speaker 1: starting point, because the Sun is what we know. It
• 16:10
  Speaker 1: is a convenient reference. But the Sun is not typical,
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  Speaker 1: not even close. The most common stars in the Milky
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  Speaker 1: Way are red dwarfs. Small, cool, dim objects with masses
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  Speaker 1: anywhere from about a tenth to about half the mass
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  Speaker 1: of the Sun. They produce so little light that even
• 16:28
  Speaker 1: nearby red dwarfs are invisible without a telescope. The closest
• 16:33
  Speaker 1: star to Earth besides the Sun is Proxima Centauri, a
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  Speaker 1: red dwarf, and it cannot be seen with the naked
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  Speaker 1: eye from four light years away. Now imagine trying to
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  Speaker 1: survey all the red dwarfs in a galaxy 100,000 light
• 16:45
  Speaker 1: years across. you cannot do it. The faint end of
• 16:51
  Speaker 1: the stellar population is genuinely, fundamentally hard to count. Surveys
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  Speaker 1: pick up the bright stars easily. They catch the mid-range
• 17:00
  Speaker 1: stars reasonably well. But the red dwarfs, the most numerous
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  Speaker 1: type in existence, slip through in enormous numbers. Every survey
• 17:10
  Speaker 1: has a detection limit, a brightness threshold below which stars
• 17:14
  Speaker 1: become invisible to the instruments. Because red dwarfs are so dim,
• 17:18
  Speaker 1: they hit that threshold at relatively short distances. What looks
• 17:22
  Speaker 1: like a complete census is actually missing most of the
• 17:25
  Speaker 1: stars by number. This is the crucial variable that makes
• 17:29
  Speaker 1: the Milky Way star count so hard to pin down.
• 17:33
  Speaker 1: If red dwarfs are common in the proportions that nearby
• 17:35
  Speaker 1: surveys suggest, the true stellar population of the Milky Way
• 17:39
  Speaker 1: is much larger than the spectroscopic measurements imply. Were they
• 17:44
  Speaker 1: three times more common than current estimates assume? The galaxy's
• 17:48
  Speaker 1: star count climbs accordingly. A different assumed mass function, a
• 17:54
  Speaker 1: different set of assumptions about how many stars of each
• 17:57
  Speaker 1: mass were born over the galaxy's history, produces a different answer.
• 18:02
  Speaker 1: Not slightly different. The range of credible estimates for the
• 18:06
  Speaker 1: number of stars in the Milky Way runs from about
• 18:09
  Speaker 1: 100 billion on the low end to perhaps a trillion
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  Speaker 1: at the high end. That is a factor of 10.
• 18:16
  Speaker 1: between the floor and the ceiling of what serious astronomers
• 18:20
  Speaker 1: publish in peer-reviewed papers about our own home galaxy. This
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  Speaker 1: is not a sign that the field is in disarray.
• 18:26
  Speaker 1: The uncertainty is baked into the physics of the problem.
• 18:30
  Speaker 1: Dark matter cannot be directly measured. The faint end of
• 18:34
  Speaker 1: the stellar mass function cannot be completely surveyed. The fraction
• 18:38
  Speaker 1: of galactic mass locked in stars versus gas versus dust
• 18:43
  Speaker 1: varies with where you look and when in galactic history
• 18:47
  Speaker 1: you are counting. Stack these uncertainties on top of each
• 18:50
  Speaker 1: other and a factor of 10 is, if anything, a
• 18:54
  Speaker 1: tight constraint given how hard the measurement is. What astronomers
• 18:58
  Speaker 1: are left with is a range, anchored by the best
• 19:01
  Speaker 1: available evidence on both sides, with the true number somewhere
• 19:06
  Speaker 1: inside it. Most researchers working in this field settle on
• 19:10
  Speaker 1: something in the neighborhood of 100 to 400 billion stars
• 19:14
  Speaker 1: as the most defensible estimate for the Milky Way. Some models,
• 19:18
  Speaker 1: weighted for more abundant, low-mass stars, push toward a trillion.
• 19:23
  Speaker 1: None of these numbers are wrong. Precisely, they reflect genuine
• 19:28
  Speaker 1: differences in assumptions about the unobservable. Here is what makes
• 19:32
  Speaker 1: this matter beyond the question of our own galaxy. The
• 19:35
  Speaker 1: Milky Way estimate is the seed from which every calculation
• 19:40
  Speaker 1: about the broader universe grows. When astronomers eventually multiply by
• 19:44
  Speaker 1: the number of galaxies in the observable universe, the uncertain
• 19:49
  Speaker 1: fact of at the bottom of that multiplication is the
• 19:52
  Speaker 1: one we just described. Every estimate of the universe's stars
• 19:57
  Speaker 1: starts here, with this uncertain foundation, and then multiplies it
• 20:02
  Speaker 1: out by a number that is itself barely better than
• 20:05
  Speaker 1: a guess. The calculation has a hidden assumption buried so
• 20:09
  Speaker 1: deep that most discussions of total star counts never surface it,
• 20:14
  Speaker 1: and it matters more than almost anything else. When astronomers
• 20:18
  Speaker 1: multiply their galaxy estimates by a star count per galaxy,
• 20:22
  Speaker 1: they need a number for how many stars a typical
• 20:25
  Speaker 1: galaxy contains. That number comes from somewhere. It comes from
• 20:30
  Speaker 1: an assumed average stellar mass. And the assumed average stellar
• 20:35
  Speaker 1: mass is where the whole calculation becomes quietly enormously uncertain.
• 20:40
  Speaker 1: If you pick the wrong average, you don't introduce a
• 20:43
  Speaker 1: small error. You introduce a wrong answer by a factor
• 20:46
  Speaker 1: that could change the headline figure by hundreds of sextillions
• 20:50
  Speaker 1: of stars. Here is the assumption. Most introductory accounts of
• 20:55
  Speaker 1: the stellar census treat a Sun-like star as roughly representative
• 20:59
  Speaker 1: of the average. This is understandable. The Sun is the
• 21:02
  Speaker 1: star we know best. We have studied it from every
• 21:05
  Speaker 1: angle in every wavelength for centuries. Its mass, its temperature,
• 21:11
  Speaker 1: its luminosity, all of it is catalogued with extraordinary precision.
• 21:16
  Speaker 1: When you need a stand-in for a typical star, the
• 21:19
  Speaker 1: Sun is the obvious choice. It is what we reach for.
• 21:23
  Speaker 1: It is also wrong. The Sun is not average. It
• 21:26
  Speaker 1: is larger and more massive than roughly 95% of all
• 21:29
  Speaker 1: stars that exist. Using it as the baseline for a
• 21:34
  Speaker 1: census is something like estimating average human height by sampling
• 21:38
  Speaker 1: professional basketball players. The sample is accessible, well characterized, and
• 21:43
  Speaker 1: almost entirely unrepresentative of the broader population. The actual average
• 21:48
  Speaker 1: star in the universe is a red dwarf. These are small, dim,
• 21:52
  Speaker 1: cool objects that burn their fuel at such a leisurely
• 21:56
  Speaker 1: rate that human language barely has words for how slowly
• 21:59
  Speaker 1: they age. The technical classification is M-type. They are the
• 22:04
  Speaker 1: most common stars in the Milky Way by a wide margin,
• 22:07
  Speaker 1: making up somewhere around 70% of all stellar objects in
• 22:11
  Speaker 1: the galaxy. Most are too faint to see with the
• 22:14
  Speaker 1: naked eye, even from relatively nearby distances. The closest star
• 22:19
  Speaker 1: to our solar system, Proxima Centauri, is a red dwarf,
• 22:23
  Speaker 1: and it cannot be seen without a telescope, despite sitting
• 22:27
  Speaker 1: just over four light-years away. Why do red dwarfs dominate
• 22:31
  Speaker 1: so completely? The answer comes down to time and fuel economy.
• 22:35
  Speaker 1: A star like the Sun fuses hydrogen at a fast
• 22:39
  Speaker 1: enough rate to burn through its core supply in roughly
• 22:42
  Speaker 1: 10 billion years. Our Sun is about halfway through that budget.
• 22:47
  Speaker 1: When it runs out, it will swell, shed its outer layers,
• 22:51
  Speaker 1: and leave behind a small dense remnant called a white dwarf.
• 22:56
  Speaker 1: The whole process takes geological timescales by human standards, but
• 23:00
  Speaker 1: by cosmic ones, it is relatively brisk. A small red
• 23:04
  Speaker 1: dwarf operates on a completely different schedule. With perhaps a
• 23:08
  Speaker 1: tenth the mass of the Sun, these stars fuse hydrogen
• 23:12
  Speaker 1: so slowly that they can sustain themselves for trillions of years.
• 23:17
  Speaker 1: Not billions. Trillions. The universe itself is only 13.8 billion
• 23:22
  Speaker 1: years old. That means not a single red dwarf that
• 23:27
  Speaker 1: has ever formed has yet had enough time to die.
• 23:30
  Speaker 1: Every one of them that ignited in the early universe
• 23:33
  Speaker 1: is still burning today. They have been accumulating in the
• 23:36
  Speaker 1: stellar census for the entire history of cosmic time, and
• 23:40
  Speaker 1: none of them have left. This is why they dominate.
• 23:43
  Speaker 1: It is not that red dwarfs form more frequently than
• 23:46
  Speaker 1: other types, though they do form often. It is that
• 23:50
  Speaker 1: they never go away. Massive blue stars live for only
• 23:54
  Speaker 1: a few million years and then explode. Sun-like stars last
• 23:58
  Speaker 1: for billions of years and then fade. Red dwarfs just
• 24:02
  Speaker 1: keep burning generation after generation of cosmic time, stacking up
• 24:07
  Speaker 1: in the population like entries in a ledger that no
• 24:10
  Speaker 1: one ever erases. So if you are counting stars, you
• 24:14
  Speaker 1: are mostly counting red dwarfs. And if you are estimating
• 24:18
  Speaker 1: how many stars exist per galaxy, by assuming a sum-like baseline,
• 24:24
  Speaker 1: you're systematically assigning too much mass to each star you count.
• 24:29
  Speaker 1: A galaxy that actually contains a trillion low-mass red dwarfs
• 24:33
  Speaker 1: will look, from a distance, like it contains far fewer
• 24:37
  Speaker 1: sum-like stars. The mass budget seems satisfied either way, but
• 24:42
  Speaker 1: the number of individual stars is completely different depending on
• 24:46
  Speaker 1: which interpretation you apply. This is where the 2010 study
• 24:51
  Speaker 1: led by Peter van Dokkum at Yale becomes significant. Van
• 24:55
  Speaker 1: Dokkum and his colleagues were studying massive elliptical galaxies, the giant,
• 24:59
  Speaker 1: roughly egg-shaped galaxies that tend to cluster at the centers
• 25:03
  Speaker 1: of galaxy groups. These galaxies are old. They formed most
• 25:07
  Speaker 1: of their stars early in cosmic history, and they have
• 25:11
  Speaker 1: not been doing much star formation since. When the team
• 25:15
  Speaker 1: analyzed the spectroscopic signatures of starlight from these galaxies, they
• 25:19
  Speaker 1: were looking for absorption features that would reveal the relative
• 25:22
  Speaker 1: contributions of different stellar types. What they found was striking.
• 25:28
  Speaker 1: The signal from red dwarf stars was far stronger than
• 25:31
  Speaker 1: existing models predicted. The data suggested that red dwarfs in
• 25:36
  Speaker 1: these elliptical galaxies were roughly three times more common than
• 25:40
  Speaker 1: the standard assumptions had built in. Not slightly more common.
• 25:44
  Speaker 1: Three times more common. If that finding holds across galaxy
• 25:49
  Speaker 1: populations broadly, the implications are not subtle. Three times more
• 25:53
  Speaker 1: red dwarfs per galaxy means three times more stars per galaxy.
• 25:58
  Speaker 1: Three times more stars per galaxy means the total star
• 26:02
  Speaker 1: count for the observable universe, whatever figure you are using,
• 26:06
  Speaker 1: needs to be tripled. Overnight, simply by correcting the assumed
• 26:11
  Speaker 1: stellar population in one class of galaxy, the headline number
• 26:15
  Speaker 1: changes from roughly 200 sextillion to something closer to 600 sextillion.
• 26:22
  Speaker 1: That is not a rounding error. That is not a footnote.
• 26:26
  Speaker 1: That is the most fundamental input to the entire calculation,
• 26:30
  Speaker 1: shifting by a factor of three. The scientific community received
• 26:35
  Speaker 1: the Van Dockum result with serious interest and significant caution.
• 26:40
  Speaker 1: Elliptical galaxies are not the only type of galaxy. Spiral
• 26:44
  Speaker 1: galaxies like the Milky Way may have different red dwarf populations.
• 26:49
  Speaker 1: The spectroscopic method used has its own systematic uncertainties. Other
• 26:53
  Speaker 1: teams have attempted to reproduce and refine the finding, with
• 26:57
  Speaker 1: results that are mixed enough that the debate has not
• 27:00
  Speaker 1: resolved cleanly. The question of exactly how many red dwarfs
• 27:04
  Speaker 1: populate the various types of galaxies remains genuinely open. but
• 27:09
  Speaker 1: the deeper point stands regardless of how that specific debate
• 27:13
  Speaker 1: eventually settles. The assumed average mass of a star is
• 27:17
  Speaker 1: not a minor technical parameter that specialists quibble over. It
• 27:22
  Speaker 1: is the hinge on which the entire census turns. Get
• 27:26
  Speaker 1: it right and the number is meaningful. Get it wrong
• 27:30
  Speaker 1: and the number is confidently, precisely, spectacularly off. Change your
• 27:36
  Speaker 1: assumption about the average star, and you don't adjust the
• 27:39
  Speaker 1: final number slightly. You multiply or divide it by something
• 27:43
  Speaker 1: that could be as large as three. In December of 1995,
• 27:48
  Speaker 1: astronomers pointed the Hubble Space Telescope at a patch of
• 27:51
  Speaker 1: sky that contained, as far as anyone could tell, absolutely nothing.
• 27:57
  Speaker 1: The region was in Ursa Major, chosen specifically because it
• 28:01
  Speaker 1: was boring. No bright 4-gram stars. No known nebulae, no
• 28:06
  Speaker 1: catalogued objects of any particular interest. From the ground it
• 28:10
  Speaker 1: looked like a gap, a small dark window between things.
• 28:14
  Speaker 1: That was the point. The team running the observation wanted
• 28:18
  Speaker 1: empty sky. They wanted to know what nothing looked like
• 28:21
  Speaker 1: when you stared at it long enough. The patch they
• 28:24
  Speaker 1: selected covered about 1 24 millionth of the full sky.
• 28:29
  Speaker 1: Hold your thumb at arm's length. The area under your
• 28:33
  Speaker 1: thumbnail is enormous compared to what Hubble was aimed at.
• 28:37
  Speaker 1: Over 10 consecutive days, the telescope collected light. 342 separate
• 28:44
  Speaker 1: exposures stacked on top of each other, combining photons that
• 28:48
  Speaker 1: had travelled for billions of years across space into a
• 28:51
  Speaker 1: single image. The total exposure time ran to more than
• 28:54
  Speaker 1: 100 hours. Hubble does not sleep. It orbited the Earth
• 28:59
  Speaker 1: while it watched, completing roughly 150 orbits of our planet
• 29:03
  Speaker 1: during those 10 days, each orbit bringing another slice of data.
• 29:08
  Speaker 1: What came back was not nothing. It was nearly 3,000
• 29:11
  Speaker 1: galaxies in a patch of sky chosen because it appeared empty.
• 29:16
  Speaker 1: Every faint smear, every dim red point, every barely resolved
• 29:22
  Speaker 1: spiral was a galaxy, not a star, not a nebula,
• 29:26
  Speaker 1: but an entire galaxy. containing hundreds of billions of stars
• 29:30
  Speaker 1: of its own. Some of them were close enough that
• 29:33
  Speaker 1: their structures were visible. Spiral arms, bright cores, rings of
• 29:38
  Speaker 1: blue star formation. Others were so distant they showed up
• 29:42
  Speaker 1: as nothing more than faint reddish smudges, their light stretched
• 29:46
  Speaker 1: and dimmed by billions of years of travel. The image
• 29:50
  Speaker 1: looked like a scatter of jewels thrown across black velvet,
• 29:54
  Speaker 1: dense in a way that nobody had expected from a
• 29:56
  Speaker 1: region that looked empty from the ground. The revelation was
• 30:00
  Speaker 1: immediate and profound. Here was empirical proof that the universe,
• 30:05
  Speaker 1: in its population of galaxies, was vastly more crowded than
• 30:09
  Speaker 1: anyone had previously understood. The 3,000 galaxies revealed in this
• 30:14
  Speaker 1: tiny patch stood as direct evidence against the assumption of emptiness.
• 30:20
  Speaker 1: What had appeared vacant to ground-based telescopes turned out to
• 30:24
  Speaker 1: be densely, almost overwhelmingly populated. Here is the logic that
• 30:29
  Speaker 1: made the image so important. That tiny patch represents 1
• 30:34
  Speaker 1: 24 millionth of the total sky. If it contains 3000 galaxies,
• 30:40
  Speaker 1: then a sky 150 million times larger should contain something
• 30:44
  Speaker 1: in the range of 400 billion galaxies. The math is simple.
• 30:49
  Speaker 1: The assumption underneath it is not. You are assuming the
• 30:52
  Speaker 1: patch you observed is representative. You are assuming that empty-looking
• 30:57
  Speaker 1: sky elsewhere contains the same density of galaxies. You are
• 31:02
  Speaker 1: assuming that detection limits are not hiding large populations below
• 31:06
  Speaker 1: the image's reach. Every one of those assumptions could be wrong.
• 31:11
  Speaker 1: If the chosen region happened to be unusually rich or
• 31:14
  Speaker 1: unusually empty, the extrapolation breaks down completely. The entire method
• 31:20
  Speaker 1: rests on the universe being, on large scales, roughly the
• 31:24
  Speaker 1: same everywhere you look. That assumption has a name. Astronomers
• 31:29
  Speaker 1: call it the cosmological principle, the idea that no region
• 31:33
  Speaker 1: of the universe is special, that matter is distributed roughly
• 31:37
  Speaker 1: uniformly when you zoom out far enough. It sounds reasonable.
• 31:42
  Speaker 1: It had theoretical support from the structure of the early
• 31:45
  Speaker 1: universe as revealed by the cosmic microwave background, but the
• 31:49
  Speaker 1: Hubble Deep Field gave it an observational test it had
• 31:52
  Speaker 1: never quite had before. If the cosmological principle holds, two
• 31:57
  Speaker 1: randomly chosen blank patches of sky should look essentially the same.
• 32:02
  Speaker 1: Three years later, they tested it. The Hubble Deep Field
• 32:07
  Speaker 1: South was imaged in 1998 on the opposite side of
• 32:11
  Speaker 1: the sky from the original, a completely independent patch of
• 32:15
  Speaker 1: apparently empty space, this time in the southern hemisphere. The
• 32:19
  Speaker 1: results were nearly identical, nearly the same density of galaxies,
• 32:24
  Speaker 1: nearly the same distribution of sizes, colors, and distances. The
• 32:29
  Speaker 1: two images looked like photographs of the same place, even
• 32:32
  Speaker 1: though they were pointing in opposite directions across the universe.
• 32:36
  Speaker 1: That was not nothing. That was confirmation. The universe, that
• 32:41
  Speaker 1: the scales the deep fields were probing, looked the same
• 32:44
  Speaker 1: in every direction. The sample was probably representative. The extrapolation
• 32:49
  Speaker 1: was probably valid. The galaxy count estimates began climbing. 200
• 32:55
  Speaker 1: billion galaxies in the observable universe became the standard figure.
• 33:00
  Speaker 1: Textbooks adopted it. Documentaries cited it. Science communication used it
• 33:06
  Speaker 1: as the foundation for the headline number on stars. 200
• 33:10
  Speaker 1: billion galaxies, each containing roughly 100 billion stars, producing a
• 33:16
  Speaker 1: total somewhere around 200 sextillion. The number felt enormous and
• 33:21
  Speaker 1: also somehow grounded. because it came from a real image
• 33:25
  Speaker 1: of real light. This figure became embedded in the popular
• 33:29
  Speaker 1: understanding of the cosmos. It shaped how scientists thought about
• 33:33
  Speaker 1: galaxy populations and set the scale for understanding cosmic structure.
• 33:38
  Speaker 1: The number itself, 200 billion, was large enough to seem
• 33:43
  Speaker 1: almost incomprehensible, yet small enough to be memorable and repeatable.
• 33:48
  Speaker 1: It represented a democratic vision of the cosmos, where galaxies
• 33:52
  Speaker 1: were distributed fairly evenly across space. Then the telescopes went deeper.
• 33:58
  Speaker 1: In 2004, Hubble produced the Ultra Deep Field, a single
• 34:03
  Speaker 1: image built from 11 days of continuous observation, aimed at
• 34:08
  Speaker 1: a region roughly 1 13th the area of the original
• 34:11
  Speaker 1: Deep Field. It went significantly further back in time. The
• 34:16
  Speaker 1: galaxies visible in the ultra-deep field were younger. Some of
• 34:20
  Speaker 1: them among the earliest structures in the universe, formed when
• 34:24
  Speaker 1: the cosmos was only a few hundred million years old.
• 34:27
  Speaker 1: Their light had traveled for nearly 13 billion years. They
• 34:32
  Speaker 1: looked different from nearby galaxies, smaller, bluer, irregular, still in
• 34:38
  Speaker 1: the early process of assembling themselves. The ultra-deep field was
• 34:43
  Speaker 1: not just a count, it was a history. In 2012,
• 34:47
  Speaker 1: astronomers released the extreme deep field, the XDF, assembled from
• 34:53
  Speaker 1: a decade of accumulated exposures pointed at roughly the same region.
• 34:59
  Speaker 1: Total observation time, approximately 50 days of stacked data. The
• 35:04
  Speaker 1: XDF revealed galaxies fainter than anything previously imaged, pushing the
• 35:09
  Speaker 1: sample to nearly 6,000 objects in a field barely larger
• 35:14
  Speaker 1: than a grain of rice held at arm's length. The
• 35:17
  Speaker 1: number of detected galaxies kept rising with each image, not
• 35:21
  Speaker 1: because the sky was different, but because the instruments were
• 35:24
  Speaker 1: more sensitive, and if each deeper image revealed more galaxies,
• 35:29
  Speaker 1: that meant the early accounts had been underestimates. Galaxies existed
• 35:34
  Speaker 1: below every previous detection threshold, invisible not because they weren't there,
• 35:40
  Speaker 1: but because the cameras weren't good enough to find them.
• 35:43
  Speaker 1: Yet each new advancement also revealed the limitations of what
• 35:46
  Speaker 1: came before. The better the instrument, the more clearly it
• 35:50
  Speaker 1: showed that previous observations had missed significant populations of faint objects.
• 35:56
  Speaker 1: This was not failure. It was the expected progression of
• 36:00
  Speaker 1: observational astronomy, where sensitivity gains unlock populations that were always
• 36:05
  Speaker 1: present but invisible to less sophisticated cameras. This created a
• 36:10
  Speaker 1: specific problem. The galaxy counts derived from the deep fields
• 36:14
  Speaker 1: were lower limits, not actual totals. For every galaxy the
• 36:19
  Speaker 1: images captured, they were presumably more too faint to detect.
• 36:23
  Speaker 1: The question was, how many more? The shape of the
• 36:26
  Speaker 1: galaxy population at the faint end, how many dim dwarf
• 36:30
  Speaker 1: galaxies exist for every bright one, was not directly measurable
• 36:34
  Speaker 1: from the images. It had to be inferred from models,
• 36:38
  Speaker 1: from physical assumptions about how galaxies form and grow. That
• 36:42
  Speaker 1: inference pointed toward a much larger population than the images
• 36:46
  Speaker 1: themselves showed. How much larger was what the next decade
• 36:50
  Speaker 1: of astronomy would debate? By the time the extreme deep
• 36:54
  Speaker 1: field was released, the 200 billion galaxy figure was already
• 36:58
  Speaker 1: starting to look like a flaw rather than an answer.
• 37:02
  Speaker 1: The deep fields had established the method validated the cosmological
• 37:07
  Speaker 1: principle and set a baseline. What they had not done
• 37:11
  Speaker 1: was capture everything that was out there. The image that
• 37:14
  Speaker 1: came back from that empty patch of sky would eventually
• 37:18
  Speaker 1: anchor a new estimate for the number of galaxies in
• 37:21
  Speaker 1: the universe, and therefore a new estimate for the number
• 37:24
  Speaker 1: of galaxies in the observable universe was around 200 billion,
• 37:29
  Speaker 1: and astronomers were reasonably comfortable with it. The Hubble Deep
• 37:33
  Speaker 1: Field had produced a number, The math had been checked,
• 37:37
  Speaker 1: and 200 billion felt like solid ground. It wasn't. In 2016,
• 37:43
  Speaker 1: a team led by Christopher Consolis at the University of
• 37:47
  Speaker 1: Nottingham published a study that did not just revise the
• 37:51
  Speaker 1: galaxy count, it multiplied it. Their conclusion, the observable universe
• 37:56
  Speaker 1: contains roughly 2 trillion galaxies, not 200 billion. Two trillion,
• 38:03
  Speaker 1: ten times more than anyone had been working with. The
• 38:07
  Speaker 1: paper did not arrive because of a new telescope or
• 38:09
  Speaker 1: a more powerful instrument. It arrived because someone asked a
• 38:14
  Speaker 1: harder question about the data that already existed. The Hubble images,
• 38:19
  Speaker 1: extraordinary as they are, have a detection threshold. Below a
• 38:24
  Speaker 1: certain brightness, galaxies simply do not show up. They are
• 38:28
  Speaker 1: there in the sense that they exist and contain stars
• 38:32
  Speaker 1: and occupy space. But the light they emit is too
• 38:35
  Speaker 1: faint by the time it crosses billions of light years
• 38:38
  Speaker 1: to register on any image Hubble has ever taken. The
• 38:42
  Speaker 1: question Consulis' team asked was, how many galaxies are we
• 38:46
  Speaker 1: missing and can we estimate that number mathematically? This kind
• 38:51
  Speaker 1: of correction has a name in astronomy. Malmquist bias describes
• 38:55
  Speaker 1: the systematic tendency of any survey to over-represent the brightest,
• 39:00
  Speaker 1: most visible objects in a population. If you survey a
• 39:04
  Speaker 1: forest by counting every tree you can see from the road,
• 39:08
  Speaker 1: you will count the tallest trees near the edge perfectly
• 39:11
  Speaker 1: and miss almost everything deep in the canopy. Your survey
• 39:15
  Speaker 1: will be accurate for what it measured and deeply misleading
• 39:19
  Speaker 1: as a description of the whole forest. The same principle
• 39:22
  Speaker 1: applies to galaxy surveys. The Hubble Deep Field captured the
• 39:27
  Speaker 1: bright end of the galaxy population with extraordinary clarity. The
• 39:31
  Speaker 1: faint end, the dwarf galaxies, the low-mass systems, the small
• 39:36
  Speaker 1: and ancient and dim, was almost entirely absent from those images.
• 39:41
  Speaker 1: What Consulis's team did was model what the faint end
• 39:45
  Speaker 1: of the galaxy population should look like. based on our
• 39:48
  Speaker 1: understanding of how galaxies form and evolve over cosmic time.
• 39:53
  Speaker 1: Galaxy formation follows physical laws. Those laws predict how many
• 39:58
  Speaker 1: small galaxies should exist for every large one, how that
• 40:01
  Speaker 1: ratio should shift across different epochs of cosmic history, and
• 40:06
  Speaker 1: what the total population should look like once you account
• 40:08
  Speaker 1: for every size class. When they ran those models against
• 40:12
  Speaker 1: the observed data and extrapolated to the full population, The
• 40:16
  Speaker 1: number that came out was not 200 billion. It was
• 40:20
  Speaker 1: 2 trillion. The implication of that revision is not subtle.
• 40:25
  Speaker 1: If you take 2 trillion galaxies and assume each one
• 40:29
  Speaker 1: contains somewhere in the range of 100 billion stars, a
• 40:33
  Speaker 1: conservative figure, and one that comes with its own complications
• 40:37
  Speaker 1: we will get to, the arithmetic produces a number with
• 40:40
  Speaker 1: 23 zeros behind it. 200 sextillion stars. That is the
• 40:47
  Speaker 1: headline figure that comes from the 2016 study. And it
• 40:51
  Speaker 1: is the number that circulates most widely when people ask
• 40:54
  Speaker 1: how many stars exist. It sounds authoritative. It has a
• 40:58
  Speaker 1: specific origin. But the chain of assumptions holding it up
• 41:02
  Speaker 1: is long. And one of the biggest links in that
• 41:05
  Speaker 1: chain had just been replaced by something 10 times larger
• 41:09
  Speaker 1: than what it replaced. It's worth sitting with how dramatic
• 41:12
  Speaker 1: that kind of revision actually is. Astronomers had been using
• 41:16
  Speaker 1: 200 billion as the galaxy count for years. Papers were
• 41:21
  Speaker 1: written with it. Textbooks printed it. And then, without a
• 41:25
  Speaker 1: single new observation, without pointing a telescope at any new
• 41:29
  Speaker 1: patch of sky, the number changed by a factor of 10.
• 41:33
  Speaker 1: That is not a rounding correction. That is a fundamental
• 41:37
  Speaker 1: shift in what we thought we knew. The methodology was
• 41:41
  Speaker 1: not reckless. Modelling the faint end of a population distribution
• 41:45
  Speaker 1: is standard practice in observational astronomy, but the result was
• 41:49
  Speaker 1: a reminder that even the most carefully anchored numbers in
• 41:53
  Speaker 1: science are sometimes anchored to something that moves. Then the
• 41:57
  Speaker 1: revision got revised. In 2021, data from the New Horizons
• 42:02
  Speaker 1: spacecraft introduced a new complication. New Horizons, built to study Pluto,
• 42:09
  Speaker 1: had by that point travelled far enough from the Sun
• 42:12
  Speaker 1: Past the zone where sunlight scattered off interplanetary dust creates
• 42:16
  Speaker 1: a faint background glow, that it could measure the cosmic
• 42:19
  Speaker 1: optical background with unusual clarity. What it found suggested the
• 42:24
  Speaker 1: universe was not as packed with faint galaxies as the
• 42:27
  Speaker 1: 2016 model predicted. The background light, which is the summed
• 42:32
  Speaker 1: glow of every galaxy too faint and distant to be
• 42:35
  Speaker 1: seen individually, did not match what two trillion galaxies would produce.
• 42:40
  Speaker 1: The implied galaxy count from New Horizons data was lower,
• 42:44
  Speaker 1: substantially lower. The new estimate pointed towards several hundred billion
• 42:49
  Speaker 1: galaxies rather than two trillion. The field did not immediately settle.
• 42:54
  Speaker 1: It rarely does. The New Horizons measurement raised its own
• 42:58
  Speaker 1: methodological questions, and the 2016 modelling approach retained defenders. What
• 43:04
  Speaker 1: the disagreement produced was not a new consensus number, but
• 43:09
  Speaker 1: a widened uncertainty range, which is, in its own way,
• 43:13
  Speaker 1: more honest than what preceded it. The confidence that had
• 43:17
  Speaker 1: surrounded 200 billion was probably not fully warranted. The confidence
• 43:22
  Speaker 1: that surrounded 2 trillion was almost certainly premature. Several hundred
• 43:28
  Speaker 1: billion to 2 trillion represents the current honest spread, with
• 43:32
  Speaker 1: different research groups weighting the evidence differently and arriving at
• 43:36
  Speaker 1: different positions within that range. None of this means astronomy
• 43:40
  Speaker 1: has broken. A field that never revises its estimates is
• 43:44
  Speaker 1: not a field that is making progress. It is a
• 43:48
  Speaker 1: field that has stopped asking harder questions. Consulis's team asked
• 43:53
  Speaker 1: a harder question, got a startling answer, and forced the
• 43:57
  Speaker 1: community to examine what the Hubble images had actually been
• 44:01
  Speaker 1: capturing and what they had been missing. New Horizons gave
• 44:05
  Speaker 1: astronomers an independent line of evidence that pulled in a
• 44:08
  Speaker 1: different direction and now both lines of evidence have to
• 44:12
  Speaker 1: be reconciled. That is the process. It is slow and
• 44:16
  Speaker 1: it is uncomfortable and it is exactly how our understanding
• 44:20
  Speaker 1: of the universe has deepened over the past century. What
• 44:23
  Speaker 1: the back and forth does reveal is something worth keeping
• 44:27
  Speaker 1: in mind as we try to build toward a total
• 44:29
  Speaker 1: star count. The galaxy count is not a settled input
• 44:33
  Speaker 1: to the calculation. It is itself an uncertain quantity, with
• 44:38
  Speaker 1: serious estimates differing by a factor of 10. Every star
• 44:42
  Speaker 1: count that derives from it inherits that uncertainty. The 200
• 44:46
  Speaker 1: sextillion figure is built on two trillium galaxies. If the
• 44:50
  Speaker 1: true number is 200 billion, the star count drops by
• 44:54
  Speaker 1: the same factor. If future instruments push it higher, the
• 44:58
  Speaker 1: star count rises accordingly. The galaxy count is the multiplier
• 45:02
  Speaker 1: on which everything else depends, and right now that multiplier
• 45:06
  Speaker 1: has error bars large enough to drive entire solar systems through.
• 45:10
  Speaker 1: Whether the number is 2 trillion or several hundred billion,
• 45:14
  Speaker 1: the range tells. You something important. We are still in
• 45:19
  Speaker 1: the early stages of understanding how many galaxies actually exist.
• 45:25
  Speaker 1: The most common way people arrive at a total star
• 45:28
  Speaker 1: count is also the most misleading. Take the Milky Way.
• 45:33
  Speaker 1: Assume it is average. Multiply by the number of galaxies.
• 45:37
  Speaker 1: It feels like a reasonable shortcut. The Milky Way is
• 45:40
  Speaker 1: the galaxy we know best. We have mapped its spiral arms,
• 45:44
  Speaker 1: measured its rotation, estimated its mass. If you want a
• 45:48
  Speaker 1: number for a typical galaxy and you only have detailed
• 45:51
  Speaker 1: knowledge of one, use the one you have The problem
• 45:55
  Speaker 1: is that the Milky Way is not typical, not even close.
• 45:59
  Speaker 1: It is a large, massive spiral, sitting comfortably in the
• 46:03
  Speaker 1: top few percent of galaxies by size. Using it as
• 46:06
  Speaker 1: a stand-in for the average galaxy is like estimating the
• 46:10
  Speaker 1: average height of all humans by measuring only professional basketball players.
• 46:15
  Speaker 1: You will get a number. It will be wrong. The
• 46:19
  Speaker 1: most common type of galaxy in the universe is the
• 46:22
  Speaker 1: dwarf galaxy. Small, faint, difficult to detect at any real distance.
• 46:28
  Speaker 1: The Magellanic Clouds, the two smudges of light visible from
• 46:32
  Speaker 1: the southern hemisphere, orbiting the Milky Way like satellites, are
• 46:36
  Speaker 1: dwarf galaxies. The Large Magellanic Cloud contains somewhere around 30
• 46:41
  Speaker 1: billion stars. The Small Magellanic Cloud holds perhaps 3 billion.
• 46:46
  Speaker 1: Compare that to the Milky Way's 300 billion and you
• 46:50
  Speaker 1: start to see the problem. These are not minor galaxies
• 46:53
  Speaker 1: at the edge of the census. They are representative of
• 46:56
  Speaker 1: the most common class of galaxy in the observable universe.
• 47:00
  Speaker 1: There are hundreds of billions of them scattered in every direction.
• 47:05
  Speaker 1: Most contain fewer than a billion stars. Many contain far
• 47:09
  Speaker 1: fewer than that. Dwarf spheroidal galaxies, the smallest faintest class,
• 47:14
  Speaker 1: can hold as few as a few million stars. A
• 47:17
  Speaker 1: rounding error compared to a spiral like ours. The challenge
• 47:21
  Speaker 1: of accurately counting dwarf galaxies compounds this problem significantly. These small,
• 47:27
  Speaker 1: faint galaxies are systematically harder to detect at cosmological distances
• 47:31
  Speaker 1: where their light grows increasingly faint. Observational surveys have improved
• 47:37
  Speaker 1: substantially over recent decades, but they remain incomplete in detecting
• 47:42
  Speaker 1: the smallest dwarf systems. Current estimates suggest the true population
• 47:47
  Speaker 1: may be even higher than previous surveys indicated. Some models
• 47:51
  Speaker 1: propose that for every observable galaxy, several more remain below
• 47:56
  Speaker 1: current detection thresholds. The full census of the universe's dwarf
• 48:01
  Speaker 1: population remains incomplete and evolving. This uncertainty at the low
• 48:06
  Speaker 1: end directly feeds into overall uncertainty about the total star count.
• 48:11
  Speaker 1: When you run the calculation honestly, weighting the galaxy population
• 48:16
  Speaker 1: by what it actually looks like rather than what the
• 48:19
  Speaker 1: Milky Way looks like, The total star count drops substantially
• 48:23
  Speaker 1: from the naive multiplication. The galaxy count, whether you use
• 48:28
  Speaker 1: two trillion or several hundred billion, is dominated numerically by
• 48:33
  Speaker 1: these small systems. If a trillion of the universe's galaxies
• 48:37
  Speaker 1: are dwarfs with an average of a few hundred million
• 48:40
  Speaker 1: stars each, they contribute a number to the total that
• 48:44
  Speaker 1: is dramatically lower per galaxy than the Milky Way figure
• 48:47
  Speaker 1: would suggest. The naive estimate swells the average because it
• 48:52
  Speaker 1: starts from the wrong baseline. But the story does not
• 48:56
  Speaker 1: stop there. On the opposite end of the scale sit
• 48:59
  Speaker 1: the giant elliptical galaxies. These are rare, but they're not negligible.
• 49:05
  Speaker 1: A large elliptical can contain 20 times the mass of
• 49:09
  Speaker 1: the Milky Way. Some of the biggest known examples, like
• 49:13
  Speaker 1: IC 1101, are so enormous that if they occupied the
• 49:18
  Speaker 1: position of the Milky Way, their outer edges would reach
• 49:21
  Speaker 1: a significant fraction of the way to Andromeda. They can
• 49:24
  Speaker 1: harbor tens of trillions of stars each. These objects are
• 49:28
  Speaker 1: not common. For every giant elliptical, there may be thousands
• 49:32
  Speaker 1: of dwarfs. But when you are building a total count,
• 49:36
  Speaker 1: The rare objects with the enormous star counts push the
• 49:39
  Speaker 1: number back upward. A single giant elliptical with 10 trillion
• 49:44
  Speaker 1: stars contributes as much to the universal total as 10,000
• 49:48
  Speaker 1: dwarf galaxies with a billion stars each. So the distribution
• 49:52
  Speaker 1: of galaxies creates a strange accounting problem. The bulk of
• 49:57
  Speaker 1: galaxies by number are small and pull the per-galaxy average down.
• 50:02
  Speaker 1: The rare giants at the high end push the total
• 50:05
  Speaker 1: back up. Neither effect cancels perfectly. What you are left
• 50:09
  Speaker 1: with is a calculation that depends sensitively on how well
• 50:13
  Speaker 1: you have characterized both ends of the galaxy distribution, the
• 50:17
  Speaker 1: faint dwarf population. That is hard to detect at cosmological distances.
• 50:22
  Speaker 1: The rare giant ellipticals, which are easy to see but
• 50:26
  Speaker 1: poorly constrained in their stellar populations. This is where a
• 50:30
  Speaker 1: second correction enters. The additional red dwarfs in massive elliptical galaxies.
• 50:36
  Speaker 1: the 2010 Van Dockum study, discussed earlier in this series,
• 50:40
  Speaker 1: found evidence that elliptical galaxies contain far more low-mass red
• 50:45
  Speaker 1: dwarf stars per unit of total stellar mass than spiral
• 50:49
  Speaker 1: galaxies like the Milky Way. If that finding holds up,
• 50:53
  Speaker 1: and the debate is still ongoing, then giant ellipticals are
• 50:57
  Speaker 1: not just more massive than spirals, they are also more
• 51:01
  Speaker 1: densely packed with faint, uncounted stars at the bottom of
• 51:04
  Speaker 1: the mass spectrum, This pushes the star count upward again,
• 51:09
  Speaker 1: partially offsetting the reduction that comes from realising most galaxies
• 51:12
  Speaker 1: are dwarfs. The corrections are pulling against each other. The
• 51:16
  Speaker 1: dwarf galaxy correction says, you have been over-counting because most
• 51:21
  Speaker 1: galaxies are small. The red dwarf correction in elliptical says,
• 51:26
  Speaker 1: you have been under-counting because the biggest galaxies are full
• 51:29
  Speaker 1: of stars you did not account for. Neither wins cleanly.
• 51:34
  Speaker 1: The net result is that the total estimate ends up
• 51:37
  Speaker 1: uncertain by roughly an order of magnitude, meaning the real
• 51:41
  Speaker 1: number could plausibly be 10 times lower or several times
• 51:45
  Speaker 1: higher than the headline figure, depending on which corrections you
• 51:48
  Speaker 1: apply and how aggressively. What does this mean in practice?
• 51:53
  Speaker 1: It means that the number 200 sextillion, the figure that
• 51:56
  Speaker 1: appears most often in popular discussions, should be understood as
• 52:00
  Speaker 1: a central estimate inside a wide range, Not a precise result.
• 52:05
  Speaker 1: Some careful calculations weighting galaxy types properly and using conservative
• 52:10
  Speaker 1: assumptions about red dwarf populations produce something closer to 70
• 52:15
  Speaker 1: billion trillion. Others, using the full 2 trillion galaxy count
• 52:20
  Speaker 1: and aggressive red dwarf corrections for ellipticals, push past 200 sextillion.
• 52:27
  Speaker 1: Some go higher. The spread between the lowest and highest
• 52:30
  Speaker 1: credible estimates spans more than a factor of 10. This
• 52:34
  Speaker 1: is not a failure of the science. It is an
• 52:37
  Speaker 1: honest reflection of what happens when you try to count
• 52:40
  Speaker 1: objects distributed across billions of light years of space using
• 52:44
  Speaker 1: a sample you can only observe from one fixed point,
• 52:48
  Speaker 1: with instruments that systematically miss the faint end of every population.
• 52:53
  Speaker 1: The Milky Way is a magnificent galaxy. and it is
• 52:57
  Speaker 1: the one we understand best. It is just not the
• 53:00
  Speaker 1: one to build a universal average from. The corrections pull
• 53:04
  Speaker 1: in opposite directions and partially cancel out. That is part
• 53:08
  Speaker 1: of why the honest answer to how many stars exist
• 53:12
  Speaker 1: is always given as a range rather than a number.
• 53:15
  Speaker 1: There is an entire category of stars that optical telescopes
• 53:19
  Speaker 1: simply cannot see, not because they are too faint or
• 53:22
  Speaker 1: too far, but because something is in the way That
• 53:26
  Speaker 1: something is dust, not the kind you find on a shelf,
• 53:30
  Speaker 1: but vast clouds of carbon grains and silicate particles scattered
• 53:34
  Speaker 1: throughout galaxies, concentrated most heavily in the same regions where
• 53:39
  Speaker 1: new stars are being born. When light from a young
• 53:42
  Speaker 1: star tries to escape its nursery, the dust intercepts it.
• 53:46
  Speaker 1: It doesn't let the light through. Instead, it absorbs the
• 53:51
  Speaker 1: energy and re-radiates it at a completely different wavelength. Infrared.
• 53:57
  Speaker 1: Far outside the range of what human eyes can detect,
• 54:00
  Speaker 1: and well outside the range of what instruments like the
• 54:03
  Speaker 1: Hubble Space Telescope were designed to capture. The star is
• 54:07
  Speaker 1: still there, still burning, still contributing to the total population
• 54:12
  Speaker 1: of the universe. But to an optical telescope, it is
• 54:16
  Speaker 1: as invisible as if it had never formed at all.
• 54:19
  Speaker 1: This matters enormously for the star count problem. The methods
• 54:24
  Speaker 1: described in previous sections, counting galaxies in the Hubble Deep Field,
• 54:29
  Speaker 1: estimating stellar populations from galaxy brightness, multiplying through to a total,
• 54:35
  Speaker 1: all rely on optical data. They are all, at some level,
• 54:39
  Speaker 1: blind to the dusty universe. And the dusty universe is
• 54:43
  Speaker 1: not a minor footnote. It is, in some periods of
• 54:47
  Speaker 1: cosmic history, most of the story. To understand why, You
• 54:52
  Speaker 1: have to think about when stars formed. The universe has
• 54:55
  Speaker 1: not always made stars at the rate it does today.
• 54:59
  Speaker 1: Star formation has a history, a rise and fall over 13.8
• 55:03
  Speaker 1: billion years. In the very early universe, conditions were not
• 55:08
  Speaker 1: yet right for large-scale star formation. Then, somewhere around 10
• 55:13
  Speaker 1: billion years ago, it peaked. Galaxies were colliding, merging, triggering
• 55:19
  Speaker 1: massive bursts of star formation at rates perhaps 10 times
• 55:22
  Speaker 1: higher than what we see now. This period, roughly between
• 55:26
  Speaker 1: 8 and 12 billion years ago, is when the bulk
• 55:30
  Speaker 1: of the universe's stellar population was assembled. It was also
• 55:34
  Speaker 1: the period when the universe was most heavily choked with dust,
• 55:38
  Speaker 1: the very process that drives intense star formation. The gravitational
• 55:43
  Speaker 1: collapse of dense molecular clouds also produces the dust that
• 55:47
  Speaker 1: hides the result. At the exact moment the universe was
• 55:51
  Speaker 1: making the most stars, it was also most effectively hiding
• 55:55
  Speaker 1: them from optical view. Astronomers using optical instruments looked back
• 55:59
  Speaker 1: at this era and saw star-forming galaxies, bright, luminous ones
• 56:04
  Speaker 1: that stood out against the darkness. But those were only
• 56:08
  Speaker 1: the galaxies where the dust had not fully obscured the light.
• 56:11
  Speaker 1: The others were there, sometimes radiating enormous amounts of energy,
• 56:16
  Speaker 1: but doing it entirely in the infrared. They looked dim
• 56:20
  Speaker 1: or even invisible in optical surveys. They were logged as
• 56:24
  Speaker 1: empty space, or as faint smudges, not as the furious
• 56:28
  Speaker 1: stellar factories they actually were. The European Space Agency launched
• 56:33
  Speaker 1: the Herschel Space Observatory in 2009 specifically to address this gap.
• 56:39
  Speaker 1: Herschel carried a 3.5 metre mirror and instruments tuned to
• 56:43
  Speaker 1: the far infrared and submillimetre wavelengths where dust-trouted galaxies shine brightest.
• 56:49
  Speaker 1: It surveyed large regions of sky, building up catalogues of
• 56:53
  Speaker 1: galaxies that had been essentially absent from every optical dataset
• 56:57
  Speaker 1: assembled before. What it found was striking. There was an
• 57:01
  Speaker 1: entire population of galaxies, dusty, intensely star-forming systems, that optical
• 57:07
  Speaker 1: surveys had missed almost completely. Some of them were among
• 57:11
  Speaker 1: the most productive star-forming environments in the observable universe, churning
• 57:17
  Speaker 1: out stars at rates hundreds of times higher than the
• 57:20
  Speaker 1: Milky Way does today. They had been invisible not because
• 57:23
  Speaker 1: they were faint, but because their energy was arriving at
• 57:27
  Speaker 1: the wrong wavelength for previous instruments to catch. Herschel also
• 57:31
  Speaker 1: did something more specific than just finding hidden galaxies. It
• 57:36
  Speaker 1: charted the star formation rate of the universe across cosmic time,
• 57:40
  Speaker 1: galaxy by galaxy, in infrared. When researchers compared that infrared-derived
• 57:46
  Speaker 1: history to the history assembled from optical data alone, the
• 57:49
  Speaker 1: optical version was an undercount. The peak of star formation,
• 57:54
  Speaker 1: when corrected for the dusty, infrared-bright galaxies that had been missing,
• 57:59
  Speaker 1: was significantly higher than optical surveys had suggested. more stars
• 58:03
  Speaker 1: were made during the universe's most active period than the
• 58:07
  Speaker 1: optical census had captured. Here is why that matters directly
• 58:12
  Speaker 1: for the total star count. If you want to know
• 58:15
  Speaker 1: how many stars exist right now, you cannot simply count
• 58:19
  Speaker 1: what's visible today. Stars that formed 10 billion years ago
• 58:23
  Speaker 1: and were massive enough have long since died. But stars
• 58:28
  Speaker 1: that formed at that same peak period and were small
• 58:31
  Speaker 1: enough are still burning. Every count of the current stellar
• 58:35
  Speaker 1: population is ultimately a count of survivors. What's left after 13.8
• 58:41
  Speaker 1: billion years of formation and death. To get that count right,
• 58:46
  Speaker 1: you need an accurate history of how many stars were
• 58:49
  Speaker 1: ever born. This was because that sets the ceiling. The
• 58:53
  Speaker 1: infrared data raised that ceiling. More stars were born in
• 58:57
  Speaker 1: the dusty early universe than the optical record showed. which
• 59:00
  Speaker 1: means more low-mass survivors from that era are still burning
• 59:04
  Speaker 1: today than the optical-only models predicted. The correction is not trivial.
• 59:10
  Speaker 1: Some analyses of Herschel data suggested that the integrated star
• 59:14
  Speaker 1: formation rate over cosmic history, when the infrared bright population
• 59:19
  Speaker 1: is included, could be as much as five times higher
• 59:22
  Speaker 1: than estimates derived from ultraviolet and optical observations alone. Five times!
• 59:29
  Speaker 1: That is not a rounding adjustment. That is a fundamental
• 59:32
  Speaker 1: revision to the input that feeds every downstream calculation about
• 59:37
  Speaker 1: total stellar numbers. It also layers on to the other
• 59:41
  Speaker 1: corrections discussed previously. You have the red dwarf problem pushing
• 59:46
  Speaker 1: the average star count per galaxy upward. You have the
• 59:50
  Speaker 1: galaxy diversity problem revealing that naive Milky Way multiplication overstates things.
• 59:56
  Speaker 1: And now you have the dust problem revealing that optical
• 59:59
  Speaker 1: surveys systematically undercounted star formation throughout cosmic history. That pushes
• 1:00:05
  Speaker 1: the number of surviving low-mass stars upward. These corrections do
• 1:00:10
  Speaker 1: not all point in the same direction. The galaxy diversity
• 1:00:14
  Speaker 1: correction pushes the total count down. The red dwarf and
• 1:00:18
  Speaker 1: dust corrections push it back up. They operate on different
• 1:00:22
  Speaker 1: parts of the problem. How many galaxies, how many stars
• 1:00:26
  Speaker 1: per galaxy, how many stars were ever formed? And when
• 1:00:30
  Speaker 1: you try to stack them together, the uncertainties compound rather
• 1:00:35
  Speaker 1: than cancel cleanly. What infrared astronomy revealed, fundamentally, is that
• 1:00:41
  Speaker 1: the universe we could see in optical light was always
• 1:00:45
  Speaker 1: a filtered version of reality. The filter was not random.
• 1:00:49
  Speaker 1: It systematically hid the most active, most productive, most dust-enshrouded
• 1:00:55
  Speaker 1: regions of star formation, concentrated right at the period when
• 1:01:00
  Speaker 1: the universe was making the most stars. Any estimate of
• 1:01:04
  Speaker 1: the total stellar population built purely from optical data was
• 1:01:08
  Speaker 1: carrying an error in a specific direction. Too low. This
• 1:01:13
  Speaker 1: was because the dustiest nurseries were dark to every instrument
• 1:01:17
  Speaker 1: pointed at them before Herschel changed the view. The infrared
• 1:01:21
  Speaker 1: universe turned out to be full of star formation that
• 1:01:24
  Speaker 1: optical surveys had been missing entirely. That missing piece is
• 1:01:28
  Speaker 1: not small. The question of how many stars exist is
• 1:01:33
  Speaker 1: actually a question about survivors. That distinction matters more than
• 1:01:38
  Speaker 1: it might seem. When astronomers produce a number, 200 sextillion,
• 1:01:44
  Speaker 1: or whatever figure they're working with that year, they are
• 1:01:47
  Speaker 1: not counting every star that has ever burned. They are
• 1:01:50
  Speaker 1: counting the ones still burning right now, at this moment
• 1:01:54
  Speaker 1: in cosmic time. And the universe has had 13.8 billion
• 1:01:59
  Speaker 1: years to eliminate a substantial fraction of everything it ever made,
• 1:02:04
  Speaker 1: to understand what the census is actually measuring. You have
• 1:02:08
  Speaker 1: to think about the universe not as a static inventory,
• 1:02:11
  Speaker 1: but as a process. Stars are born, they burn, they die.
• 1:02:16
  Speaker 1: The count we are attempting is a snapshot of an
• 1:02:19
  Speaker 1: ongoing attrition that has been running since the first stars
• 1:02:23
  Speaker 1: ignited a few hundred million years after the Big Bang.
• 1:02:27
  Speaker 1: The critical variable is how long different types of stars live.
• 1:02:32
  Speaker 1: This is where the numbers become genuinely strange. A blue supergiant,
• 1:02:37
  Speaker 1: one of those enormous, spectacularly luminous stars that burns hot
• 1:02:43
  Speaker 1: and bright and makes for dramatic photographs. lives for only
• 1:02:47
  Speaker 1: a few million years. That sounds like a long time
• 1:02:52
  Speaker 1: until you set it against the age of the universe.
• 1:02:55
  Speaker 1: A few million years is less than one-tenth of one
• 1:02:58
  Speaker 1: percent of cosmic history. A star like that forms, blazes
• 1:03:03
  Speaker 1: at extraordinary luminosity, exhausts its fuel, collapses in a supernova
• 1:03:09
  Speaker 1: before the universe has barely registered its existence. Our own sun,
• 1:03:14
  Speaker 1: a medium-weight star by the standards of what's out there,
• 1:03:18
  Speaker 1: has a lifespan of roughly 10 billion years. It formed
• 1:03:22
  Speaker 1: about 4.6 billion years ago and has approximately the same
• 1:03:26
  Speaker 1: time remaining. A small red dwarf, one of those dim,
• 1:03:30
  Speaker 1: cool stars that barely clears the threshold for sustained fusion,
• 1:03:35
  Speaker 1: can burn steadily for a trillion years or more. The
• 1:03:38
  Speaker 1: current age of the universe is not even 2% of
• 1:03:41
  Speaker 1: a red dwarf's total lifespan. What this means for the
• 1:03:45
  Speaker 1: census is profound. The universe forms stars at very different
• 1:03:49
  Speaker 1: rates at different points in its history. In the very
• 1:03:52
  Speaker 1: early universe, star formation was relatively slow. The raw material existed,
• 1:03:58
  Speaker 1: but the conditions had not yet fully organized into the
• 1:04:01
  Speaker 1: dense clouds that collapse into stars. Then, roughly 10 billion
• 1:04:06
  Speaker 1: years ago, something changed. The rate of star formation across
• 1:04:11
  Speaker 1: the universe spiked dramatically. Galaxies were colliding. Gas was dense
• 1:04:17
  Speaker 1: and plentiful. New stars were igniting at perhaps ten times
• 1:04:22
  Speaker 1: the pace we see today. This was the peak of
• 1:04:25
  Speaker 1: cosmic star formation. It produced enormous numbers of stars across
• 1:04:29
  Speaker 1: an enormous range of masses. Here is the problem. Many
• 1:04:33
  Speaker 1: of those stars were massive. The same conditions that trigger
• 1:04:37
  Speaker 1: intense star formation also tend to produce a population weighted
• 1:04:41
  Speaker 1: toward the heavier end. Massive stars are not subtle. They
• 1:04:45
  Speaker 1: burn ferociously, live briefly, die violently in supernovae that seed
• 1:04:51
  Speaker 1: the surrounding interstellar medium with heavy elements. The iron in
• 1:04:55
  Speaker 1: your blood was forged in a star that exploded billions
• 1:04:59
  Speaker 1: of years ago, probably during or shortly after that peak
• 1:05:03
  Speaker 1: formation era. Those stars contributed enormously to the chemical history
• 1:05:08
  Speaker 1: of the universe, but they did not stick around to
• 1:05:11
  Speaker 1: be counted. Every massive star born during the formation peak,
• 1:05:16
  Speaker 1: roughly 10 billion years ago, has been dead for almost
• 1:05:20
  Speaker 1: as long as the Earth has existed. This creates a
• 1:05:22
  Speaker 1: gap between two very different numbers. The first is the
• 1:05:26
  Speaker 1: total number of stars the universe has ever produced across
• 1:05:30
  Speaker 1: its entire history. Call it the total enrollment. The second
• 1:05:35
  Speaker 1: is the number currently burning, the current population. These are
• 1:05:39
  Speaker 1: not the same number, and the gap between them is
• 1:05:41
  Speaker 1: not small. Estimates of how many stars have ever formed
• 1:05:45
  Speaker 1: throughout cosmic history run substantially higher than estimates of how
• 1:05:49
  Speaker 1: many exist right now. The dead outnumber the living, and
• 1:05:53
  Speaker 1: they have for a long time. What you're left with
• 1:05:56
  Speaker 1: after 13.8 billion years of this process is a population
• 1:06:01
  Speaker 1: increasingly dominated by stars that were never going to die
• 1:06:05
  Speaker 1: quickly in the first place. Red dwarfs, those dim, slow-burning
• 1:06:10
  Speaker 1: stars that account for roughly 70% of all stars by
• 1:06:14
  Speaker 1: number in the current universe, have been accumulating since the
• 1:06:18
  Speaker 1: earliest epochs of star formation without losing a single member.
• 1:06:22
  Speaker 1: No red dwarf has ever died of old age. The
• 1:06:26
  Speaker 1: universe is not old enough. Every red dwarf that has
• 1:06:29
  Speaker 1: ever formed is, to the best of our knowledge, still
• 1:06:33
  Speaker 1: burning somewhere. They are the survivors by default, not because
• 1:06:37
  Speaker 1: they are hardy in any active sense, but because the
• 1:06:40
  Speaker 1: universe simply hasn't had time to kill them yet. This
• 1:06:44
  Speaker 1: matters enormously for how astronomers do the counting. There is
• 1:06:48
  Speaker 1: a concept in stellar physics called the initial mass function.
• 1:06:53
  Speaker 1: That describes the distribution of masses among stars at the
• 1:06:56
  Speaker 1: moment of their birth. It is a statistical relationship that
• 1:07:00
  Speaker 1: tells you, roughly, what fraction of new stars are born
• 1:07:04
  Speaker 1: massive versus medium versus small. The initial mass function is
• 1:07:08
  Speaker 1: not uniform, it is heavily weighted towards the low end.
• 1:07:12
  Speaker 1: Most stars are born small, but the exact shape of
• 1:07:16
  Speaker 1: that distribution, particularly at the very low end where red
• 1:07:20
  Speaker 1: dwarfs live, is still not perfectly pinned down, and the
• 1:07:24
  Speaker 1: shape matters for the census in a specific way. When
• 1:07:27
  Speaker 1: you're counting survivors rather than total enrolment, a population that
• 1:07:31
  Speaker 1: started with more massive stars will have lost more members
• 1:07:34
  Speaker 1: over time. The census is sensitive to the initial mass
• 1:07:38
  Speaker 1: function in ways that it would not be if every
• 1:07:41
  Speaker 1: star lived forever. Think of it this way. Imagine trying
• 1:07:45
  Speaker 1: to count the survivors of a very long war by
• 1:07:47
  Speaker 1: looking at who's still standing. If you don't know what
• 1:07:50
  Speaker 1: fraction of the original army was in the infantry versus
• 1:07:54
  Speaker 1: in the cavalry versus in some other unit, If those
• 1:07:57
  Speaker 1: different units suffered radically different casualty rates, then your count
• 1:08:01
  Speaker 1: of survivors tells you only part of the story. The
• 1:08:05
  Speaker 1: universe's massive stars were the infantry. They took the heaviest losses, earliest,
• 1:08:10
  Speaker 1: and are now almost entirely gone from the rolls. What
• 1:08:14
  Speaker 1: remains is overwhelmingly the lower mass population that never faced
• 1:08:18
  Speaker 1: the same attrition. The star formation rate today is a
• 1:08:21
  Speaker 1: fraction of what it was at the peak. The universe
• 1:08:24
  Speaker 1: is still making new stars, The Milky Way produces a
• 1:08:28
  Speaker 1: few solar masses worth of new stars per year, but
• 1:08:31
  Speaker 1: the pace has been declining for billions of years. The
• 1:08:35
  Speaker 1: great era of star birth is behind us. What that
• 1:08:39
  Speaker 1: means for the long-term count is that the stellar population
• 1:08:43
  Speaker 1: is gradually aging and thinning. The massive stars that are
• 1:08:47
  Speaker 1: still forming today will die within millions of years. The
• 1:08:51
  Speaker 1: Sun-like stars forming now will still be around in 10
• 1:08:55
  Speaker 1: billion years. but the dominant trend is downward. We are
• 1:08:59
  Speaker 1: past the peak, living in the long slow decline of
• 1:09:03
  Speaker 1: universal star formation. The census we're attempting to take is
• 1:09:07
  Speaker 1: a count taken well into the falling side of that arc.
• 1:09:11
  Speaker 1: Every star that was born massive enough to run through
• 1:09:13
  Speaker 1: its fuel quickly has already died, What we're counting when
• 1:09:17
  Speaker 1: we attempt the census is what's left over after 13.8
• 1:09:21
  Speaker 1: billion years of attrition. The phrase number of stars in
• 1:09:25
  Speaker 1: the universe is doing something slightly dishonest every time it
• 1:09:29
  Speaker 1: gets used. Not dishonest in a careless way. The astronomers
• 1:09:33
  Speaker 1: who produce these estimates are serious people doing careful work.
• 1:09:38
  Speaker 1: The dishonesty is built into the language itself, into the
• 1:09:41
  Speaker 1: word universe, which sounds like it means everything. but in
• 1:09:45
  Speaker 1: practice means something considerably smaller. When a scientist says there
• 1:09:49
  Speaker 1: are 200 sextillion stars in the universe, what they mean
• 1:09:53
  Speaker 1: precisely is that there are roughly that many stars in
• 1:09:57
  Speaker 1: the observable universe. Those two things are not the same.
• 1:10:01
  Speaker 1: The difference between them might be the most important qualifier
• 1:10:04
  Speaker 1: in all of astronomy, and it almost never makes it
• 1:10:07
  Speaker 1: into the headline. The observable universe has a hard boundary.
• 1:10:12
  Speaker 1: It is not a wall or an edge in the
• 1:10:14
  Speaker 1: physical sense. Nothing stops at it. Space continues past it
• 1:10:19
  Speaker 1: without interruption. What defines the boundary is time. Light travels fast,
• 1:10:26
  Speaker 1: but it does not travel instantly. The universe has been
• 1:10:29
  Speaker 1: around for about 13.8 billion years. The farthest light that
• 1:10:34
  Speaker 1: could have reached us since the Big Bang defines the
• 1:10:37
  Speaker 1: limit of what we can, in principle, observe. Anything beyond
• 1:10:41
  Speaker 1: that distance has not had enough time to send us
• 1:10:44
  Speaker 1: a signal. Its light is still on the way. It
• 1:10:47
  Speaker 1: hasn't arrived yet, and in many cases, it never will.
• 1:10:51
  Speaker 1: Here is where the numbers get strange. You might expect
• 1:10:55
  Speaker 1: the observable universe to extend about 13.8 billion light years
• 1:11:00
  Speaker 1: in every direction, one light year of radius for every
• 1:11:03
  Speaker 1: year the universe has existed. But the actual figure is larger.
• 1:11:08
  Speaker 1: The cosmic horizon sits at roughly 46 billion light years.
• 1:11:13
  Speaker 1: The reason is expansion. The universe has been stretching the
• 1:11:17
  Speaker 1: whole time light has been traveling through it. A galaxy
• 1:11:21
  Speaker 1: that emitted light toward us 13 billion years ago was
• 1:11:25
  Speaker 1: much closer to us when it sent that light. It
• 1:11:28
  Speaker 1: has since been carried far away by the expansion of space.
• 1:11:32
  Speaker 1: The light still made it to us because it was
• 1:11:35
  Speaker 1: already in transit. but the galaxy that sent it is
• 1:11:38
  Speaker 1: now vastly more distant than 13.8 billion light years. The
• 1:11:44
  Speaker 1: observable universe is a sphere about 93 billion light years across,
• 1:11:49
  Speaker 1: and it is not a fixed region. Its contents are
• 1:11:52
  Speaker 1: moving outward continuously. Beyond 46 billion light years, the universe continues.
• 1:11:59
  Speaker 1: We know this because there is nothing special about our positions.
• 1:12:03
  Speaker 1: The same physics that governs the matter we can see
• 1:12:06
  Speaker 1: should govern whatever lies beyond our horizon. The structures we
• 1:12:10
  Speaker 1: observe near the edge of the observable universe– galaxies, clusters,
• 1:12:15
  Speaker 1: filaments of cosmic web– do not suddenly stop at the boundary.
• 1:12:19
  Speaker 1: They continue. The boundary is ours, not theirs. It is
• 1:12:23
  Speaker 1: a limit on what we can detect, not a limit
• 1:12:26
  Speaker 1: on what exists. What we genuinely do not know is
• 1:12:30
  Speaker 1: how far beyond that boundary the universe extends. The range
• 1:12:34
  Speaker 1: of possibilities is extreme. The universe could be finite, curving
• 1:12:39
  Speaker 1: back on itself like the surface of a sphere, but
• 1:12:42
  Speaker 1: on a scale so much larger than the observable portion
• 1:12:46
  Speaker 1: that the curvature is undetectable. Current measurements of cosmic geometry
• 1:12:51
  Speaker 1: suggest the universe is either flat or very close to flat,
• 1:12:55
  Speaker 1: which means If it is finite, it is very large.
• 1:12:59
  Speaker 1: Some estimates place the full universe at a minimum of
• 1:13:01
  Speaker 1: 250 times the diameter of the observable portion. Others go
• 1:13:06
  Speaker 1: much further. And some serious cosmological models allow for a
• 1:13:11
  Speaker 1: universe that is infinite in extent, with matter and galaxies
• 1:13:15
  Speaker 1: and stars extending without limit in every direction. If the
• 1:13:19
  Speaker 1: universe is infinite, then the number of stars is infinite.
• 1:13:23
  Speaker 1: Not very large. Not beyond counting. infinite. A question about
• 1:13:28
  Speaker 1: total count becomes unanswerable, not because our instruments are insufficient,
• 1:13:33
  Speaker 1: but because the answer is not a finite number. This
• 1:13:36
  Speaker 1: is not speculation dressed up as science. Infinite universes are
• 1:13:41
  Speaker 1: perfectly consistent with current physical theory. We cannot rule it out,
• 1:13:46
  Speaker 1: and we may never be able to. If the universe
• 1:13:49
  Speaker 1: is finite but much larger than the portion we can see,
• 1:13:53
  Speaker 1: then the actual star count is larger than any published
• 1:13:56
  Speaker 1: figure by a corresponding factor. A universe 200 times larger
• 1:14:01
  Speaker 1: in volume than the observable portion contains 200 times as
• 1:14:05
  Speaker 1: many stars. Assuming the cosmos is roughly uniform on large scales,
• 1:14:11
  Speaker 1: which observations suggest it is, the headline figure of 200
• 1:14:15
  Speaker 1: sextillion becomes 40 sextillion trillion, a number that doesn't even
• 1:14:21
  Speaker 1: have a convenient name. The problem is not solvable with
• 1:14:25
  Speaker 1: better telescopes. This is the part that matters. Light from
• 1:14:29
  Speaker 1: beyond the cosmic horizon has not reached us and it
• 1:14:32
  Speaker 1: will not reach us. The universe is expanding. The most
• 1:14:37
  Speaker 1: distant regions are receding from us faster than light can
• 1:14:40
  Speaker 1: cross the growing gap between us and them. This is
• 1:14:43
  Speaker 1: not a temporary technological limitation. It is a fundamental feature
• 1:14:48
  Speaker 1: of how the universe is structured. No instrument no matter
• 1:14:52
  Speaker 1: how sensitive, can gather light that is not arriving. The
• 1:14:56
  Speaker 1: information simply is not here. Every count of stars that
• 1:15:00
  Speaker 1: has been discussed in any documentary, any textbook, any press release,
• 1:15:06
  Speaker 1: sits inside this limitation. The number is always, without exception,
• 1:15:12
  Speaker 1: a count of the observable universe. The universe, the whole thing,
• 1:15:17
  Speaker 1: whatever it is, almost certainly contains more stars than that.
• 1:15:22
  Speaker 1: Probably far more. Possibly infinitely more. This does not make
• 1:15:27
  Speaker 1: the observable universe figure useless. It is still an enormous
• 1:15:31
  Speaker 1: and meaningful number, arrived at through genuinely impressive scientific work.
• 1:15:37
  Speaker 1: But it should be held with an awareness of what
• 1:15:39
  Speaker 1: it is not claiming. It is an inventory of our
• 1:15:42
  Speaker 1: cosmic neighbourhood, drawn up carefully from inside a bubble of
• 1:15:46
  Speaker 1: space that represents an unknown fraction of the full total.
• 1:15:50
  Speaker 1: The honest version of the statement would say, there are
• 1:15:53
  Speaker 1: roughly 200 sextillion stars in the portion of the universe
• 1:15:58
  Speaker 1: we can observe from Earth. And beyond that, we do
• 1:16:02
  Speaker 1: not know. Everything beyond that horizon exists in a region
• 1:16:06
  Speaker 1: where the light simply hasn't had time to reach us.
• 1:16:10
  Speaker 1: And whatever is out there, however many stars are burning
• 1:16:13
  Speaker 1: in those unreachable distances, does not enter any count we
• 1:16:17
  Speaker 1: can make. In 2013, The European Space Agency launched a
• 1:16:22
  Speaker 1: spacecraft with a goal that would have sounded impossible a
• 1:16:25
  Speaker 1: generation earlier. Map the positions of one billion stars with
• 1:16:30
  Speaker 1: enough precision to measure their distances directly. The spacecraft was
• 1:16:35
  Speaker 1: called Gaia. It carried two telescopes, a billion pixel camera array,
• 1:16:41
  Speaker 1: enough precision to detect an angular shift roughly equivalent to
• 1:16:45
  Speaker 1: the width of a human hair seen from across a continent.
• 1:16:48
  Speaker 1: It was placed at a gravitational sweet spot about a
• 1:16:51
  Speaker 1: million miles from Earth, where it could spin slowly and
• 1:16:55
  Speaker 1: scan the full sky repeatedly, building up observations over years.
• 1:17:00
  Speaker 1: The engineering required to do this was extraordinary. Any vibration,
• 1:17:05
  Speaker 1: any thermal expansion of the instrument, any drift in its
• 1:17:08
  Speaker 1: orientation would corrupt the measurements. The team at ESA spent
• 1:17:13
  Speaker 1: years accounting for every possible source of error before the
• 1:17:17
  Speaker 1: first observation was taken. What Gaia actually measures is worth
• 1:17:21
  Speaker 1: slowing down to understand. The most basic output is position,
• 1:17:26
  Speaker 1: where in the sky each star sits, to a precision
• 1:17:30
  Speaker 1: that previous instruments could not approach. But position alone is
• 1:17:34
  Speaker 1: not distance. To get distance, Gaia uses a method called parallax.
• 1:17:39
  Speaker 1: the same geometric trick that lets your brain judge depth
• 1:17:43
  Speaker 1: when you close one eye and then the other. As
• 1:17:46
  Speaker 1: the Earth moves around the Sun over the course of
• 1:17:48
  Speaker 1: a year, nearby stars appear to shift very slightly against
• 1:17:52
  Speaker 1: the backdrop of more distant ones. That shift is tiny,
• 1:17:56
  Speaker 1: for stars even a few hundred light-years away. It amounts
• 1:18:00
  Speaker 1: to fractions of a milli-arc-second, a unit so small it
• 1:18:05
  Speaker 1: has no intuitive equivalent in daily life. Gaia is precise
• 1:18:09
  Speaker 1: enough to measure it anyway, and from that shift, a
• 1:18:12
  Speaker 1: direct distance follows. No assumptions, no models, just geometry. Beyond parallax,
• 1:18:19
  Speaker 1: Gaia tracks proper motion, the slow drift of each star
• 1:18:23
  Speaker 1: across the sky as it moves through the galaxy on
• 1:18:26
  Speaker 1: its own orbit. Stars are not fixed, they travel, and
• 1:18:30
  Speaker 1: over decades, that travel becomes detectable. By measuring where a
• 1:18:34
  Speaker 1: star was at the start of the mission, and where
• 1:18:37
  Speaker 1: it sits years later. Gaia can calculate the star's velocity
• 1:18:41
  Speaker 1: across the plane of the sky. Combine that with radial
• 1:18:44
  Speaker 1: velocity measurements, how fast a star is moving toward or
• 1:18:48
  Speaker 1: away from us. derived from the Doppler shift in its spectrum,
• 1:18:53
  Speaker 1: and you have a three-dimensional velocity vector for each star
• 1:18:56
  Speaker 1: in the catalogue. This turns a static map into something
• 1:19:00
  Speaker 1: closer to a film. Gaia is not just recording where
• 1:19:03
  Speaker 1: things are, it is recording where they are going. The
• 1:19:07
  Speaker 1: spectroscopic data adds another layer, By breaking each star's light
• 1:19:12
  Speaker 1: into its component wavelengths, Gaia can infer chemical composition and
• 1:19:16
  Speaker 1: surface temperature. A star's spectrum is a fingerprint. Certain elements
• 1:19:21
  Speaker 1: absorb light at specific frequencies, leaving dark lines at predictable positions.
• 1:19:27
  Speaker 1: From those lines, astronomers can read off whether the star
• 1:19:31
  Speaker 1: is rich in metals, elements heavier than hydrogen and helium,
• 1:19:35
  Speaker 1: or depleted of them. That tells you roughly when in
• 1:19:38
  Speaker 1: cosmic history it formed. Old stars born in the early
• 1:19:42
  Speaker 1: universe before many heavy elements existed are metal-poor. Younger stars,
• 1:19:48
  Speaker 1: formed from gas already enriched by earlier generations of supernovae,
• 1:19:52
  Speaker 1: carry more of those heavier elements in their spectra. Gaia's
• 1:19:56
  Speaker 1: spectroscopic catalogue is allowing astronomers to age-date stellar populations across
• 1:20:02
  Speaker 1: the galaxy in a way that was not possible before.
• 1:20:05
  Speaker 1: By 2022, Gaia had produced a catalogue containing about 1.7
• 1:20:11
  Speaker 1: billion individual stars, each entry carrying position, proper motion, parallax distance,
• 1:20:18
  Speaker 1: in many cases spectroscopic data. The picture of the Milky
• 1:20:22
  Speaker 1: Way that emerged from this catalogue was sharper than anything
• 1:20:26
  Speaker 1: seen before. Spiral arms that had been inferred from indirect
• 1:20:30
  Speaker 1: evidence now had precise stellar populations attached to them, stellar streams.
• 1:20:36
  Speaker 1: The thin rivers of stars moving together through the galaxy
• 1:20:39
  Speaker 1: turned out to be remnants of dwarf galaxies that the
• 1:20:42
  Speaker 1: Milky Way had pulled apart and absorbed long ago. The
• 1:20:46
  Speaker 1: galaxy's merger history was written in the motions of these streams,
• 1:20:50
  Speaker 1: and Gaia was reading it directly. Some of these disrupted
• 1:20:54
  Speaker 1: structures had been suspected but never cleanly mapped. The catalogue
• 1:20:58
  Speaker 1: confirmed them and found others that nobody had predicted. The
• 1:21:01
  Speaker 1: distribution of stellar ages across different regions of the galaxy
• 1:21:06
  Speaker 1: also came into focus. The central bulge is dominated by
• 1:21:10
  Speaker 1: old stars, metal-rich and densely packed, formed in a burst
• 1:21:14
  Speaker 1: of activity billions of years ago. The disk where our
• 1:21:19
  Speaker 1: sun sits shows a mixture of ages, with younger stars
• 1:21:22
  Speaker 1: concentrated in the spiral arms where gas is still dense
• 1:21:26
  Speaker 1: enough to fuel new formation. The outer halo is a
• 1:21:29
  Speaker 1: different environment entirely. sparse, ancient, dominated by the metal-poor survivors
• 1:21:36
  Speaker 1: of the galaxy's earliest epochs. Gaia gave astronomers detailed census
• 1:21:41
  Speaker 1: data on all of these regions simultaneously. For the first time,
• 1:21:46
  Speaker 1: the structure could be studied not just spatially but temporally.
• 1:21:50
  Speaker 1: Here is the problem that Gaia cannot solve. Its catalogue
• 1:21:54
  Speaker 1: is magnitude limited. There is a brightness threshold below which
• 1:21:58
  Speaker 1: the instrument cannot detect a star reliably. A large fraction
• 1:22:02
  Speaker 1: of the Milky Way's stars sit below it. Red dwarfs,
• 1:22:06
  Speaker 1: the most common type of star in the galaxy, are
• 1:22:09
  Speaker 1: faint enough that Gaia's catalogue is incomplete for all but
• 1:22:12
  Speaker 1: the nearest ones. The catalogue captures the bright, the close
• 1:22:16
  Speaker 1: and the well-characterised with extraordinary precision. The faint, distant, low-mass
• 1:22:22
  Speaker 1: end of the stellar population remains undercounted. This is not
• 1:22:26
  Speaker 1: a flaw in the mission design. It is a hard
• 1:22:28
  Speaker 1: physical limit. you cannot measure what your detectors cannot see.
• 1:22:33
  Speaker 1: What astronomers do instead is use the well-observed portion as
• 1:22:36
  Speaker 1: a calibration sample. Within the volume of space where Gaia's
• 1:22:41
  Speaker 1: coverage is essentially complete, they can measure the true distribution
• 1:22:45
  Speaker 1: of stellar types, including how many red dwarfs exist relative
• 1:22:50
  Speaker 1: to more massive stars. They then extrapolate that measured ratio
• 1:22:54
  Speaker 1: outward into the regions where coverage falls off. It is
• 1:22:58
  Speaker 1: the same statistical logic used everywhere else in this problem.
• 1:23:02
  Speaker 1: You measure what you can and infer the rest. The
• 1:23:05
  Speaker 1: Gaia data has made that measured anchor far more reliable
• 1:23:09
  Speaker 1: than anything that existed before. The extrapolation is still an extrapolation.
• 1:23:15
  Speaker 1: The uncertainty about the full red dwarf population is narrower
• 1:23:18
  Speaker 1: than it was a decade ago. But it has not
• 1:23:21
  Speaker 1: collapsed to zero. Gaia has given astronomers the most detailed
• 1:23:26
  Speaker 1: stellar census ever attempted. What it has revealed about the
• 1:23:30
  Speaker 1: Milky Way has refined the estimate, but the stars it
• 1:23:34
  Speaker 1: cannot see are still the ones the estimate is most
• 1:23:37
  Speaker 1: uncertain about. Ask ten astronomers how many stars exist in
• 1:23:41
  Speaker 1: the observable universe, and you will get answers that span
• 1:23:45
  Speaker 1: more than an order of magnitude. All of them will
• 1:23:48
  Speaker 1: be correct in the sense that they are defensible from
• 1:23:51
  Speaker 1: the available data. That should give you pause. This is
• 1:23:55
  Speaker 1: not a field where the experts are confused about basics.
• 1:23:59
  Speaker 1: These are people who have spent careers building the most
• 1:24:02
  Speaker 1: sophisticated measurement tools in human history, running them at the
• 1:24:06
  Speaker 1: edges of what physics allows. The range of their answers
• 1:24:09
  Speaker 1: covers a factor of 10 or more. The reason that
• 1:24:12
  Speaker 1: range exists is not disagreement about the method. It is
• 1:24:16
  Speaker 1: disagreement about the inputs. And when inputs are uncertain and
• 1:24:20
  Speaker 1: you multiply several of them together, the uncertainty compounds fast.
• 1:24:25
  Speaker 1: Start with the low end. Some researchers, weighting galaxy populations carefully,
• 1:24:31
  Speaker 1: using conservative figures for how many stars a typical galaxy holds.
• 1:24:36
  Speaker 1: Sticking with older assumptions about red dwarf abundance, arrive at
• 1:24:40
  Speaker 1: roughly 70 billion trillion stars. Written out that is a 7,
• 1:24:46
  Speaker 1: followed by 22 zeros. The team that produced this figure
• 1:24:50
  Speaker 1: drawing heavily on luminosity measurements and careful corrections for galaxy diversity.
• 1:24:56
  Speaker 1: Tried to avoid using the Milky Way as a template,
• 1:24:59
  Speaker 1: they sampled across galaxy types, weighted dwarf galaxies more heavily
• 1:25:03
  Speaker 1: than most prior estimates, deliberately held back on red dwarf
• 1:25:07
  Speaker 1: corrections that they felt were not yet supported by enough
• 1:25:11
  Speaker 1: direct evidence. The result is the most conservative serious estimate
• 1:25:15
  Speaker 1: in the literature. 70 billion trillion is Still a number
• 1:25:20
  Speaker 1: that defeats comprehension, but it is sitting at the cautious
• 1:25:25
  Speaker 1: end of the range. Move up from there. The estimate
• 1:25:29
  Speaker 1: most commonly cited in textbooks and popular science articles lands
• 1:25:33
  Speaker 1: around 200 sextillion, a two followed by 23 zeros. This
• 1:25:39
  Speaker 1: is roughly three times the conservative figure. Where does that
• 1:25:42
  Speaker 1: extra factor of three come from? Most of it is
• 1:25:45
  Speaker 1: the galaxy count. The conservative estimate uses something closer to
• 1:25:50
  Speaker 1: 200 billion galaxies, which was the prevailing figure before 2016.
• 1:25:55
  Speaker 1: The 200 sextillion figure tends to use a higher galaxy
• 1:25:58
  Speaker 1: count paired with an average star count per galaxy that
• 1:26:02
  Speaker 1: runs toward the upper range of what the Milky Way
• 1:26:04
  Speaker 1: data supports. If you put 300 billion stars in each
• 1:26:09
  Speaker 1: of 700 billion galaxies, you land somewhere near that number.
• 1:26:13
  Speaker 1: The galaxy count and the per galaxy average are doing
• 1:26:17
  Speaker 1: the heavy lifting. Both of them carry real uncertainty. Then
• 1:26:21
  Speaker 1: there is the upper end. Some estimates push toward one septillion,
• 1:26:26
  Speaker 1: ten times even the 200 sextillion figure. The biggest driver
• 1:26:31
  Speaker 1: of these higher numbers is the red dwarf correction introduced
• 1:26:34
  Speaker 1: by Pieter van Dokkum and his colleagues at Yale in 2010.
• 1:26:38
  Speaker 1: Their study of massive elliptical galaxies suggested that red dwarf
• 1:26:42
  Speaker 1: stars in those systems might be three times more common
• 1:26:46
  Speaker 1: than standard models assumed. Elliptical galaxies are rare, but when
• 1:26:50
  Speaker 1: they appear, they are enormous. A single giant elliptical can
• 1:26:54
  Speaker 1: contain 20 times the stellar mass of the Milky Way.
• 1:26:58
  Speaker 1: If those behemoths are packed with three times as many
• 1:27:01
  Speaker 1: red dwarfs as anyone previously estimated, The contribution to the
• 1:27:06
  Speaker 1: total count from that category of galaxy alone is enormous.
• 1:27:10
  Speaker 1: Combine the 2016 2 Trillium galaxy figure with the Van
• 1:27:15
  Speaker 1: Dockum red dwarf correction applied across the galaxy population. You
• 1:27:20
  Speaker 1: start pushing the total towards the Septillium range. So you
• 1:27:25
  Speaker 1: have three inputs. The galaxy count, the average stars per galaxy,
• 1:27:31
  Speaker 1: the red dwarf multiplier, and none of them is nailed down.
• 1:27:36
  Speaker 1: The galaxy count varies by a factor of 10 depending
• 1:27:40
  Speaker 1: on which study you use and whether you trust the
• 1:27:43
  Speaker 1: 2016 Consulis estimate of 2 trillion or the downward revision
• 1:27:48
  Speaker 1: that came later from New Horizons data suggesting a few
• 1:27:52
  Speaker 1: hundred billion is more defensible. The average star count per
• 1:27:56
  Speaker 1: galaxy varies by a factor of 3 depending on how
• 1:28:00
  Speaker 1: you handle the galaxy mass distribution and what you assume
• 1:28:04
  Speaker 1: about stellar populations in dwarf versus giant systems. The red
• 1:28:09
  Speaker 1: dwarf correction varies by a factor of three depending on
• 1:28:12
  Speaker 1: whether you apply the Van Dokkum finding broadly or restrict
• 1:28:16
  Speaker 1: it only to ellipticals where it was actually measured. If
• 1:28:20
  Speaker 1: you multiply the ranges together, 10 times 3 times 3,
• 1:28:24
  Speaker 1: you get a factor of 90 spread from the most
• 1:28:27
  Speaker 1: conservative inputs to the most aggressive. is why the numbers
• 1:28:32
  Speaker 1: in the literature look so different from each other they
• 1:28:35
  Speaker 1: are not using different physics they are making different judgement
• 1:28:39
  Speaker 1: calls about uncertain quantities, and those judgement calls compound. What
• 1:28:45
  Speaker 1: the estimates agree on is more instructive than where they diverge.
• 1:28:49
  Speaker 1: Every serious figure in the literature accepts that the number
• 1:28:53
  Speaker 1: is at least in the tens of sextillions. Nobody who
• 1:28:57
  Speaker 1: has done the calculation carefully He's arguing for something as
• 1:29:00
  Speaker 1: low as a trillion or as high as a Google.
• 1:29:04
  Speaker 1: The basic framework, count galaxies, estimate stars per galaxy, apply
• 1:29:09
  Speaker 1: population corrections, produces a range that is genuinely constrained at
• 1:29:14
  Speaker 1: both ends. The floor is solid. The ceiling is hazier
• 1:29:17
  Speaker 1: than scientists would like, but it is still a ceiling.
• 1:29:21
  Speaker 1: The uncertainty is large by everyday standards. By the standards
• 1:29:25
  Speaker 1: of measuring a universe from inside it with instruments that
• 1:29:28
  Speaker 1: can only collect the light that happens to arrive here,
• 1:29:32
  Speaker 1: the fact that the range is only a factor of
• 1:29:34
  Speaker 1: 10 is actually a kind of tru- umph. The parts
• 1:29:39
  Speaker 1: of the calculation that are well constrained are the parts
• 1:29:42
  Speaker 1: where direct observation is possible. Gaia has tightened the Milky
• 1:29:47
  Speaker 1: Way's stellar distribution significantly for the stars it can see.
• 1:29:51
  Speaker 1: Hubble and its successors have produced galaxy counts in sampled
• 1:29:55
  Speaker 1: sky regions that are reproducible and internally consistent. The luminosity function,
• 1:30:02
  Speaker 1: the relationship between a galaxy's brightness and its likely stellar population,
• 1:30:06
  Speaker 1: is reasonably well calibrated for large and medium galaxies. These
• 1:30:11
  Speaker 1: anchors hold the estimate in place. What pulls the range
• 1:30:15
  Speaker 1: apart is everything at the edges. The faint end of
• 1:30:18
  Speaker 1: the galaxy population that detection limits cannot reach. The dim
• 1:30:23
  Speaker 1: end of the stellar mass function that surveys systematically under count.
• 1:30:27
  Speaker 1: The question of whether the red dwarf correction measured in
• 1:30:30
  Speaker 1: nearby ellipticals applies to the rest of the universe. Those
• 1:30:35
  Speaker 1: are not small questions. They are the questions the estimate
• 1:30:38
  Speaker 1: is most sensitive to. The spread is not sloppiness. It
• 1:30:43
  Speaker 1: is an honest map of where the knowledge runs out.
• 1:30:46
  Speaker 1: Two hundred sextillion is a number that defeats any attempt
• 1:30:50
  Speaker 1: to feel it. Write it out and it looks like this.
• 1:30:54
  Speaker 1: Two hundred sextillion. That is not a number the human
• 1:30:58
  Speaker 1: brain was built to process. Our intuitions about large quantities
• 1:31:02
  Speaker 1: top out somewhere around a few thousand. Beyond that, everything
• 1:31:07
  Speaker 1: just becomes a lot. A million feels big. A billion
• 1:31:12
  Speaker 1: feels bigger. A trillion is already beyond anything a person
• 1:31:16
  Speaker 1: experiences directly in a lifetime. 200 sextillion is a trillion
• 1:31:21
  Speaker 1: times a trillion times 200. There is no lived experience
• 1:31:26
  Speaker 1: that connects to it. So astronomers and science communicators reach
• 1:31:30
  Speaker 1: for analogies. The most common one involves sand. Take every
• 1:31:35
  Speaker 1: grain of sand on every beach on Earth, not just
• 1:31:38
  Speaker 1: the visible surfaces, but digging down to the depth where
• 1:31:41
  Speaker 1: sand still exists. across every coastline and every desert on
• 1:31:46
  Speaker 1: every continent. Count every single grain. That number, by the
• 1:31:51
  Speaker 1: best geological estimates, sits somewhere around 7.5 quintillion. That is
• 1:31:57
  Speaker 1: a big number. It is also roughly 25,000 times smaller
• 1:32:02
  Speaker 1: than the estimated number of stars in the observable universe.
• 1:32:07
  Speaker 1: The beaches and the deserts are not enough. Not even close.
• 1:32:11
  Speaker 1: Try time instead. If you started counting at one number
• 1:32:15
  Speaker 1: per second, no breaks, no sleep, just counting, reaching 200
• 1:32:20
  Speaker 1: sextillion would take approximately 6.3 trillion years. The universe is
• 1:32:26
  Speaker 1: currently about 13.8 billion years old. You would need to
• 1:32:31
  Speaker 1: count for roughly 450 entire ages of the universe, stacked
• 1:32:36
  Speaker 1: end to end, to finish. Stars would form and die
• 1:32:39
  Speaker 1: in the time it would take you to count the
• 1:32:41
  Speaker 1: ones already accounted for. The exercise is almost meaningless and
• 1:32:45
  Speaker 1: that is the point. The number does not fit inside
• 1:32:49
  Speaker 1: any framework we have for time or space or quantity.
• 1:32:52
  Speaker 1: What the number implies about life is where things become
• 1:32:56
  Speaker 1: genuinely difficult to dismiss. Take 200 sextillion as fixed. Apply
• 1:33:02
  Speaker 1: a very conservative filter. Assume only one in a billion
• 1:33:06
  Speaker 1: stars has a planet orbiting at the right distance for
• 1:33:09
  Speaker 1: liquid water to exist. That is a brutal filter. One
• 1:33:14
  Speaker 1: in a billion. The result is still 200 trillion planets
• 1:33:18
  Speaker 1: sitting in what astronomers call the habitable zone. 200 trillion.
• 1:33:23
  Speaker 1: Now apply another filter. One in a million of those
• 1:33:27
  Speaker 1: planets actually develops any form of life. Single-celled, microbial, anything
• 1:33:32
  Speaker 1: that counts as biological. That gives you 200 million worlds
• 1:33:37
  Speaker 1: with life. Apply one more. One in a million of
• 1:33:40
  Speaker 1: those worlds produces something that could be called intelligent. The
• 1:33:44
  Speaker 1: number that falls out is 200. 200 civilizations, under assumptions
• 1:33:49
  Speaker 1: so conservative they border on hostile to the idea of life.
• 1:33:54
  Speaker 1: Push the filters even slightly towards generosity and the numbers
• 1:33:58
  Speaker 1: climb fast. None of this proves anything. The filters are guesses.
• 1:34:03
  Speaker 1: The assumptions could be wrong in either direction. Life might
• 1:34:07
  Speaker 1: require conditions so specific, that even 200 sextillion stars is
• 1:34:12
  Speaker 1: not enough to produce it more than once. But the
• 1:34:15
  Speaker 1: arithmetic is hard to sit with. The scale of the
• 1:34:18
  Speaker 1: stellar population makes a universe with only one inhabited world
• 1:34:22
  Speaker 1: feel like a very specific and improbable outcome. Now the
• 1:34:27
  Speaker 1: number has a different problem. Most of those 200 sextillion
• 1:34:31
  Speaker 1: stars are not just unvisited. They are unreachable in a
• 1:34:35
  Speaker 1: deeper sense. The universe is expanding. On small scales, the
• 1:34:41
  Speaker 1: scale of galaxies, of galaxy clusters, gravity holds things together
• 1:34:47
  Speaker 1: and the expansion doesn't matter. But at very large scales,
• 1:34:51
  Speaker 1: the space between galaxy clusters is growing. The farther away
• 1:34:55
  Speaker 1: something is, the faster it appears to be receding. At
• 1:35:00
  Speaker 1: a certain distance, the recession speed exceeds the speed of light.
• 1:35:05
  Speaker 1: This is not a violation of physics. No object is
• 1:35:08
  Speaker 1: moving through space faster than light. The space itself is expanding,
• 1:35:13
  Speaker 1: carrying the galaxies with it, and there is no speed
• 1:35:16
  Speaker 1: limit on the expansion of space. The boundary where recession
• 1:35:20
  Speaker 1: speed equals light speed defines what cosmologists call the cosmic horizon.
• 1:35:26
  Speaker 1: Galaxies beyond that horizon are receding faster than any signal
• 1:35:30
  Speaker 1: from us could ever catch them. Light we send toward
• 1:35:33
  Speaker 1: them today will never arrive, and crucially, Light they send
• 1:35:38
  Speaker 1: toward us today will never arrive either. The distance is
• 1:35:42
  Speaker 1: growing faster than the light can cross it. Some of
• 1:35:45
  Speaker 1: the galaxies included in the observable universe estimate sit near
• 1:35:49
  Speaker 1: that boundary. Their light left them billions of years ago,
• 1:35:53
  Speaker 1: when the universe was younger and smaller, and that ancient
• 1:35:56
  Speaker 1: light has been crossing space ever since, finally arriving at
• 1:36:00
  Speaker 1: Earth's telescopes. We see them. We count them. We include
• 1:36:05
  Speaker 1: their stars in the census. But since that light departed,
• 1:36:10
  Speaker 1: those galaxies have been carried outward by expansion, and they
• 1:36:13
  Speaker 1: are now beyond the horizon. We will never receive any
• 1:36:17
  Speaker 1: new information from them. Any signal they send from now
• 1:36:21
  Speaker 1: on will never reach us. This creates a strange situation.
• 1:36:25
  Speaker 1: The observable universe is defined by the light that has
• 1:36:28
  Speaker 1: already arrived, the accumulated record of billions of years of
• 1:36:33
  Speaker 1: photons crossing space. but the sources of some of that
• 1:36:36
  Speaker 1: light are already gone from our future. They exist in
• 1:36:40
  Speaker 1: our past sky. They do not exist in our future one.
• 1:36:44
  Speaker 1: They almost certainly still exist, burning through their fuel on
• 1:36:48
  Speaker 1: schedules that have nothing to do with our ability to
• 1:36:51
  Speaker 1: observe them. But we are done hearing from them. The
• 1:36:55
  Speaker 1: census included them, and then the universe closed the file.
• 1:37:00
  Speaker 1: The 200 sextillion figure is already a kind of fossil record.
• 1:37:04
  Speaker 1: a snapshot built from light of many different ages, some
• 1:37:08
  Speaker 1: of it ancient, travelling from galaxies that have since moved
• 1:37:11
  Speaker 1: past the point of return. When astronomers say the observable
• 1:37:15
  Speaker 1: universe contains this many stars, they mean this many stars
• 1:37:19
  Speaker 1: sent light toward us that has actually arrived. What those
• 1:37:23
  Speaker 1: galaxies look like now, how many of their stars have
• 1:37:26
  Speaker 1: since died or formed, is beyond any measurement we can make.
• 1:37:30
  Speaker 1: The observable universe is not a photograph of the present.
• 1:37:34
  Speaker 1: It is a collage of different pasts, assembled from light
• 1:37:38
  Speaker 1: that has been travelling for varying lengths of time, from
• 1:37:42
  Speaker 1: sources at varying stages of their own histories. Some of
• 1:37:46
  Speaker 1: the galaxies whose starlight we count in our estimates are
• 1:37:49
  Speaker 1: already beyond our future reach, carried past the cosmic horizon
• 1:37:53
  Speaker 1: by expansion, their stars visible only as ancient messengers from
• 1:37:58
  Speaker 1: a universe we can no longer access. After all of
• 1:38:02
  Speaker 1: the counting methods, the galaxy surveys, the infrared corrections and
• 1:38:07
  Speaker 1: the red dwarf adjustments, there is one final truth that
• 1:38:11
  Speaker 1: the numbers do not advertise. We cannot actually see most
• 1:38:15
  Speaker 1: of the stars we are counting, not in any direct sense,
• 1:38:19
  Speaker 1: not in any way that would satisfy someone who asked
• 1:38:22
  Speaker 1: for proof. The headline figure, 200 sextillion stars in the
• 1:38:27
  Speaker 1: observable universe, is built all entirely from inference. The gap
• 1:38:33
  Speaker 1: between what we can detect and what we believe exists
• 1:38:36
  Speaker 1: is not a small gap, it is enormous. Here is
• 1:38:40
  Speaker 1: the actual number. When astronomers perform the full calculation carefully,
• 1:38:45
  Speaker 1: accounting for brightness limits, for distance, for absorption and scattering
• 1:38:49
  Speaker 1: across billions of light years, the number of stars whose
• 1:38:53
  Speaker 1: light we can directly detect with any instrument that currently
• 1:38:56
  Speaker 1: exists or could plausibly exist comes out to roughly 80 quintillion.
• 1:39:01
  Speaker 1: That is about 4% of the estimated total. The other
• 1:39:05
  Speaker 1: 96% are invisible to us. Not invisible in the way
• 1:39:10
  Speaker 1: a star is invisible when clouds cover the sky. Invisible
• 1:39:14
  Speaker 1: in a deeper sense. Too faint. Too remote. to embedded
• 1:39:19
  Speaker 1: in host galaxies too dim to resolve at any individual
• 1:39:22
  Speaker 1: stellar level. We know they are there the way you
• 1:39:25
  Speaker 1: might know a crowd is behind a frosted window. You
• 1:39:29
  Speaker 1: can see the light. You cannot see the faces. Those
• 1:39:33
  Speaker 1: invisible stars are not missing from the count. They are
• 1:39:36
  Speaker 1: inferred into it. When astronomers point a telescope at a
• 1:39:40
  Speaker 1: distant galaxy and measure its total brightness and its color,
• 1:39:44
  Speaker 1: they are not counting individual stars. They are reading the
• 1:39:48
  Speaker 1: collective output of billions of them at once and working
• 1:39:51
  Speaker 1: backward through models of stellar populations to estimate how many
• 1:39:55
  Speaker 1: stars must be inside to produce that signal. The stars
• 1:39:59
  Speaker 1: themselves are completely beyond resolution. What reaches the detector is
• 1:40:04
  Speaker 1: one blended smear of ancient light, and everything said about
• 1:40:08
  Speaker 1: the stars inside that galaxy is a deduction, not an observation.
• 1:40:13
  Speaker 1: This is careful science. It is not guesswork. But it
• 1:40:17
  Speaker 1: is a very different thing from counting. This creates a
• 1:40:21
  Speaker 1: strange situation. The number 200 sextillion carries a kind of
• 1:40:26
  Speaker 1: false precision. It gets stated plainly. It appears in textbooks
• 1:40:31
  Speaker 1: and documentaries without much qualification attached. But the honest version
• 1:40:36
  Speaker 1: of that number comes with a qualifier that rarely makes
• 1:40:39
  Speaker 1: it into the headline. We think. We think there are
• 1:40:43
  Speaker 1: 200 sextillion stars. We think this because the galaxies we
• 1:40:48
  Speaker 1: can observe suggest it, and because the physics of how
• 1:40:51
  Speaker 1: stars form suggests the populations inside those galaxies are distributed
• 1:40:56
  Speaker 1: in particular ways, and because the infrared data and the
• 1:41:00
  Speaker 1: red dwarf corrections and the galaxy count estimates all feed
• 1:41:04
  Speaker 1: into models that produce roughly that figure when you run them.
• 1:41:07
  Speaker 1: The chain of reasoning is solid. The chain is also long,
• 1:41:11
  Speaker 1: and every link introduces uncertainty. now push past the observable
• 1:41:16
  Speaker 1: universe entirely. All of this, the galaxy surveys, the deep
• 1:41:21
  Speaker 1: field images, the infrared corrections, all of it applies only
• 1:41:26
  Speaker 1: to the observable universe. That is the region of space
• 1:41:29
  Speaker 1: from which light has had time to reach us in
• 1:41:32
  Speaker 1: the 13.8 billion years since the Big Bang. Beyond it,
• 1:41:36
  Speaker 1: the universe continues. We do not know how far. We
• 1:41:41
  Speaker 1: have no way of knowing. The physics that describes what
• 1:41:45
  Speaker 1: we can see gives us no special reason to believe
• 1:41:48
  Speaker 1: the observable universe is representative of the whole. It might be.
• 1:41:53
  Speaker 1: It might not be. The boundaries of the observable universe
• 1:41:57
  Speaker 1: are not walls that stop creation. They are simply the
• 1:42:01
  Speaker 1: edges of what light has managed to reach us from
• 1:42:04
  Speaker 1: during the universe's expansion. Beyond them are certainly galaxies and
• 1:42:09
  Speaker 1: stars continuing without our knowledge. The universe did not stop
• 1:42:14
  Speaker 1: creating at the edge of our vision, but the distances
• 1:42:17
  Speaker 1: are so great that no photon from those regions will
• 1:42:20
  Speaker 1: ever touch our instruments. They have been traveling toward us
• 1:42:24
  Speaker 1: for billions of years and still have billions more to
• 1:42:28
  Speaker 1: go if the universe continues expanding as observations suggest. We
• 1:42:33
  Speaker 1: are like sailors able to see to the horizon with
• 1:42:37
  Speaker 1: no way to know what lies beyond. If the universe
• 1:42:40
  Speaker 1: is infinite, and nothing in current physics rules this out,
• 1:42:44
  Speaker 1: then the number of stars is also infinite. Not a
• 1:42:48
  Speaker 1: very large number, infinite. Asking how many stars exist in
• 1:42:52
  Speaker 1: an infinite universe is like asking how many even numbers
• 1:42:56
  Speaker 1: there are. The answer is not on any scale that
• 1:42:59
  Speaker 1: numbers are designed to handle. The question stops being a
• 1:43:03
  Speaker 1: question about astronomy and becomes something else entirely. Even if
• 1:43:07
  Speaker 1: the universe is finite, Its total extent could dwarf the
• 1:43:12
  Speaker 1: observable portion by factors that make the comparison meaningless. The
• 1:43:17
  Speaker 1: observable universe could be one small bubble inside something a
• 1:43:21
  Speaker 1: trillion times larger, or a trillion trillion times larger, and
• 1:43:26
  Speaker 1: there would be no way to tell from inside it.
• 1:43:29
  Speaker 1: Every star counted in every estimate exists within our cosmic horizon.
• 1:43:34
  Speaker 1: Everything beyond that horizon is not dark to us because
• 1:43:37
  Speaker 1: we haven't looked hard enough. It is beyond reach in principle.
• 1:43:42
  Speaker 1: The light from those regions is not on its way.
• 1:43:45
  Speaker 1: It departed long ago from sources that were already beyond
• 1:43:48
  Speaker 1: the distance light could bridge in the time available. So
• 1:43:52
  Speaker 1: what should we do with the 200 sextillion figure? Not
• 1:43:55
  Speaker 1: dismiss it. The number came from real work by serious
• 1:43:59
  Speaker 1: people using the best tools available and it represents something genuine.
• 1:44:04
  Speaker 1: a reckoning with the observable universe that is more rigorous
• 1:44:08
  Speaker 1: than anything attempted before it. But hold it the way
• 1:44:11
  Speaker 1: you would hold any approximation built from incomplete data. It
• 1:44:15
  Speaker 1: is a bound, not a census. It is the count
• 1:44:19
  Speaker 1: from inside a temporary window that the universe has left open,
• 1:44:23
  Speaker 1: and what lies outside the frame is not zero. There
• 1:44:27
  Speaker 1: is something almost clarifying about this. Every star in every
• 1:44:31
  Speaker 1: estimate was counted from a single fixed point. One planet,
• 1:44:36
  Speaker 1: orbiting one star in one galaxy among trillions at one
• 1:44:40
  Speaker 1: particular moment in cosmic time. The estimate is our best
• 1:44:44
  Speaker 1: attempt to extrapolate from that position outward to everything. It
• 1:44:49
  Speaker 1: reaches astonishingly far, and then it stops, at a horizon
• 1:44:53
  Speaker 1: we did not choose, beyond which the universe simply continues
• 1:44:57
  Speaker 1: without us. The number is not wrong. It is bounded,
• 1:45:01
  Speaker 1: and the universe is not. The 206-tillion figure is the
• 1:45:06
  Speaker 1: best reckoning available from inside a universe that has not
• 1:45:09
  Speaker 1: finished revealing itself, and the distance between that number and
• 1:45:13
  Speaker 1: the real answer is something no instrument we could ever
• 1:45:16
  Speaker 1: build would be able to close. The precise number of
• 1:45:20
  Speaker 1: stars in the universe is unknown, and may remain unknown indefinitely,
• 1:45:25
  Speaker 1: and yet the century-long attempt to count them has been
• 1:45:28
  Speaker 1: one of the most productive failures in the history of science.
• 1:45:32
  Speaker 1: Consider what the attempt actually required. To estimate how many
• 1:45:37
  Speaker 1: stars exist, astronomers had to build instruments capable of imaging
• 1:45:41
  Speaker 1: a patch of sky smaller than a pinhead held at
• 1:45:44
  Speaker 1: arm's length, and counting thousands of galaxies inside it. They
• 1:45:49
  Speaker 1: had to develop statistical methods rigorous enough to scale from
• 1:45:53
  Speaker 1: that pinhead to the full celestial sphere. They had to
• 1:45:56
  Speaker 1: understand how dust absorbs light, how galaxies cluster, how dark
• 1:46:01
  Speaker 1: matter shapes the structures that stars form inside of. None
• 1:46:05
  Speaker 1: of that knowledge existed at the beginning of the project.
• 1:46:09
  Speaker 1: Most of it was discovered because the counting problem demanded it.
• 1:46:13
  Speaker 1: The Hubble Deep Field is the clearest example. When astronomers
• 1:46:17
  Speaker 1: pointed the telescope at a region of sky chosen specifically
• 1:46:21
  Speaker 1: because it appeared to contain nothing, and held that exposure
• 1:46:25
  Speaker 1: open for 10 days, they were not expecting to solve
• 1:46:28
  Speaker 1: the star count problem. They were testing a method. What
• 1:46:32
  Speaker 1: they got back was nearly 3,000 galaxies in an area
• 1:46:35
  Speaker 1: of sky that looked completely empty to any previous instrument.
• 1:46:40
  Speaker 1: That single image changed the estimated number of galaxies in
• 1:46:43
  Speaker 1: the observable universe. It also changed how astronomers thought about
• 1:46:48
  Speaker 1: blank sky. There is no blank sky. Point any sufficiently
• 1:46:52
  Speaker 1: sensitive instrument at any direction, wait long enough, and the
• 1:46:56
  Speaker 1: galaxies appear. The attempt to count stars taught us that
• 1:47:00
  Speaker 1: the universe is packed far more densely with structure than anyon.
• 1:47:05
  Speaker 1: E had assumed. Infrared astronomy developed along a parallel track,
• 1:47:10
  Speaker 1: driven by the same problem. Optical telescopes could not see
• 1:47:15
  Speaker 1: through the dust clouds that surrounded many of the most
• 1:47:18
  Speaker 1: active star-forming regions in the early universe. The stars were there.
• 1:47:23
  Speaker 1: The signal just couldn't get through. When the ESA's Herschel
• 1:47:28
  Speaker 1: Observatory mapped the sky in infrared wavelengths, it found populations
• 1:47:32
  Speaker 1: of galaxies churning out new stars at rates that previous
• 1:47:36
  Speaker 1: surveys had completely missed. The early universe, it turned out,
• 1:47:41
  Speaker 1: was doing most of its star forming behind thick curtains
• 1:47:44
  Speaker 1: of dust. That finding pushed estimates of the total historical
• 1:47:49
  Speaker 1: star formation rate significantly upward. It also produced an entirely
• 1:47:54
  Speaker 1: new branch of observational astronomy. The attempt to count stars,
• 1:47:59
  Speaker 1: in other words, built the tools that revealed a universe
• 1:48:02
  Speaker 1: hidden inside the one we thought we already understood. The
• 1:48:06
  Speaker 1: red dwarf problem forced a different kind of reckoning. For
• 1:48:10
  Speaker 1: most of the 20th century, astronomers use sun-like stars as
• 1:48:14
  Speaker 1: their mental template for what a star is. The sun
• 1:48:18
  Speaker 1: is what we know. It seemed reasonable to generalize from it.
• 1:48:22
  Speaker 1: But when researchers began seriously pressing on the question of
• 1:48:26
  Speaker 1: how many stars exist per unit of galaxy mass, the
• 1:48:30
  Speaker 1: template broke down. Red dwarfs, stars with a fraction of
• 1:48:34
  Speaker 1: the sun's mass and a tiny fraction of its brightness,
• 1:48:38
  Speaker 1: turned out to dominate the stellar population by sheer number.
• 1:48:42
  Speaker 1: Roughly 70% of all stars are red dwarfs. They are
• 1:48:47
  Speaker 1: so dim that even nearby ones are invisible to the
• 1:48:50
  Speaker 1: naked eye. Accounting for them properly nearly changes the entire calculation.
• 1:48:56
  Speaker 1: The 2010 study suggesting red dwarfs in elliptical galaxies might
• 1:49:01
  Speaker 1: be three times more common than assumed didn't just adjust
• 1:49:05
  Speaker 1: a footnote, it threatened to triple the total. Astronomers are
• 1:49:09
  Speaker 1: still working through the implications. Galaxy formation theory advanced for
• 1:49:15
  Speaker 1: the same reason. To estimate how many stars exist, you
• 1:49:19
  Speaker 1: need to know how many galaxies exist and how many
• 1:49:22
  Speaker 1: stars each type of galaxy contains. That requires understanding why
• 1:49:27
  Speaker 1: galaxies form the way they do, why some become vast
• 1:49:31
  Speaker 1: spirals and others become compact ellipticals, Why dwarf galaxies outnumber
• 1:49:37
  Speaker 1: large galaxies by such enormous margins? And how dark matter
• 1:49:42
  Speaker 1: halos shape the environments where stars ignite? These are among
• 1:49:46
  Speaker 1: the deepest questions in cosmology, and they were sharpened significantly
• 1:49:51
  Speaker 1: by the practical need to arrive at a defensible star count.
• 1:49:55
  Speaker 1: The census project and fundamental physics fed each other. The
• 1:49:59
  Speaker 1: James Webb Space Telescope has pushed all of this further.
• 1:50:03
  Speaker 1: JWST sees in infrared with a sensitivity and resolution that
• 1:50:08
  Speaker 1: no previous observatory matched, and it has already begun returning
• 1:50:13
  Speaker 1: findings that surprised the astronomers who built it. Massive, well-structured
• 1:50:18
  Speaker 1: galaxies are showing up at redshifts that placed them in
• 1:50:21
  Speaker 1: the very early universe, earlier than the standard models of
• 1:50:25
  Speaker 1: galaxy formation predicted they should exist. These galaxies formed their
• 1:50:31
  Speaker 1: stars fast, faster than the physics was supposed to allow.
• 1:50:35
  Speaker 1: What that means for total star counts is still being
• 1:50:39
  Speaker 1: worked out, but the direction of the revision is familiar.
• 1:50:43
  Speaker 1: More stars. Earlier. In places and at times where the
• 1:50:47
  Speaker 1: model said things should still be forming. Every new telescope
• 1:50:52
  Speaker 1: in the last 30 years has done roughly the same. E-thing.
• 1:50:57
  Speaker 1: It has opened a new window onto the early universe
• 1:51:00
  Speaker 1: and the universe on the other side of that window
• 1:51:03
  Speaker 1: has always turned out to be more populated, more structured
• 1:51:08
  Speaker 1: and more active than the one before. There is something
• 1:51:11
  Speaker 1: worth sitting with in all of this. The number 200
• 1:51:16
  Speaker 1: sextillion or wherever the current estimate lands is not just
• 1:51:20
  Speaker 1: a very large integer. It is an argument. It is
• 1:51:24
  Speaker 1: nearly impossible given that number. to maintain with a straight
• 1:51:28
  Speaker 1: face that the particular arrangement of chemistry that produce life
• 1:51:32
  Speaker 1: on this planet is unique. If even a small fraction
• 1:51:36
  Speaker 1: of those stars host planets, and the evidence increasingly suggests
• 1:51:40
  Speaker 1: that most do, the raw count of potentially habitable worlds
• 1:51:44
  Speaker 1: runs into the hundreds of trillions. The stellar census does
• 1:51:48
  Speaker 1: not prove life exists elsewhere, but it does make the alternative.
• 1:51:53
  Speaker 1: out of 200 sextillion stars and whatever orbits around them,
• 1:51:58
  Speaker 1: life happened exactly once, require a kind of special pleading
• 1:52:03
  Speaker 1: that the numbers make harder and harder to justify. Why?
• 1:52:07
  Speaker 1: The project is not finished. JWST will continue returning data
• 1:52:12
  Speaker 1: on the early universe for years. The Euclid mission, launched
• 1:52:17
  Speaker 1: in 2023, is mapping the large-scale structure of the cosmos
• 1:52:21
  Speaker 1: with a precision designed to answer questions about dark matter
• 1:52:25
  Speaker 1: and dark energy that feed directly into galaxy count estimates.
• 1:52:30
  Speaker 1: The Nancy Grace Roman Space Telescope, when it begins operations,
• 1:52:34
  Speaker 1: will survey the sky in infrared at a scale that
• 1:52:37
  Speaker 1: Hubble could not achieve. Each of these instruments will refine
• 1:52:42
  Speaker 1: some part of the calculation. The galaxy count will be revised,
• 1:52:46
  Speaker 1: The stellar initial mass function will be measured more precisely
• 1:52:50
  Speaker 1: in more galaxy types. The red dwarf question will move
• 1:52:54
  Speaker 1: from estimation towards something closer to measurement. On any clear
• 1:52:59
  Speaker 1: night away from city lights, the same 4,500 or so
• 1:53:04
  Speaker 1: stars that have always been visible are sitting above anyone
• 1:53:08
  Speaker 1: who cares to look up. They are a small and
• 1:53:11
  Speaker 1: highly unrepresentative sample of a population so large that the
• 1:53:16
  Speaker 1: full count requires 23 digits to write out. Most of
• 1:53:20
  Speaker 1: the stars in the observable universe are invisible to the
• 1:53:23
  Speaker 1: naked eye. Most are too dim, too distant, too obscured.
• 1:53:29
  Speaker 1: The ones we see are the bright exceptions, the outliers,
• 1:53:33
  Speaker 1: the stars whose light happens to be strong enough to
• 1:53:36
  Speaker 1: reach an unaided human eye across interstellar distance. The number
• 1:53:41
  Speaker 1: is still an estimate. The uncertainty is still enormous. And
• 1:53:45
  Speaker 1: every clear night, the same stars are rising that have
• 1:53:49
  Speaker 1: always risen, more of them than any mind can hold,
• 1:53:53
  Speaker 1: each one burning without any regard for whether it has
• 1:53:57
  Speaker 1: been counted.