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Behemoth _ Is This the Largest Star in the Universe_ · Boring Science

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Speaker 1: If you place the largest known star where our sun sits,
0:04
Speaker 1: its surface would swallow Jupiter whole and keep going. We're
0:08
Speaker 1: talking about objects so massive, they make our entire solar
0:13
Speaker 1: system look like a marble in a stadium. But here's
0:17
Speaker 1: the problem. We can't agree which one actually holds the title.
0:22
Speaker 1: Right now, there's a star out there called Stevenson 218.
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Speaker 1: Sitting nearly 19,000 light-years away in the constellation Scutum, current
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Speaker 1: measurements put its radius at roughly 2,150 times that of
0:38
Speaker 1: our Sun. That makes it the reigning champion. Except those
0:45
Speaker 1: measurements might be completely wrong. You see, measuring stars this
0:50
Speaker 1: extreme isn't like measuring your height against a wall. These
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Speaker 1: are objects so distant and so large that they constantly
0:58
Speaker 1: pulse and shift, shedding mass like a dog sheds fur
1:03
Speaker 1: in summer. Their atmospheres are so diffuse that scientists can't
1:08
Speaker 1: even agree where one star ends and space begins. And
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Speaker 1: the distances involved. Those numbers come with error margins that
1:17
Speaker 1: could fit entire solar systems inside them. So when you
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Speaker 1: ask if Stevenson 2-18 is truly the largest star in
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Speaker 1: the universe, you're really asking a much more complicated question.
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Speaker 1: You're asking how we measure the unmeasurable, how we know
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Speaker 1: what we think we know, and whether the answer even
1:40
Speaker 1: matters when tomorrow's telescope might rewrite everything. If you find
1:45
Speaker 1: comfort in these gentle explorations of space and science, I'd
1:49
Speaker 1: be grateful if you'd consider subscribing and giving this video
1:53
Speaker 1: a like. Let's start with something familiar to give you
2:02
Speaker 1: a sense of scale. Our sun is big, really big.
2:08
Speaker 1: About 109 Earths could line up across its diameter. You
2:13
Speaker 1: could fit 1.3 million Earths inside its volume. It's a
2:18
Speaker 1: massive ball of nuclear fusion that's been burning for 4.6
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Speaker 1: billion years, and will keep going for another 5 billion
2:26
Speaker 1: or so. Now forget all that, because the Sun is
2:31
Speaker 1: actually tiny. In the cosmic hierarchy of stellar size, our
2:36
Speaker 1: home star barely registers. It's what astronomers call a yellow dwarf,
2:43
Speaker 1: which sounds diminutive because it is. There are stars out
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Speaker 1: there that make the sun look like a grain of
2:50
Speaker 1: sand on a beach. Red supergiants and hypergiants. These are
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Speaker 1: the true monsters of the cosmos. Take VY Canis Majoris,
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Speaker 1: located about 3,900 light-years away in the constellation Canis Major.
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Speaker 1: This red hypergiant is so large that if you replace
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Speaker 1: the sun with it, its surface would extend somewhere between
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Speaker 1: the orbits of Jupiter and Saturn. We're talking about a
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Speaker 1: star that's roughly 1,420 times the sun's radius. You could
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Speaker 1: fit nearly 3 billion suns inside its volume. But VY
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Speaker 1: Canis Majoris isn't even close to claiming the crown anymore.
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Speaker 1: The contest has shifted over the years as our instruments
3:41
Speaker 1: improved and our measurements became more precise. For a while,
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Speaker 1: UY Scuti held the title. This red supergiant in the
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Speaker 1: constellation Scutum was measured at around 1,708 solar radii back
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Speaker 1: when astronomers thought it sat about 9,500 light-years away. calculations
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Speaker 1: were straightforward. Measure the star's angular size, estimate its distance,
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Speaker 1: and do the math. Then the Gaia satellite happened. Gaia
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Speaker 1: is a space observatory launched by the European Space Agency,
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Speaker 1: designed to create the most accurate three-dimensional map of our galaxy.
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Speaker 1: It measures stellar parallax, the apparent shift in a star's position,
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Speaker 1: as Earth orbits the Sun with unprecedented precision. When Gaia
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Speaker 1: turned its instruments toward UY Scuti, the results were startling.
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Speaker 1: The star wasn't nearly as far away as everyone thought.
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Speaker 1: Instead of 9,500 light years, it was closer to 5,900.
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Speaker 1: That might sound like a minor correction, but in astronomy,
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Speaker 1: distance is everything. If a star is closer than you thought,
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Speaker 1: it's smaller than you calculated. UY Scuti's radius estimates crashed
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Speaker 1: from over 1,700 solar radii down to about 755. Just
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Speaker 1: like that, the champion had been dethroned by better data.
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Speaker 1: This is the fundamental challenge with measuring stellar giants. The
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Speaker 1: math seems simple enough, If you know a star's angular
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Speaker 1: diameter and its distance from Earth, you can calculate its
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Speaker 1: actual size. But both of those measurements are maddeningly difficult
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Speaker 1: to nail down with precision. Angular diameter comes from observing
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Speaker 1: how much of the sky a star appears to cover.
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Speaker 1: For most stars, this is an impossibly small measurement. Even
5:56
Speaker 1: the largest stars appear as mere points of light to
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Speaker 1: ground-based telescopes. Astronomers use interferometry, combining light from multiple telescopes
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Speaker 1: to effectively create one enormous virtual telescope, but the technique
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Speaker 1: has limitations. Stellar atmospheres aren't solid surfaces with clear boundaries.
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Speaker 1: They're diffuse clouds of gas that fade gradually into space.
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Speaker 1: Where exactly does the star end? Different wavelengths of light
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Speaker 1: sample different depths in a star's atmosphere. Measure in visible
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Speaker 1: light and you might get one radius. Measure in infrared
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Speaker 1: and you could get something completely different. The star isn't
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Speaker 1: actually changing size between measurements. You're just seeing different layers
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Speaker 1: of its extended atmosphere. Distance measurements face their own problems.
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Speaker 1: Parallax works beautifully for nearby stars. But once you get
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Speaker 1: beyond a few thousand light years, The angles become so
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Speaker 1: small that even Gaia struggles. You're trying to measure something
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Speaker 1: smaller than a human hair viewed from a mile away.
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Speaker 1: The error margins balloon. For Stevenson 218, that distance estimate
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Speaker 1: of 18,900 light years carries enormous uncertainty. The star sits
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Speaker 1: in a region called Stevenson 2, an open cluster of
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Speaker 1: massive stars. Astronomers can estimate the cluster's distance by observing
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Speaker 1: multiple stars and averaging their measurements, but individual stars within
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Speaker 1: the cluster might sit thousands of light-years closer or farther
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Speaker 1: than the average. Compound that distance uncertainty with the challenge
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Speaker 1: of measuring the star's angular size, and you end up
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Speaker 1: with a radius estimate that's really more of a guess
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Speaker 1: dressed up in scientific notation. That figure of 2,150 solar
8:04
Speaker 1: radii was derived from the star's luminosity and temperature using
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Speaker 1: the Stefan-Boltzmann law, a fundamental equation that relates a star's
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Speaker 1: energy output to its surface area and temperature. But here's
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Speaker 1: the catch. That calculation assumed a temperature of 3,200 Kelvin.
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Speaker 1: Change that temperature by just a few hundred degrees. and
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Speaker 1: the radius changes dramatically. And measuring the temperature of a
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Speaker 1: star that's constantly pulsating, shedding mass, and hidden behind clouds
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Speaker 1: of its own ejected material. Good luck getting that number right.
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Speaker 1: The fact is, Stevenson 2-18 might not be the largest
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Speaker 1: star at all. It might not even be close. There's
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Speaker 1: another contender that deserves attention. WO-HG-64, a red supergiant sitting
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Speaker 1: in the Large Magellanic Cloud, a satellite galaxy orbiting the
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Speaker 1: Milky Way. This beast was discovered in the 1970s, and
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Speaker 1: has been fascinating astronomers ever since. Current estimates put its
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Speaker 1: radius around 1,540 solar radii, smaller than Stevenson 2-18 on paper,
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Speaker 1: but with one crucial advantage. WOHG 64 might have the
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Speaker 1: most well-defined radius of any stellar giant we've measured. The
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Speaker 1: star is surrounded by a torus, a donut-shaped ring of
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Speaker 1: dust and gas that it's been shedding for thousands of years.
9:51
Speaker 1: This circumstellar environment creates a kind of cosmic ruler. By
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Speaker 1: studying how light interacts with that dust torus, astronomers can
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Speaker 1: constrain the star's size with more confidence than the usual
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Speaker 1: interferometry measurements allow. But even WOHG64 comes with complications. In 2014,
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Speaker 1: something strange happened. the star appeared to undergo a dramatic transformation,
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Speaker 1: shifting from a red supergiant to what might be a
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Speaker 1: yellow hypergiant. If true, this could signal that WOHG 64
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Speaker 1: is approaching the end of its life, preparing for a
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Speaker 1: supernova that could happen tomorrow or 100,000 years from now.
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Speaker 1: Yellow hypergiants are rare. so rare that we know of
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Speaker 1: fewer than a dozen in the entire observable universe. They
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Speaker 1: represent a brief transition phase in the evolution of the
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Speaker 1: most massive stars, lasting perhaps only a few thousand years.
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Speaker 1: If WOHG 64 has entered this phase, its size might
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Speaker 1: be changing rapidly. The star we measured yesterday might not
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Speaker 1: be the star we're looking at today, This brings us
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Speaker 1: to a fundamental truth about stellar giants. They're not stable.
11:19
Speaker 1: Our sun is remarkably consistent. Its radius varies by less
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Speaker 1: than a thousandth of a percent over its 11-year activity cycle.
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Speaker 1: You could measure it today, measure it again in a decade,
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Speaker 1: and get essentially the same number. Red supergiants and hypergiants
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Speaker 1: don't work that way. These stars pulsate. Their outer layers
11:45
Speaker 1: expand and contract in cycles that can last months or years.
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Speaker 1: UY Scuti, for instance, is classified as a semi-regular variable
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Speaker 1: with a pulsation period of about 740 days. Over the
12:01
Speaker 1: course of roughly two years, its radius swings from around 1,500
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Speaker 1: solar radii to 1,900 solar radii, and back again. Betelgeuse,
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Speaker 1: the bright red star in Orion's shoulder that everyone knows,
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Speaker 1: does the same thing. It's a pulsating variable that cycles
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Speaker 1: through multiple periods of expansion and contraction. When Betelgeuse famously
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Speaker 1: dimmed in late 2019, astronomers scrambled to figure out whether
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Speaker 1: the star was about to explode. Turned out it had
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Speaker 1: just ejected a massive cloud of dust that temporarily blocked
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Speaker 1: our view. This variability makes the question, how big is
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Speaker 1: this star, almost meaningless without adding, when did you measure it?
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Speaker 1: Stevenson 2-18 might be 2,150 solar radii today, but six
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Speaker 1: months from now it could be significantly larger or smaller.
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Speaker 1: We don't have enough observations spanning enough time to map
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Speaker 1: its pulsation cycle, so we're essentially taking a snapshot and
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Speaker 1: assuming it represents the average. The largest stars also shed
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Speaker 1: mass at prodigious rates. VY Canis Majoris loses material 100
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Speaker 1: times faster than Betelgeuse, which itself loses mass much faster
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Speaker 1: than the Sun. During ejection events, VY Canis Majoris can
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Speaker 1: increase its mass loss rate by a factor of 10
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Speaker 1: or more, flinging blobs of material into space that each
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Speaker 1: weigh more than twice the mass of Jupiter. Stevenson 2-18
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Speaker 1: has been estimated to lose about 1.35 times 10 to
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Speaker 1: the minus 5 solar masses per year. That might sound small,
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Speaker 1: but over time, it adds up. The star is literally evaporating,
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Speaker 1: surrounding itself with shells of ejected gas and dust that
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Speaker 1: make it even harder to measure its true photospheric radius.
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Speaker 1: These massive stars exist right at the edge of what
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Speaker 1: physics allows. There's a fundamental limit called the Eddington limit,
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Speaker 1: which represents the point where a star's outward radiation pressure
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Speaker 1: equals its inward gravitational pull. get too close to that limit,
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Speaker 1: and the star becomes unstable. Its own light starts blowing
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Speaker 1: away its outer layers. Hypergiants live dangerously close to this boundary.
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Speaker 1: They're so luminous that they're nearly tearing themselves apart from
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Speaker 1: the inside out. This is why they have such short lifespans,
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Speaker 1: only a few million years compared to the sun's 10 billion.
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Speaker 1: They burn bright and die young, going supernova in spectacular fashion. Actually,
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Speaker 1: that's not quite right. We think they go supernova. Turns out,
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Speaker 1: there's a problem. Astronomers have observed plenty of core collapse supernova,
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Speaker 1: the explosions that mark the deaths of massive stars. When
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Speaker 1: they look back at archival images to see what star
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Speaker 1: was there before the explosion, they find red supergiants. but
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Speaker 1: only small ones. Stars below about 18 solar masses and 100,000
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Speaker 1: solar luminosities. The real monsters, stars like Stevenson 218, VY
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Speaker 1: Canis Majoris, and their ilk, don't seem to explode. Or
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Speaker 1: if they do, we're not seeing it. This is called
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Speaker 1: the Red Supergiant Problem, and it's one of the more
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Speaker 1: frustrating puzzles in modern astrophysics. Some theories suggest the most
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Speaker 1: massive red supergiants don't explode in traditional supernovae at all. Instead,
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Speaker 1: they might evolve backward, shrinking and heating up to become hotter,
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Speaker 1: bluer stars before their final collapse. Others propose that these
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Speaker 1: giants do explode, but they're shrouded in so much dust
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Speaker 1: that we simply can't see the fireworks. Recent observations support
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Speaker 1: that second idea. In 2021, astronomers managed to observe a
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Speaker 1: red supergiant during its last 130 days before going supernova.
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Speaker 1: They detected bright radiation and violent ejections of material, behavior
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Speaker 1: that earlier observations had missed. The star was preparing for
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Speaker 1: death in ways that weren't obvious, until someone looked at
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Speaker 1: the right time with the right instruments. Another study found
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Speaker 1: that a supernova explosion had originated from a red supergiant
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Speaker 1: wrapped in an unexpected blanket of dust. The dust absorbed
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Speaker 1: and scattered the light from the explosion, making it appear
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Speaker 1: much dimmer than it should have been. If this is common,
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Speaker 1: it would explain why we're not seeing the largest red
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Speaker 1: supergiants explode. They're dying right in front of us, but
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Speaker 1: we're missing it because they've hidden themselves behind veils of
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Speaker 1: their own making. All of this matters for one simple reason.
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Speaker 1: If we can't trust our measurements, we can't crown a champion. Stevenson, 2-18,
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Speaker 1: holds the current title with a radius of 2,150 solar radii.
17:46
Speaker 1: But that number is built on assumptions about temperature and
17:49
Speaker 1: distance that might be wrong. UY Scuti was once the
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Speaker 1: largest until better data cut it down to size. VY
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Speaker 1: Canis Majoris has been in and out of the running
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Speaker 1: for years. WOHG 64 might have the best defined measurements,
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Speaker 1: but it's also possibly in the middle of a transformation
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Speaker 1: that's changing its size. And those are just the stars
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Speaker 1: we know about. The Milky Way contains somewhere between 100
18:23
Speaker 1: and 400 billion stars, and we've only measured a tiny
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Speaker 1: fraction of them. There could be a red hypergiant 10
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Speaker 1: times larger than Stevenson 2-18, hiding in some dusty corner
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Speaker 1: of the galaxy, completely unknown because it's too obscured or
18:41
Speaker 1: too distant for our current instruments to detect. Even if
18:45
Speaker 1: we expand our view to the observable universe, with its
18:50
Speaker 1: estimated two trillion galaxies, we've barely scratched the surface. The
18:57
Speaker 1: largest star in the universe might be in a galaxy
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Speaker 1: so far away that its light hasn't reached us yet.
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Speaker 1: or it might have already died, exploding millions of years
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Speaker 1: ago while its final light is still traveling through space.
19:12
Speaker 1: But here's what we do know. We know that stars
19:15
Speaker 1: can grow to truly staggering sizes, dwarfing our sun by
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Speaker 1: factors of thousands. We know these giants are rare, unstable,
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Speaker 1: and short-lived. We know they shape their environments through powerful
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Speaker 1: stellar winds and spectacular deaths. And we know that every
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Speaker 1: time we improve our instruments, we discover something that challenges
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Speaker 1: what we thought we understood. The question isn't really whether
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Speaker 1: Stevenson 218 is the largest star in the universe. It's
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Speaker 1: whether we're even asking the right question. Size is just
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Speaker 1: one measure of stellar extremity, and perhaps not the most
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Speaker 1: interesting one. What matters more is luminosity, the raw power output,
20:06
Speaker 1: and on that scale, Stars like Stevenson 2-18 aren't even
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Speaker 1: in the running. Take R136a1. It's a blue hypergiant in
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Speaker 1: the Large Magellanic Cloud, and it absolutely dwarfs every red
20:21
Speaker 1: supergiant in terms of sheer energy. Its luminosity clocks in
20:26
Speaker 1: at nearly 8.7 million times that of the Sun. That's
20:30
Speaker 1: roughly 20 times brighter than Stevenson 2-18. But its radius?
20:37
Speaker 1: only about 35 solar radii. You could fit dozens of
20:42
Speaker 1: R136A1s inside Stevenson 2-18's photosphere. So which one is more extreme?
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Speaker 1: The bloated red giant that barely produces heat, or the
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Speaker 1: compact blue furnace pumping out radiation at levels that defy comprehension?
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Speaker 1: Temperature throws another wrench into the comparisons. Stevenson 2-18 has
21:06
Speaker 1: a surface temperature around 3,200 Kelvin. That's cooler than a
21:11
Speaker 1: candle flame. R-1361, meanwhile, burns at approximately 53,000 Kelvin, hot
21:19
Speaker 1: enough that most of its energy radiates as ultraviolet light
21:23
Speaker 1: our eyes can't even detect. The difference isn't subtle. It's
21:29
Speaker 1: the difference between a dying ember and a welding torch.
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Speaker 1: Mass complicates things further. Red hypergiants might be enormous in volume,
21:40
Speaker 1: but they're not particularly massive compared to their blue counterparts.
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Speaker 1: Stevenson 2-18 likely started its life with somewhere between 15
21:50
Speaker 1: and 25 solar masses. R136a1. It weighs in at roughly
21:51
Speaker 1: 215 solar masses, nearly 10 times heavier. That mass dictates
22:04
Speaker 1: everything about how a star lives and dies. Here's the
22:08
Speaker 1: uncomfortable truth. When we focus solely on size, we're measuring
22:13
Speaker 1: the least stable, least representative moment in a massive star's
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Speaker 1: life cycle. Red supergiants and hypergiants are swollen, dying stars
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Speaker 1: in their final throws. They've exhausted their core hydrogen, burned
22:30
Speaker 1: through their helium, and they're desperately fusing heavier elements in
22:35
Speaker 1: shells around an inert core. Their outer layers have expanded
22:40
Speaker 1: to ridiculous proportions, not because they're powerful, but because they're
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Speaker 1: losing control. It's like declaring the largest human being whoever
22:50
Speaker 1: holds the most water weight. Technically accurate, but it misses
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Speaker 1: the point of what makes someone physically imposing. The pulsations
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Speaker 1: make it even messier, UY Scuti doesn't maintain a constant size.
23:07
Speaker 1: It's a semi-regular variable, which means it expands and contracts
23:12
Speaker 1: over a period of roughly 740 days. During that cycle,
23:18
Speaker 1: its radius can swing between 1500 and 1900 solar radii.
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Speaker 1: So when someone asks how big UY Scuti is, The
23:30
Speaker 1: answer depends entirely on what day you're asking. Catch it
23:34
Speaker 1: at maximum expansion and you get one number. Catch it
23:38
Speaker 1: during contraction and you get another. Neither is more correct
23:43
Speaker 1: than the other. Betelgeuse does the same thing. Multiple pulsation
23:48
Speaker 1: cycles overlap, some lasting hundreds of days, others spanning years.
23:54
Speaker 1: And they interfere with each other in ways that make
23:57
Speaker 1: the star's behaviour chaotic. and unpredictable. That's why Betelgeuse's famous
24:04
Speaker 1: dimming event in 2019 caught everyone off guard. The star
24:10
Speaker 1: wasn't dying. It was just experiencing an unusually deep minimum
24:15
Speaker 1: in its pulsation cycle, possibly combined with a massive dust
24:19
Speaker 1: ejection event that blocked some of its light. These stars
24:24
Speaker 1: aren't static objects. They're dynamic, unstable systems that are constantly
24:30
Speaker 1: changing size, and any measurement we take is just a
24:34
Speaker 1: snapshot of one particular moment. Distance remains the biggest problem.
24:39
Speaker 1: The farther away a star is, the harder it becomes
24:43
Speaker 1: to measure accurately. Stevenson 2-18 sits roughly 18,900 light years
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Speaker 1: from Earth. That number comes from analysing the star's location,
24:55
Speaker 1: relative to the Stevenson 2 cluster and making assumptions about
25:00
Speaker 1: the cluster's distance. But if that distance estimate is off
25:04
Speaker 1: by even 10%, the star's calculated radius changes dramatically. WOHG
25:12
Speaker 1: 64 faces the opposite challenge. It's in the Large Magellanic Cloud,
25:19
Speaker 1: which is actually helpful for distance measurements, because we have
25:23
Speaker 1: good data on how far away that satellite galaxy is.
25:28
Speaker 1: But WOHG 64 is wrapped in so much dust that
25:33
Speaker 1: measuring its actual photosphere becomes a nightmare. The dust scatters
25:39
Speaker 1: and absorbs light, creating a glowing envelope that's much larger
25:44
Speaker 1: than the star itself. When astronomers measure the apparent size,
25:49
Speaker 1: They're seeing that envelope, not the star's surface. The recent
25:54
Speaker 1: discovery that WOHG64 might be transitioning from a red supergiant
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Speaker 1: to a yellow hypergiant adds another layer of uncertainty. If
26:06
Speaker 1: the star is genuinely changing temperature and luminosity, its radius
26:10
Speaker 1: is changing too. The measurements we have might already be
26:15
Speaker 1: outdated by the time they're published. VY Canis Majoris illustrates
26:20
Speaker 1: the problem perfectly. Early measurements suggested a radius exceeding 2,000
26:26
Speaker 1: solar radii. Then better instruments brought that number down to
26:31
Speaker 1: around 1,420 solar radii. That's a 30% reduction, enough to
26:38
Speaker 1: knock it out of the top spot entirely. The star
26:42
Speaker 1: didn't change. Our ability to measure it did. And that's
26:46
Speaker 1: assuming we're even measuring the right thing. Some stars have
26:51
Speaker 1: such extensive circumstellar envelopes that defining where the star ends
26:56
Speaker 1: becomes philosophical. Is the boundary at the photosphere, where most
27:01
Speaker 1: of the light originates, or is it at the outer
27:05
Speaker 1: edge of the extended atmosphere? where material is still loosely
27:09
Speaker 1: bound to the star but barely denser than the surrounding space.
27:14
Speaker 1: Different research teams use different definitions, and that produces different results.
27:21
Speaker 1: One paper might report a radius of 1,800 solar radii,
27:26
Speaker 1: while another reports 1,200 for the same star. And both
27:32
Speaker 1: could be technically correct depending on what they're measuring. The
27:37
Speaker 1: variability problem intersects with the measurement problem in frustrating ways.
27:43
Speaker 1: When you're observing a star that's 19,000 light years away,
27:47
Speaker 1: you can't just watch it continuously. You get snapshots separated
27:52
Speaker 1: by weeks, months, or years. If the star is pulsating
27:57
Speaker 1: with a period of 740 days, and you only observe
28:01
Speaker 1: it twice, you might catch it at maximum size both times,
28:06
Speaker 1: or minimum size both times, or somewhere in between. Your
28:11
Speaker 1: measurements will be correct for those specific moments, but won't
28:15
Speaker 1: tell you much about the star's average size. This is
28:19
Speaker 1: why infrared observations have become so important. Red supergiants emit
28:25
Speaker 1: most of their energy in the infrared, and they're often
28:28
Speaker 1: surrounded by dust that glows brightly in those wavelengths. Infrared
28:34
Speaker 1: telescopes can peer through the dust and get clearer views
28:38
Speaker 1: of the star itself. But even infrared observations have limitations,
28:45
Speaker 1: especially when you're trying to resolve the surface of a
28:49
Speaker 1: star that appears as a tiny point of light even
28:53
Speaker 1: through our most powerful telescopes. Only a handful of stars
28:57
Speaker 1: are close enough and large enough that we can actually
29:01
Speaker 1: image their surfaces directly. Betelgeuse's one. Observations from instruments like
29:08
Speaker 1: the Very Large Telescope Interferometer have produced actual images showing
29:13
Speaker 1: bright spots and dark regions on Betelgeuse's photosphere, giant convection
29:19
Speaker 1: cells where hot material rises and cool material sinks. Those
29:25
Speaker 1: images are blurry and low resolution by Earth standards, but
29:30
Speaker 1: they're revolutionary for stellar astronomy. For more distant giants like
29:34
Speaker 1: Stevenson 218, we don't have that luxury. We can't see
29:39
Speaker 1: the surface. We can only infer its properties from the
29:43
Speaker 1: total amount of light reaching us, the star's color, and
29:47
Speaker 1: mathematical models of how stellar atmospheres behave. Those models are
29:53
Speaker 1: sophisticated and well-tested, but they're still models. They make assumptions
29:59
Speaker 1: about things like the star's chemical composition, its rotation rate,
30:05
Speaker 1: and the structure of its atmosphere. Change those assumptions even slightly,
30:12
Speaker 1: and you get different results. The chemical composition issue is
30:17
Speaker 1: particularly tricky. Red supergiants have processed a lot of their
30:21
Speaker 1: original hydrogen into heavier elements. and they've mixed some of
30:26
Speaker 1: that processed material up to their surfaces through convection. Different
30:31
Speaker 1: amounts of elements like carbon, nitrogen and oxygen affect the
30:36
Speaker 1: star's opacity. How easily light can escape from the interior.
30:41
Speaker 1: Higher opacity means the star has to expand more to
30:45
Speaker 1: release the same amount of energy. So two stars with
30:49
Speaker 1: identical masses and luminosities could have different radii just because
30:55
Speaker 1: they have different chemical make-ups. Then there's the question of
30:59
Speaker 1: what happens in the final moments. The pre-supernova observations from
31:05
Speaker 1: 2021 showed that red supergiants can undergo violent changes in
31:11
Speaker 1: their last few months or years of life. If a
31:15
Speaker 1: star ejects a huge amount of mass right before exploding,
31:20
Speaker 1: its radius could change dramatically in a very short time.
31:25
Speaker 1: We might observe it at one size, and then it
31:28
Speaker 1: could swell or shrink substantially before we get another measurement.
31:33
Speaker 1: None of this means the measurements are useless. They're not.
31:38
Speaker 1: We've learned an enormous amount about stellar physics from studying
31:42
Speaker 1: these extreme objects, but it does mean that declaring a
31:46
Speaker 1: single largest star requires an asterisk the size of Jupiter.
31:52
Speaker 1: Stevenson 218 is the largest star we currently know about
31:57
Speaker 1: based on our current measurements and our current understanding of
32:01
Speaker 1: its properties. That statement is probably true, but it comes
32:06
Speaker 1: with the implicit understanding that we might be wrong about
32:10
Speaker 1: its distance, its temperature, its variability, or what we're actually
32:15
Speaker 1: measuring when we define its radius. U.Y. Scooty held the
32:21
Speaker 1: title before Stevenson 2-18, and it might reclaim it if
32:26
Speaker 1: future observations revise the numbers again. WOHG 64 might turn
32:33
Speaker 1: out to be larger than both of them once we
32:36
Speaker 1: account for its recent transformation, or we might discover something
32:40
Speaker 1: completely new that makes all of them look small by comparison.
32:45
Speaker 1: The galaxy is vast, and we've barely scratched it. Red
32:50
Speaker 1: hypergiants are rare, maybe a few dozen in the entire
32:54
Speaker 1: Milky Way. But that's still plenty of room for undiscovered
32:58
Speaker 1: giants hiding in dusty, star-forming regions we haven't surveyed yet.
33:04
Speaker 1: The infrared satellites like WISE and SPITZER have found thousands
33:09
Speaker 1: of candidates that haven't been studied in detail. Some of
33:13
Speaker 1: them could be larger than anything we currently know about.
33:17
Speaker 1: And that's just our galaxy. WOH G64 proves that other
33:23
Speaker 1: galaxies have their own populations of extreme stars. The Andromeda Galaxy,
33:30
Speaker 1: the Triangulum Galaxy, the dozens of dwarf galaxies orbiting the
33:34
Speaker 1: Milky Way, each one potentially hosts red hypergiants we haven't
33:39
Speaker 1: identified yet. What's clear is that size alone doesn't capture
33:44
Speaker 1: what makes these stars extraordinary. Their luminosities are staggering. Their
33:49
Speaker 1: mass loss rates are extreme, their evolutionary timescales are incredibly short,
33:57
Speaker 1: and their deaths are among the most energetic events in
34:00
Speaker 1: the universe. Focusing only on radius means missing the bigger picture.
34:07
Speaker 1: These stars represent something more than astronomical curiosities. They're laboratories
34:14
Speaker 1: for extreme physics. When you compress the entire periodic table
34:20
Speaker 1: into a sphere thousands of times the diameter of the Sun,
34:24
Speaker 1: then watch it hemorrhage mass at rates that would evaporate
34:29
Speaker 1: Earth in centuries, you're witnessing processes that push against the
34:34
Speaker 1: known limits of matter and energy. The Eddington limit defines
34:40
Speaker 1: one such boundary. It's the point where radiation pressure equals
34:44
Speaker 1: gravitational force. For most stars, this balance sits comfortably in
34:50
Speaker 1: the background, but hypergiants dance right on that edge. Picture
34:56
Speaker 1: it this way. The core of a red hypergiant generates
35:00
Speaker 1: so much energy that the photons streaming outward exert physical
35:05
Speaker 1: pressure on the surrounding material. At a certain luminosity, that
35:11
Speaker 1: outward push matches the inward pull of gravity. cross that
35:16
Speaker 1: threshold and the star starts tearing itself apart from the
35:20
Speaker 1: inside out. Not catastrophically, at least not immediately, but steadily, relentlessly.
35:30
Speaker 1: The photosphere lifts off in layers like an onion shedding
35:34
Speaker 1: its skin. This is why mass loss rates matter so much.
35:39
Speaker 1: Stevenson 2-18 sheds material at roughly 1.35 times 10 to
35:45
Speaker 1: the negative fifth solar masses per year. That number looks
35:50
Speaker 1: abstract until you convert it. The star is ejecting about
35:55
Speaker 1: 25 Earth's worth of material annually. VY Canis Majoris operates
36:02
Speaker 1: at even higher rates during its most violent episodes, blowing
36:07
Speaker 1: off Jupiter mass chunks in single outbursts. Where does all
36:12
Speaker 1: that material go? It forms nebulae. complex, asymmetric structures that
36:19
Speaker 1: extend for light-years around the parent star. VY Canis Majoris
36:24
Speaker 1: sits inside a reflection nebula containing several tenths of a
36:28
Speaker 1: solar mass. The inner shell spans 140 astronomical units. The
36:35
Speaker 1: outer reaches stretch to 12,000 astronomical units. For perspective, Voyager
36:42
Speaker 1: 1 has travelled about 160 astronomical units from the Sun
36:47
Speaker 1: in over four decades. It would take 3,000 years to
36:51
Speaker 1: traverse VYCMA's nebula at that speed. These nebulae aren't uniform clouds.
36:58
Speaker 1: They're intricate tangles of knots, arcs and filaments, each one
37:02
Speaker 1: a fossil record of past eruptions. Some regions glow bright
37:07
Speaker 1: in infrared. Others appear dark. obscured by dense concentrations of dust.
37:13
Speaker 1: Spectroscopic analysis reveals complex chemistry, molecules you'd never expect to
37:19
Speaker 1: find in the vacuum of space, water, silicon monoxide, titanium oxide,
37:25
Speaker 1: all forged in the dying breaths of a star that's
37:28
Speaker 1: literally boiling itself away. WO-HG64's dust envelope spans roughly one
37:36
Speaker 1: light year in diameter. It contains between three and nine
37:40
Speaker 1: solar masses of expelled material. Think about that. The star
37:45
Speaker 1: has thrown off more mass than our entire solar system
37:48
Speaker 1: contains in rocky planets, and that ejected material now forms
37:53
Speaker 1: a cocoon so thick that the star itself becomes difficult
37:57
Speaker 1: to observe at optical wavelengths. This is where the yellow
38:01
Speaker 1: hypergiant transformation becomes fascinating. In 2014, Observations suggested WOHG64 had
38:12
Speaker 1: changed colour, not over millions of years, over a human lifetime.
38:18
Speaker 1: The spectrum shifted towards shorter wavelengths, indicating a hotter surface temperature.
38:25
Speaker 1: Stars aren't supposed to do that. Red supergiants should cool
38:29
Speaker 1: as they age, not heat up. But the models allow
38:33
Speaker 1: for it under specific circumstances. If a red supergiant sheds
38:39
Speaker 1: enough mass fast enough, it can expose hotter interior layers.
38:44
Speaker 1: The outer envelope thins. The effective temperature rises. The star
38:50
Speaker 1: moves blueward on the HR diagram, becoming a yellow hypergiant before, potentially,
38:57
Speaker 1: evolving into a luminous blue variable, or even a Wolf-Rayet star.
39:03
Speaker 1: This isn't just theoretical speculation. Rho Cassiopeiae, another yellow hypergiant,
39:10
Speaker 1: went through something similar in 2000. It dimmed dramatically, ejected
39:16
Speaker 1: a shell of material, then recovered. The star had blown
39:21
Speaker 1: off part of its outer layers in a titanic outburst.
39:26
Speaker 1: When the dust settled, literally, the spectrum showed it had
39:31
Speaker 1: changed temperature and composition. Yellow hypergiants occupy an unstable region
39:37
Speaker 1: of parameter space. They're too hot to have the deep
39:40
Speaker 1: convective envelopes that characterize red supergiants, but too cool to
39:45
Speaker 1: have the radiatively driven winds of blue supergiants. This makes
39:49
Speaker 1: them prone to violent instabilities. Their atmospheres pulsate. Their mass
39:55
Speaker 1: loss fluctuates wildly. Some undergo periodic eruptions that would make
40:01
Speaker 1: solar flares look like candle flickers. Only about a dozen
40:06
Speaker 1: confirmed yellow hypergiants exist in our galaxy. They represent a
40:11
Speaker 1: brief transitional phase. maybe just a few thousand years out
40:16
Speaker 1: of a star's multi-million year lifetime. Catching one in the
40:22
Speaker 1: act of transformation is like witnessing lightning strike. It happens fast,
40:28
Speaker 1: and you need to be looking in exactly the right
40:31
Speaker 1: place at exactly the right time. The implications for WOHG64
40:37
Speaker 1: are stark. If it has transitioned from red to yellow,
40:41
Speaker 1: It's entering the final chapter before core collapse. Yellow hypergiants
40:47
Speaker 1: don't last. They either explode as supernovae, collapse directly into
40:52
Speaker 1: black holes, or undergo some other catastrophic endpoint we haven't
40:57
Speaker 1: fully characterized yet. This brings us to the red supergiant problem.
41:03
Speaker 1: Theory predicts that most core collapse supernova should come from
41:07
Speaker 1: red supergiants. They're the natural evolutionary endpoint for massive stars.
41:14
Speaker 1: But when astronomers catalogue supernova progenitors, stars identified in pre-explosion images,
41:21
Speaker 1: they find a puzzling gap. Red supergiants explode, sure, but
41:26
Speaker 1: only the smaller ones, below about 18 solar masses and 100,000
41:32
Speaker 1: solar luminosities. The true behemoths, the VY Canis Majorices, The
41:38
Speaker 1: Stevenson 218s, the massive red hypergiants pushing 40 solar masses,
41:45
Speaker 1: we almost never see them explode, at least not in
41:49
Speaker 1: ways we recognize as standard supernovae. Several explanations have been proposed.
41:56
Speaker 1: Maybe the most massive red supergiants don't explode at all.
42:01
Speaker 1: Maybe they collapse directly into black holes, swallowing their own
42:05
Speaker 1: light before it can escape. Or maybe they do explode,
42:10
Speaker 1: but the explosions are obscured by the dense, circumstellar material
42:15
Speaker 1: they've spent millennia ejecting. The 2021 pre-supernova observations support the
42:21
Speaker 1: second scenario. Astronomers watched a red supergiant for 130 days
42:27
Speaker 1: before it detonated. During that time, the star showed violent activity,
42:34
Speaker 1: bright emissions, mass ejections, Signs of internal upheaval. Then it exploded.
42:42
Speaker 1: But here's the critical detail. When they went back and
42:46
Speaker 1: looked at archival images, they found the star had been
42:50
Speaker 1: surrounded by dense shells of previously ejected material. When the
42:55
Speaker 1: supernova shockwave hit those shells, it lit them up. The
43:00
Speaker 1: explosion was there. It just looked different because the environment
43:05
Speaker 1: around the star was so cluttered with debris. Another recent
43:10
Speaker 1: discovery reinforced this. A supernova that initially seemed to lack
43:15
Speaker 1: a progenitor turned out to have originated from a red
43:19
Speaker 1: supergiant wrapped in thick dust clouds. The dust absorbed and
43:24
Speaker 1: scattered the light, making the explosion harder to detect at
43:28
Speaker 1: optical wavelengths. But in infrared, where dust glows brightly, the
43:34
Speaker 1: supernova blazed clear. This might solve the red supergiant problem.
43:40
Speaker 1: The massive ones do explode. We're just missing them because
43:44
Speaker 1: we've been looking in the wrong wavelengths and not accounting
43:47
Speaker 1: for how much junk these stars dump into their surroundings
43:51
Speaker 1: before they die. But it raises another question. If red
43:56
Speaker 1: hypergiants are ejecting solar masses of material in their final millennia,
44:02
Speaker 1: what triggers those ejections? What causes a star to suddenly
44:07
Speaker 1: decide it needs to shed a Jupiter's worth of mass
44:11
Speaker 1: in a single outburst? The answer probably involves instabilities deep
44:16
Speaker 1: in the stellar interior. As the core burns through successive
44:20
Speaker 1: layers of fusion, hydrogen, helium, carbon, neons, Oxygen. Silicon. The
44:28
Speaker 1: energy generation rate fluctuates. Sometimes it surges. The outer layers
44:34
Speaker 1: can't respond fast enough, and they puff outward. Other times,
44:39
Speaker 1: energy production drops. The outer layers contract. These oscillations aren't gentle.
44:47
Speaker 1: They're violent, convulsive events that ripple through hundreds of solar
44:51
Speaker 1: radii of stellar material. Add in rotation, magnetic fields, and
44:57
Speaker 1: the complex interplay between radiation pressure and opacity, and you
45:02
Speaker 1: get a star that's fundamentally unstable. Not on the edge
45:07
Speaker 1: of exploding, not yet, but constantly adjusting, pulsating, throwing off
45:13
Speaker 1: mass in irregular bursts that make it nearly impossible to
45:17
Speaker 1: pin down a single size at any given moment. This
45:21
Speaker 1: is why VY Canis Majoris, UY Scooty, and Betelgeuse all
45:26
Speaker 1: show variations in their measured radii, depending on when and
45:30
Speaker 1: how you observe them. They're not lying still for the portrait.
45:35
Speaker 1: They're breathing, swelling, and contracting over timescales of months to years.
45:41
Speaker 1: UY Scuti has a pulsation period of roughly 740 days.
45:47
Speaker 1: During that cycle, its radius changes by hundreds of solar radii.
45:52
Speaker 1: Betelgeuse undergoes multiple overlapping cycles, short-period pulsations, long-period variations, and
46:01
Speaker 1: occasional dramatic dimming events, like the one in 2019 that
46:06
Speaker 1: sparked speculation it might explode. It didn't, obviously. The dimming
46:12
Speaker 1: turned out to be caused by a massive dust cloud
46:15
Speaker 1: the star had ejected, which temporarily blocked our view of
46:19
Speaker 1: part of the photosphere. But the incident highlighted just how
46:23
Speaker 1: dynamic these objects are. They're not static spheres sitting peacefully
46:28
Speaker 1: in space. They're roiling cauldrons of plasma, constantly churning, ejecting, pulsating.
46:36
Speaker 1: Measuring their size requires choosing a specific definition of where
46:40
Speaker 1: the star ends. Is it the photosphere, the visible surface,
46:46
Speaker 1: the point where optical depth reaches a certain value? the
46:50
Speaker 1: radius that contains a specific percentage of the total mass.
46:55
Speaker 1: Different definitions give different answers, sometimes varying by hundreds of
47:00
Speaker 1: solar radii. This ambiguity doesn't make the measurements meaningless. It
47:06
Speaker 1: just means we need to be precise about what we're
47:10
Speaker 1: measuring and acknowledge the inherent fuzziness in trying to draw
47:14
Speaker 1: hard boundaries around objects that don't have them. The extreme
47:18
Speaker 1: luminosities add another layer of complexity. Stevenson 2-18 radiates 436,516
47:28
Speaker 1: times the Sun's luminosity. That's not a typo, nearly half
47:33
Speaker 1: a million solar luminosities pouring out of a single star.
47:39
Speaker 1: At that output level, the radiation field becomes dense enough
47:43
Speaker 1: to exert significant physical effects on the surrounding material. Photons
47:49
Speaker 1: carry momentum. When you have that many photons streaming outward,
47:55
Speaker 1: they push. This is radiation pressure, and it's what drives
47:59
Speaker 1: stellar winds in hot, massive stars. For red hypergiants, the
48:05
Speaker 1: mechanism is slightly different. The cooler temperatures allow dust grains
48:10
Speaker 1: to form in the outer atmosphere. Those dust grains are
48:14
Speaker 1: excellent at absorbing radiation. The photons hit the dust, transfer
48:20
Speaker 1: momentum and accelerate the grains outward. The dust drags the
48:25
Speaker 1: surrounding gas along with it, creating a wind. The efficiency
48:30
Speaker 1: of this process depends critically on the dust composition and
48:34
Speaker 1: grain size. Get the conditions right and you can drive
48:39
Speaker 1: winds with velocities of tens of kilometres per second. carrying
48:44
Speaker 1: away significant fractions of the solar mass per million years.
48:49
Speaker 1: Get them wrong, and the wind stalls. The dust evaporates
48:54
Speaker 1: before it can escape. The mass loss drops. This is
48:59
Speaker 1: another source of variability. Changes in the stellar temperature or
49:04
Speaker 1: density can affect dust formation, which affects mass loss, which
49:09
Speaker 1: affects the circumstellar environment, which affects what we observe. It's
49:15
Speaker 1: a coupled system where everything influences everything else. and small
49:20
Speaker 1: perturbations can cascade into large observable changes. Now imagine trying
49:27
Speaker 1: to model all of this. You need to track nuclear
49:30
Speaker 1: reactions in the core, energy transport through radiative and convective zones,
49:37
Speaker 1: pulsational dynamics in the envelope, dust formation in the outer atmosphere,
49:43
Speaker 1: radiative transfer through optically thick dust shells, and the interaction
49:49
Speaker 1: between stellar winds and the surrounding interstellar medium. Each component
49:55
Speaker 1: requires sophisticated physics, and they all have to work together consistently.
50:01
Speaker 1: Current models can reproduce some aspects of red hypergiant behaviour,
50:07
Speaker 1: They predict roughly the right luminosities, mass loss rates, and
50:12
Speaker 1: evolutionary timescales. But they struggle with the details, the irregular variability,
50:19
Speaker 1: the asymmetric nebulae, the occasional dramatic outbursts. These phenomena emerge
50:26
Speaker 1: from processes we don't fully understand yet, which means that
50:30
Speaker 1: when we look at Stevenson 2-18 and declare it the
50:34
Speaker 1: largest known star, We're making that claim based on incomplete
50:38
Speaker 1: information processed through imperfect models. It's probably correct. The measurements
50:45
Speaker 1: are solid enough. But there's always the possibility that we're
50:49
Speaker 1: missing something fundamental about how these objects work. And that's
50:55
Speaker 1: the reality we're dealing with. The largest star in the
50:59
Speaker 1: universe might not stay the largest. It might shrink next
51:04
Speaker 1: year when better data arrives. It might pulsate to twice
51:09
Speaker 1: its current size next month. It might have already exploded,
51:15
Speaker 1: and we won't know for another 18,000 years. Which brings
51:19
Speaker 1: us to something you've probably been wondering. If measuring these
51:23
Speaker 1: objects is so difficult and uncertain, why do we even bother?
51:28
Speaker 1: What's the point of chasing after numbers that might be wrong?
51:32
Speaker 1: Here's what makes it worth the effort. These stars aren't
51:36
Speaker 1: just cosmic curiosities. They're laboratories. Natural experiments running at scales
51:43
Speaker 1: we could never hope to replicate. The physics happening inside Stevenson,
51:48
Speaker 1: 218 right now, is pushing against theoretical boundaries we've spent
51:54
Speaker 1: decades establishing. Consider the Eddington Limit. It's the point where
52:00
Speaker 1: a star's radiation pressure equals its gravitational pull. Push past
52:06
Speaker 1: that limit, and the star should tear itself apart. The
52:10
Speaker 1: outward force of light becomes stronger than the inward pull
52:15
Speaker 1: of gravity. Game over. Stevenson 2-18 is dancing right on
52:21
Speaker 1: that edge. Its luminosity puts it perilously close to the
52:25
Speaker 1: Eddington limit. How close depends on its mass. which we
52:29
Speaker 1: don't know precisely, but we can make educated guesses based
52:33
Speaker 1: on evolutionary models. Those models suggest it started life with
52:38
Speaker 1: 30 to 40 solar masses. It's shed some of that
52:43
Speaker 1: through stellar winds. Current estimates put it somewhere around 20
52:48
Speaker 1: solar masses. Run the numbers with those values, and you
52:52
Speaker 1: get a star that's operating at 80 to 90% of
52:54
Speaker 1: its Eddington limit. That's uncomfortably high. It means the radiation
53:02
Speaker 1: field is strong enough to accelerate particles outward at significant
53:07
Speaker 1: fractions of the escape velocity. Small perturbations in temperature or
53:12
Speaker 1: density could tip the balance. The star could enter a
53:16
Speaker 1: phase of enhanced mass loss, losing substantial amounts of material
53:21
Speaker 1: in a relatively short time. This actually happens. We've observed
53:27
Speaker 1: it in other hypergiants. Eta Carinae famously ejected several solar
53:32
Speaker 1: masses of material in the 1840s during what's called the
53:36
Speaker 1: Great Eruption. The star dimmed, then brightened, then dimmed again,
53:42
Speaker 1: all while throwing off mass at rates that would empty
53:46
Speaker 1: the sun in a few thousand years. It created a
53:50
Speaker 1: spectacular bipolar nebula that's still expanding today. VY Canis Majoris
53:57
Speaker 1: shows similar behaviour, though on a somewhat smaller scale. High
54:02
Speaker 1: resolution imaging reveals an asymmetric nebula surrounding the star, composed
54:08
Speaker 1: of discrete knots and filaments of ejected material. Some of
54:13
Speaker 1: these knots contain more mass than Jupiter. They were expelled
54:17
Speaker 1: in discrete events, possibly triggered by pulsational instabilities. or convective
54:23
Speaker 1: upwellings that brought extra energy to the surface. Stevenson 2-18
54:29
Speaker 1: likely experiences similar episodes. We just haven't watched it long
54:34
Speaker 1: enough to catch one in action. The star's mass loss
54:38
Speaker 1: rate is estimated at around 1.35 times 10 to the
54:44
Speaker 1: minus 5 solar masses per year. That's based on infrared
54:49
Speaker 1: excess and MESA emission. from the surrounding material. Those masas
54:55
Speaker 1: are particularly interesting. They form in the dense molecular gas
55:00
Speaker 1: being swept away by the stellar wind, and their properties
55:04
Speaker 1: tell us about the velocity, density, and temperature structure of
55:10
Speaker 1: that wind. The fact that Stevenson 2-18 shows strong master
55:14
Speaker 1: emissions suggests it's currently in a phase of active mass loss.
55:19
Speaker 1: The wind is thick enough and structured enough to create
55:22
Speaker 1: the conditions masses need. This might be a temporary state.
55:27
Speaker 1: The mass loss could drop in the future as the
55:30
Speaker 1: star's properties evolve. Or it could increase if the star
55:35
Speaker 1: enters an eruptive phase. None of this is captured in
55:40
Speaker 1: simple radius measurements. When we say Stevenson 2-18, has a
55:46
Speaker 1: radius of 2,150 solar radii. We're summarizing an enormously complex
55:53
Speaker 1: object with a single number. It's like describing a hurricane
55:58
Speaker 1: with its maximum wind speed. Technically accurate, but missing most
56:03
Speaker 1: of the interesting details. The real story is in how
56:07
Speaker 1: that radius changes over time, how it responds to internal dynamics,
56:13
Speaker 1: how it couples to the surrounding environment. Red hypergiants are
56:18
Speaker 1: intrinsically unstable. Their envelopes are barely bound. Pulsations can temporarily
56:25
Speaker 1: lift material well beyond the nominal photosphere, creating an extended
56:30
Speaker 1: pseudo-photosphere that fades back down when the pulsation cycle reverses.
56:36
Speaker 1: We see this behaviour in other well-studied red supergiants. Betelgeuse,
56:42
Speaker 1: for instance, goes through regular pulsation cycles that change its
56:46
Speaker 1: radius by several percent. It also experiences longer-term variations that
56:53
Speaker 1: can alter its brightness by more than a magnitude. The
56:58
Speaker 1: famous dimming event in 2019 to 2020 was likely caused
57:03
Speaker 1: by a localized cooling of the photosphere combined with dust formation,
57:08
Speaker 1: but the exact mechanism is still debated. Stevenson 2-18 is
57:15
Speaker 1: probably doing all of this and more. We just don't
57:19
Speaker 1: have the observational coverage to see it clearly. The star
57:23
Speaker 1: is faint, distant, and heavily obscured by dust. Getting high-quality
57:29
Speaker 1: photometry is challenging. Getting spectroscopy is harder. Getting interferometric measurements
57:36
Speaker 1: to directly resolve the stellar disk is currently impossible with
57:40
Speaker 1: existing facilities. So we work with what we have. We
57:45
Speaker 1: combine different types of observations, each with its own uncertainties
57:51
Speaker 1: and systematics. We build models that attempt to capture the
57:56
Speaker 1: essential physics while making simplifying assumptions about the details. We
58:02
Speaker 1: compare predictions to observations and adjust parameters until things roughly match.
58:10
Speaker 1: It's an iterative process that gradually refines our understanding without
58:16
Speaker 1: ever reaching perfect certainty. This is normal in astrophysics. We're
58:21
Speaker 1: trying to understand objects that are fundamentally inaccessible to direct study.
58:27
Speaker 1: We can't send probes. We can't take samples. We can
58:32
Speaker 1: only collect the light they send us and try to
58:35
Speaker 1: decode the information it carries. For nearby stars, we have
58:40
Speaker 1: enough photons to work with that we can extract detailed information.
58:46
Speaker 1: For distant hypergiants, every photon is precious, and we have
58:51
Speaker 1: to squeeze as much science out of them as possible.
58:55
Speaker 1: The payoff comes when we discover something unexpected. When observations
59:01
Speaker 1: don't match predictions. When a star does something it's not
59:05
Speaker 1: supposed to do according to our models. that's when we
59:09
Speaker 1: learn the most. Because it tells us our understanding is incomplete.
59:14
Speaker 1: There's new physics to uncover. WOHG64 recently provided exactly this
59:22
Speaker 1: kind of surprise. Astronomers observing it in 2014 noticed something odd.
59:28
Speaker 1: The star appeared to have changed colour, not dramatically, but
59:32
Speaker 1: enough to be statistically significant. Follow-up observations confirmed it. WOHG64
59:40
Speaker 1: had transitioned from a red supergiant to a yellow hypergiant.
59:44
Speaker 1: This is not supposed to happen. Or rather, it's supposed
59:48
Speaker 1: to happen, but over timescales of thousands of years, not decades.
59:54
Speaker 1: Red supergiants can evolve back toward hotter temperatures as they
59:58
Speaker 1: burn through their nuclear fuel and adjust their internal structure,
1:00:03
Speaker 1: but seeing it happen in real time was unprecedented. It
1:00:08
Speaker 1: suggested that either WOHG64 was going through an unusually rapid
1:00:15
Speaker 1: evolutionary phase, or our timescales were wrong. The implications are significant.
1:00:22
Speaker 1: If massive stars can transition between spectral types on human timescales,
1:00:27
Speaker 1: it means they're more dynamic than we thought. It means
1:00:31
Speaker 1: the neat evolutionary sequences we've constructed might be oversimplified. Real
1:00:38
Speaker 1: stars don't necessarily follow smooth tracks through temperature luminosity space.
1:00:44
Speaker 1: They can jump around, responding to internal instabilities or mass
1:00:49
Speaker 1: loss events that temporarily alter their surface properties. This matters
1:00:55
Speaker 1: for understanding stellar populations. When we look at a star
1:00:59
Speaker 1: cluster and count how many red supergiants versus blue supergiants
1:01:04
Speaker 1: it contains, we're using those counts to infer the cluster's
1:01:09
Speaker 1: age and the initial mass function of its stars. If
1:01:14
Speaker 1: stars can rapidly transition between categories, those inferences become less reliable.
1:01:21
Speaker 1: We need to account for the possibility that what we're
1:01:24
Speaker 1: seeing is a snapshot of a dynamic process, rather than
1:01:29
Speaker 1: a static distribution. Stevenson 2 to 18 could do the
1:01:34
Speaker 1: same thing. It could transition to a yellow hypergiant, or
1:01:39
Speaker 1: even a luminous blue variable, if conditions are right. The
1:01:43
Speaker 1: trigger might be enhanced mass loss that removes the outer envelope,
1:01:48
Speaker 1: exposing hotter layers beneath. Or it might be a change
1:01:52
Speaker 1: in the core that alters the energy generation rate and
1:01:56
Speaker 1: forces a readjustment of the entire stellar structure. Predicting exactly
1:02:01
Speaker 1: when or if this will happen requires understanding the star's
1:02:06
Speaker 1: current internal state. Which brings us back to the models.
1:02:11
Speaker 1: They need to track nuclear burning in the core, energy
1:02:15
Speaker 1: transport through the radiative and convective zones. the structure and
1:02:20
Speaker 1: stability of the envelope, and the interaction with the circumstellar environment.
1:02:27
Speaker 1: Each component has uncertainties. Combine them all together and the
1:02:32
Speaker 1: uncertainties multiply. Current best models suggest Stevenson 218 is probably
1:02:39
Speaker 1: burning helium in its core, with hydrogen burning in a shell.
1:02:45
Speaker 1: It's past the main sequence but hasn't yet reached the
1:02:48
Speaker 1: carbon-burning phase. It might spend another few hundred thousand years
1:02:54
Speaker 1: in this state before helium exhaustion forces the next transition,
1:02:59
Speaker 1: or it might speed through in a tenth of that
1:03:02
Speaker 1: time if mass loss accelerates. The range of possibilities is
1:03:07
Speaker 1: wide enough that making firm predictions is difficult. What we
1:03:12
Speaker 1: can say with more confidence is what happens eventually. All
1:03:18
Speaker 1: roads lead to core collapse. Once the core builds up
1:03:22
Speaker 1: enough iron peak elements that fusion becomes endothermic rather than exothermic,
1:03:28
Speaker 1: the pressure support vanishes. Gravity takes over. The core implodes
1:03:34
Speaker 1: in less than a second, reaching nuclear densities and bouncing
1:03:39
Speaker 1: back in a shockwave that tears through the overlying layers.
1:03:44
Speaker 1: Neutrinos carry away most of the energy. What's left drives
1:03:49
Speaker 1: the supernova explosion that we see as a brief, brilliant flash,
1:03:54
Speaker 1: except we might not see it. This is the red
1:03:58
Speaker 1: supergiant problem. Theory predicts that stars like Stevenson 2-18 should
1:04:05
Speaker 1: explode as type 2p supernova, the most common variety of
1:04:10
Speaker 1: core collapse supernovae. Observations show that most Type II P
1:04:15
Speaker 1: supernovae do come from red supergiants, but only the relatively
1:04:21
Speaker 1: small ones. Red supergiants above about 18 solar masses and 100,000
1:04:28
Speaker 1: solar luminosities are rarely, if ever, observed as supernova progenitors.
1:04:35
Speaker 1: Stevenson 218 is well above both those thresholds. If it
1:04:40
Speaker 1: exploded tomorrow, it would be one of the most luminous
1:04:43
Speaker 1: and massive red supergiant supernovae ever observed. But statistically, we
1:04:49
Speaker 1: don't see many like it. Either they're not exploding, or
1:04:53
Speaker 1: they're exploding in ways we don't recognise, or they're obscured
1:04:57
Speaker 1: by dust. Recent work suggests dust obscuration might be part
1:05:02
Speaker 1: of the answer. When astronomers went back and examined the
1:05:06
Speaker 1: sites of known supernovae using infrared observations that can penetrate dust,
1:05:13
Speaker 1: they found several cases where the progenitor star was surrounded
1:05:18
Speaker 1: by thick circumstellar material. This material absorbed the optical light
1:05:24
Speaker 1: from the supernova, making it appear fainter than it actually was.
1:05:29
Speaker 1: In some cases, The supernova was almost completely hidden in
1:05:34
Speaker 1: the optical but bright in the infrared. If Stevenson 2-18
1:05:39
Speaker 1: is surrounded by substantial amounts of dust, which seems likely
1:05:44
Speaker 1: given its high mass loss rate and infrared excess, then
1:05:48
Speaker 1: its eventual supernova might be similarly obscured. We'd see the
1:05:53
Speaker 1: infrared brightening, but optical telescopes might miss it entirely. This
1:05:59
Speaker 1: would explain why high luminosity red supergiants appear to be
1:06:03
Speaker 1: missing from supernova progenitor lists. They're there. We're just not
1:06:09
Speaker 1: seeing them properly. Alternative explanations involve failed supernovae. Maybe the
1:06:15
Speaker 1: most massive red supergiants don't explode. Maybe the shockwave stalls
1:06:20
Speaker 1: before it can eject the envelope. The core collapses all
1:06:24
Speaker 1: the way to a black hole, swallowing the rest of
1:06:27
Speaker 1: the star in the process. no bright explosion, just a
1:06:32
Speaker 1: star that quietly disappears. This has been proposed as a
1:06:37
Speaker 1: solution to the problem, and there's some theoretical support for it.
1:06:42
Speaker 1: Simulations show that under certain conditions, the shock can indeed fail,
1:06:48
Speaker 1: leading to direct black hole formation without a visible supernova.
1:06:53
Speaker 1: But we don't have definitive observational evidence for failed supernova yet,
1:06:58
Speaker 1: It's hard to prove something didn't happen. You need long-term
1:07:03
Speaker 1: monitoring of massive stars to catch them disappearing without exploding.
1:07:07
Speaker 1: And even then, there are other explanations for why a
1:07:11
Speaker 1: star might seem to vanish. There's another possibility. Evolution backward.
1:07:17
Speaker 1: Not all red supergiants march straight toward core collapse. Some
1:07:22
Speaker 1: take detours. The most massive ones might lose so much
1:07:26
Speaker 1: mass that they shed their hydrogen envelopes entirely, exposing deeper layers.
1:07:33
Speaker 1: When that happens, the effective temperature rises. The star moves
1:07:38
Speaker 1: leftward on the Hertzsprung-Russell diagram, transitioning from red to yellow
1:07:44
Speaker 1: to blue. It becomes a yellow hypergiant, then possibly a
1:07:49
Speaker 1: luminous blue variable, before eventually meeting its end. This isn't speculation.
1:07:56
Speaker 1: We've observed stars doing exactly this. WOHG64 provides a compelling example.
1:08:04
Speaker 1: Located in the Large Magellanic Cloud, this star was classified
1:08:08
Speaker 1: as a red supergiant for decades. Recent observations showed something unexpected.
1:08:15
Speaker 1: Its spectrum changed. The temperature increased. the star appeared to
1:08:21
Speaker 1: be transitioning toward a yellow hypergiant phase. If confirmed, this
1:08:27
Speaker 1: would represent direct observational evidence of backward evolution in one
1:08:32
Speaker 1: of the largest known stars. The implications are significant. If
1:08:37
Speaker 1: massive red supergiants commonly evolve back to higher temperatures, then
1:08:42
Speaker 1: the number of stars that actually explode while red decreases.
1:08:48
Speaker 1: The red supergiant problem becomes less of a problem. The
1:08:52
Speaker 1: missing high-mass progenitors aren't missing. They've just moved on to
1:08:57
Speaker 1: different evolutionary stages before exploding. But this raises questions about Stevenson,
1:09:03
Speaker 1: 2 to 18 specifically. Will it follow the same path?
1:09:09
Speaker 1: The answer depends on how much mass it loses, and
1:09:12
Speaker 1: how quickly. Current estimates suggest its mass loss rate is
1:09:18
Speaker 1: among the highest known for any red supergiant, roughly 1.35
1:09:23
Speaker 1: times 10 to the negative fifth solar masses per year.
1:09:28
Speaker 1: That's phenomenal. At that rate, the star could shed a
1:09:32
Speaker 1: significant portion of its envelope in just a few hundred
1:09:36
Speaker 1: thousand years. Whether that's fast enough to trigger backward evolution
1:09:41
Speaker 1: before core collapse is uncertain. The timeline is tight. Carbon
1:09:47
Speaker 1: burning lasts only thousands of years. Silicon burning is even briefer.
1:09:54
Speaker 1: Once the core starts fusing silicon into iron, you're measuring
1:09:58
Speaker 1: the remaining lifetime in days. If the mass loss hasn't
1:10:03
Speaker 1: removed the hydrogen envelope by then, the star explodes as
1:10:07
Speaker 1: a red supergiant. If it has, you get a yellow
1:10:11
Speaker 1: or blue supergiant supernova instead. There's observational evidence for both outcomes.
1:10:19
Speaker 1: Some supernovae show progenitors consistent with red supergiants. Others show
1:10:25
Speaker 1: yellow or blue supergiants. The distribution suggests that the evolutionary
1:10:30
Speaker 1: path depends on initial mass, metallicity, and rotation rate. Higher
1:10:37
Speaker 1: mass stars with higher metallicity and faster rotation are more
1:10:41
Speaker 1: likely to lose their envelopes and evolve blueward. Lower mass
1:10:46
Speaker 1: red supergiants with lower metallicity tend to stay red until
1:10:51
Speaker 1: the end. Stevenson 2-18 sits in a cluster with moderately
1:10:57
Speaker 1: high metallicity. Its rotation rate is unknown. Its current mass
1:11:03
Speaker 1: is estimated at around 16 to 20 solar masses, though
1:11:07
Speaker 1: uncertainties are Given those parameters, backward evolution is possible, but
1:11:14
Speaker 1: not guaranteed. The star could go either way. Let's zoom
1:11:20
Speaker 1: out for a moment. Why does any of this matter?
1:11:23
Speaker 1: Because understanding how the largest stars end their lives helps
1:11:27
Speaker 1: us understand the chemical evolution of galaxies. When massive stars explode,
1:11:33
Speaker 1: they inject heavy elements into the interstellar medium. Carbon, oxygen, silicon, iron,
1:11:42
Speaker 1: all the stuff that planets and people are made of.
1:11:45
Speaker 1: The explosion's energy also triggers new rounds of star formation
1:11:50
Speaker 1: by compressing nearby gas clouds. The cycle continues. If the
1:11:55
Speaker 1: most massive stars collapse directly to black holes without exploding,
1:12:01
Speaker 1: then those elements stay locked inside. They don't enrich the
1:12:06
Speaker 1: surrounding medium. the chemical evolution of the galaxy slows. Fewer
1:12:13
Speaker 1: heavy elements mean fewer rocky planets, fewer opportunities for life.
1:12:19
Speaker 1: On the other hand, if these stars do explode, but
1:12:22
Speaker 1: we're just not seeing the explosions because of dust, then
1:12:26
Speaker 1: the enrichment continues as expected. We're simply missing the observational confirmation.
1:12:34
Speaker 1: Recent infrared surveys are starting to close that gap. By
1:12:38
Speaker 1: observing supernovae in wavelengths that penetrate dust, astronomers can identify
1:12:44
Speaker 1: obscured explosions that would otherwise go unnoticed. The early results
1:12:51
Speaker 1: are promising. Several high-luminosity red supergiants have now been identified
1:12:58
Speaker 1: as supernova progenitors, including some that were heavily dust-enshrouded. This
1:13:03
Speaker 1: suggests that the red supergiant problem might be largely an
1:13:07
Speaker 1: observational bias. The stars are exploding, we're just not always
1:13:13
Speaker 1: seeing it in the optical. Infrared astronomy is revealing what
1:13:18
Speaker 1: was hidden, but there's still the failed supernova scenario to consider.
1:13:24
Speaker 1: Even if most red supergiants explode, maybe the absolute most
1:13:29
Speaker 1: massive ones don't. Maybe there's a threshold above which the
1:13:33
Speaker 1: shockwave always stalls. Theory suggests this might happen for stars
1:13:38
Speaker 1: with helium cores above about 45 solar masses. The exact
1:13:44
Speaker 1: number depends on details of the explosion mechanism that we
1:13:48
Speaker 1: still don't fully understand. If Stevenson 218 has a helium
1:13:53
Speaker 1: core that massive, it might be a failed supernova candidate.
1:13:58
Speaker 1: The core collapses. A black hole forms. The envelope falls inward.
1:14:04
Speaker 1: Within hours or days, the entire star disappears from view.
1:14:10
Speaker 1: No bright explosion. Just a brief transient as the outer
1:14:14
Speaker 1: layers accrete onto the newly formed black hole. Detecting a
1:14:19
Speaker 1: failed supernova is challenging. You need to be monitoring the
1:14:23
Speaker 1: star beforehand so you know it was there. Then it
1:14:27
Speaker 1: needs to vanish without producing a standard supernova light curve.
1:14:32
Speaker 1: A few candidates have been identified, but confirmation is difficult.
1:14:37
Speaker 1: It's hard to prove that a star didn't explode versus
1:14:40
Speaker 1: being obscured by dust or having an unusually faint explosion.
1:14:45
Speaker 1: Long-term monitoring programs like the Large Binocular Telescope's search for
1:14:50
Speaker 1: failed supernovae are addressing this. they observe thousands of massive
1:14:56
Speaker 1: stars in nearby galaxies, looking for ones that disappear. The
1:15:02
Speaker 1: results so far are intriguing, but not definitive. Some stars
1:15:08
Speaker 1: have indeed vanished, but whether they underwent failed supernovae or
1:15:13
Speaker 1: just faded below detection limits remains unclear. Now, here's something
1:15:19
Speaker 1: that doesn't get discussed often enough. The uncertainty in all
1:15:24
Speaker 1: these predictions comes down to physics we still don't fully understand.
1:15:29
Speaker 1: The supernova mechanism itself remains partially mysterious. We know the
1:15:35
Speaker 1: broad outline. Core collapses. Bounce happens. Shockwave forms. Neutrinos carry
1:15:43
Speaker 1: away energy. But the details of how the shockwave revives
1:15:47
Speaker 1: after stalling, how neutrino heating works, how magnetic fields and
1:15:52
Speaker 1: rotation affect the outcome. These are active areas of research.
1:15:57
Speaker 1: Current simulations can produce successful explosions for stars up to
1:16:02
Speaker 1: about 25 solar masses, depending on assumptions. Above that, the
1:16:08
Speaker 1: results become uncertain. Stevenson 218's current mass is estimated at
1:16:14
Speaker 1: 16 to 20 solar masses. but its initial mass on
1:16:19
Speaker 1: the main sequence was higher, maybe 25 to 40 solar masses.
1:16:26
Speaker 1: It's lost a significant amount through stellar winds over its lifetime.
1:16:31
Speaker 1: What matters for the explosion mechanism is the mass of
1:16:34
Speaker 1: the core and envelope at the time of collapse, not
1:16:38
Speaker 1: the initial mass. If the star has lost enough mass
1:16:43
Speaker 1: that its helium core is below the critical threshold, it
1:16:46
Speaker 1: should explode successfully. If not, failure becomes more likely. We
1:16:53
Speaker 1: don't know which scenario applies because we don't know the
1:16:57
Speaker 1: current mass structure precisely enough. There's another wrinkle. Rotation. Fast
1:17:04
Speaker 1: rotating massive stars can produce different outcomes than slowly rotating ones.
1:17:10
Speaker 1: Rotation affects how mass is distributed, how magnetic fields behave,
1:17:15
Speaker 1: and how energy is transported during collapse. In extreme cases,
1:17:21
Speaker 1: rapid rotation can lead to jet-driven explosions that look very
1:17:26
Speaker 1: different from standard supernova. These might be connected to long-duration
1:17:32
Speaker 1: gamma-ray bursts, though that link is still being investigated. We
1:17:37
Speaker 1: don't have direct measurements of Stevenson 2-18's rotation rate, Its
1:17:44
Speaker 1: spectral lines are broad, which could indicate rotation, but could
1:17:49
Speaker 1: also result from turbulence or pulsation. Without resolved imaging or
1:17:55
Speaker 1: detailed spectroscopic monitoring, pinning down the rotation is difficult. If
1:18:00
Speaker 1: the star is rotating rapidly, its fate could involve more
1:18:04
Speaker 1: exotic outcomes. Collapsar models, where the core collapses to a
1:18:10
Speaker 1: black hole while the envelope forms an accretion disk, become possible.
1:18:15
Speaker 1: These can produce energetic explosions along the rotation axis, while
1:18:21
Speaker 1: most of the mass falls back onto the black hole.
1:18:25
Speaker 1: The result is a supernova that's asymmetric, possibly accompanied by
1:18:30
Speaker 1: a gamma ray burst if we're looking down the jet.
1:18:34
Speaker 1: This is speculative for Stevenson 2-18 specifically, but it illustrates
1:18:41
Speaker 1: how many variables are in play. The star's ultimate fate
1:18:45
Speaker 1: depends on current mass, core structure, mass loss history, rotation rate,
1:18:52
Speaker 1: magnetic field strength, and possibly other factors we haven't even
1:18:58
Speaker 1: identified yet. What we can say is this. Within the
1:19:02
Speaker 1: next few hundred thousand years, give or take, Stevenson 218
1:19:07
Speaker 1: will undergo core collapse. Whether we see a bright supernova,
1:19:13
Speaker 1: a dust-obscured explosion, a failed supernova, or something more exotic
1:19:19
Speaker 1: depends on factors we're still working to constrain. The odds
1:19:23
Speaker 1: favour some kind of explosion. Most red supergiants do explode,
1:19:28
Speaker 1: even if we don't always see it. The question is
1:19:32
Speaker 1: what kind of explosion and how bright it will be.
1:19:36
Speaker 1: If it happens while the star is still red and
1:19:39
Speaker 1: surrounded by dust, We might need infrared telescopes to catch it.
1:19:44
Speaker 1: If it evolves blueward first, the explosion will be more
1:19:48
Speaker 1: visible in optical wavelengths. Either way, when Stevenson 218 finally detonates,
1:19:56
Speaker 1: it will be one of the more luminous supernovae in
1:19:59
Speaker 1: the Milky Way. The exact peak brightness depends on the
1:20:03
Speaker 1: explosion energy and how much mass is ejected, but estimates
1:20:08
Speaker 1: suggest it could reach absolute magnitudes around negative 18 or brighter.
1:20:16
Speaker 1: From Earth, at a distance of 18,900 light years, that
1:20:21
Speaker 1: would translate to an apparent magnitude of roughly plus 4
1:20:25
Speaker 1: or 5, making it visible to the naked eye. Not
1:20:28
Speaker 1: as bright as historical supernovae like SN 1006 or SN 1054,
1:20:32
Speaker 1: which were much closer but still noticeable. A new star
1:20:40
Speaker 1: appearing in scutum, shining steadily for weeks before fading. Astronomers
1:20:46
Speaker 1: worldwide would point every available instrument at it. Neutrino detectors
1:20:52
Speaker 1: would catch the burst from the core collapse seconds before
1:20:56
Speaker 1: the light arrived. Gravitational wave observatories might detect signals if
1:21:02
Speaker 1: the collapse is asymmetric enough. The data would provide unprecedented
1:21:09
Speaker 1: insights into how the most massive stars die. We'd measure
1:21:15
Speaker 1: the explosion energy, the mass of ejected material, the composition
1:21:20
Speaker 1: of the ejector, the structure of the circumstellar medium. All
1:21:25
Speaker 1: of that would feed back into models of stellar evolution
1:21:28
Speaker 1: and supernova physics, refining our understanding. But that's in the future.
1:21:35
Speaker 1: For now, Stevenson 218 continues burning helium in its core,
1:21:41
Speaker 1: shedding mass into space, pulsating irregularly. It's in a relatively
1:21:46
Speaker 1: stable phase that could last another 100,000 years or end tomorrow.
1:21:54
Speaker 1: The uncertainty is part of what makes these stars so fascinating.
1:21:59
Speaker 1: They exist at the edge of stability. where small changes
1:22:03
Speaker 1: in internal conditions can have dramatic effects. In a sense,
1:22:09
Speaker 1: studying Stevenson 218 and its peers is like studying extreme
1:22:14
Speaker 1: weather on Earth. The basic physics is understood, but predicting
1:22:19
Speaker 1: exactly when and how a hurricane will intensify or a
1:22:23
Speaker 1: tornado will form remains challenging. The systems are complex, with
1:22:29
Speaker 1: many interacting factors. Small perturbations can cascade into large effects.
1:22:35
Speaker 1: Stellar astrophysics faces similar challenges. We have the fundamental equations,
1:22:41
Speaker 1: general relativity, hydrodynamics, nuclear physics, radiation transport. But solving those
1:22:49
Speaker 1: equations for a star with 2,150 times the sun's radius,
1:22:56
Speaker 1: blowing off mass at prodigious rates, with a core undergoing
1:23:00
Speaker 1: nuclear fusion in multiple shells, while the envelope pulsates and convects,
1:23:07
Speaker 1: that requires approximations. And approximations introduce uncertainties. The next generation
1:23:14
Speaker 1: of telescopes will help. The extremely large telescope, currently under construction,
1:23:21
Speaker 1: will have the resolving power to image the surfaces of
1:23:25
Speaker 1: nearby red supergiants. we'll see convection cells, mass ejections, asymmetries
1:23:32
Speaker 1: in the photosphere. That observational data will constrain models in
1:23:38
Speaker 1: ways that aren't currently possible. For Stevenson 2-18, being so
1:23:44
Speaker 1: far away, direct imaging remains out of reach even with
1:23:48
Speaker 1: future instruments. But we can study analogues. Betelgeuse is a
1:23:53
Speaker 1: red supergiant that's much closer and has been imaged in detail.
1:23:58
Speaker 1: While smaller and less luminous than Stevenson 218, it provides
1:24:03
Speaker 1: insights into how these stars behave. The great dimming of
1:24:08
Speaker 1: 2019 to 2020, when Betelgeuse dropped to half its normal brightness,
1:24:15
Speaker 1: was likely caused by a massive dust cloud ejected from
1:24:19
Speaker 1: the star's surface. Studying that event in detail, gave us
1:24:24
Speaker 1: clues about mass loss mechanisms that probably operate in all
1:24:29
Speaker 1: red supergiants. If Stevenson 2-18 undergoes similar dimming events, we
1:24:35
Speaker 1: could detect them through photometric monitoring. Long-term light curves would
1:24:40
Speaker 1: reveal the pattern of mass ejections, helping us estimate the
1:24:45
Speaker 1: total mass loss rate more accurately. That, in turn, would
1:24:50
Speaker 1: improve predictions about the star's evolutionary trajectory. The technology exists.
1:24:57
Speaker 1: What's needed is sustained effort, monitoring programs that track these
1:25:02
Speaker 1: stars over years and decades. When something changes, when a
1:25:07
Speaker 1: star dims or brightens or shows spectral variations, we need
1:25:12
Speaker 1: rapid follow-up with multiple instruments, spectroscopy to measure velocities and compositions,
1:25:19
Speaker 1: Infrared imaging to map the dust distribution. Radio observations to
1:25:25
Speaker 1: detect masses in the outflowing material. All of this data
1:25:29
Speaker 1: builds a more complete picture. Not just of one star,
1:25:34
Speaker 1: but of how the largest stars in the universe live
1:25:37
Speaker 1: and die. Stevenson 218 is the current record holder, but
1:25:43
Speaker 1: records are made to be broken. There might be even
1:25:47
Speaker 1: larger stars out there. hidden in distant clusters we haven't
1:25:51
Speaker 1: surveyed yet. Or maybe Stevenson 218 really is near the
1:25:56
Speaker 1: upper limit of what's physically possible. Either way, understanding this
1:26:02
Speaker 1: star means understanding the extremes of stellar evolution, and it
1:26:07
Speaker 1: teaches us where the boundaries lie between what's possible and
1:26:11
Speaker 1: what the laws of physics will never permit. But Stevenson
1:26:15
Speaker 1: 218 doesn't exist in isolation. There's a whole cast of
1:26:20
Speaker 1: stellar titans competing for the title of largest star, and
1:26:24
Speaker 1: the rankings shift every time new data arrives. UY Scuti
1:26:30
Speaker 1: held the crown for years. Located about 5,900 light years
1:26:36
Speaker 1: away in Scutum, the same constellation as Stevenson 2-18, it
1:26:40
Speaker 1: was measured at over 1,700 solar radii in some studies.
1:26:48
Speaker 1: that'd make it even larger than Stevenson 2-18. Except those
1:26:54
Speaker 1: measurements relied on distance estimates that turned out to be wrong.
1:26:58
Speaker 1: When the Gaia spacecraft measured UY Scuti's parallax directly, the
1:27:03
Speaker 1: calculated distance dropped significantly. Closer star, lower luminosity. Lower luminosity
1:27:11
Speaker 1: means smaller radius to match the observed brightness. The new
1:27:16
Speaker 1: estimate puts Ui Scuti around 755 solar radii. Still enormous,
1:27:24
Speaker 1: still one of the largest known stars, but nowhere near
1:27:28
Speaker 1: the behemoth it was once thought to be. That's the
1:27:32
Speaker 1: problem with declaring champions in this category. The measurements change,
1:27:37
Speaker 1: sometimes dramatically. Vuicanus Majoris went through a similar revision. For
1:27:44
Speaker 1: decades it was considered the largest star known, with estimates
1:27:48
Speaker 1: reaching 2,000 solar radii or more. Modern analysis puts it
1:27:53
Speaker 1: closer to 1,420 solar radii. Huge, yes. Record-breaking. Not anymore.
1:28:02
Speaker 1: Then there's WOHG-64, located in the Large Magellanic Cloud. It's
1:28:08
Speaker 1: one of the few candidates that might actually rival Stevenson 2-18.
1:28:15
Speaker 1: Estimates place it around 1,540 solar radii, though like all
1:28:21
Speaker 1: these measurements, there's substantial uncertainty. What makes WOHG64 particularly interesting
1:28:31
Speaker 1: is that it's undergoing a dramatic transformation right now. Recent
1:28:36
Speaker 1: observations suggest it's transitioning from a red supergiant to a
1:28:40
Speaker 1: yellow hypergiant. That's exceptionally rare. Most stars don't change spectral
1:28:46
Speaker 1: types on human timescales, but WOHG 64 appears to be
1:28:53
Speaker 1: doing exactly that, possibly shedding enormous amounts of mass in
1:28:57
Speaker 1: the process. If that's true, we're watching a star fundamentally
1:29:02
Speaker 1: alter its structure in real time. Yellow hypergiants occupy a narrow,
1:29:08
Speaker 1: unstable region of the Hertzsprung-Russell diagram. They're stars in transition,
1:29:15
Speaker 1: either expanding into red supergiants or contracting back toward hotter
1:29:20
Speaker 1: temperatures after a red supergiant phase. The transition period might
1:29:25
Speaker 1: last only a few thousand years, which means catching a
1:29:29
Speaker 1: star in this state is like winning the astronomical lottery.
1:29:34
Speaker 1: And if WOHG 64 is indeed becoming a yellow hypergiant,
1:29:40
Speaker 1: that tells us something about its future. These transitions often
1:29:45
Speaker 1: precede supernova explosions. The star might be in its final years,
1:29:51
Speaker 1: stripping away its outer layers before the core collapse that
1:29:55
Speaker 1: will end its life. Which raises a question. Could Stevenson
1:30:01
Speaker 1: 2-18 go through a similar transition? Probably not. The evolutionary
1:30:07
Speaker 1: path depends on initial mass and current mass loss rate.
1:30:12
Speaker 1: WOHG 64 appears to be losing mass fast enough to
1:30:16
Speaker 1: expose hotter layers beneath the photosphere. Stevenson 2-18's mass loss rate,
1:30:23
Speaker 1: while high, might not be extreme enough for that particular path.
1:30:28
Speaker 1: More likely, it will remain a red supergiant until core collapse.
1:30:34
Speaker 1: But we can't be certain. That's the recurring theme with
1:30:38
Speaker 1: these stars. We observe them. We measure them. We build models.
1:30:43
Speaker 1: But they're so far away, so extreme in their properties.
1:30:48
Speaker 1: so short-lived on cosmic timescales, that definitive answers remain elusive.
1:30:55
Speaker 1: The competition for largest star also includes candidates we can
1:30:59
Speaker 1: barely observe. There are stars in distant clusters we haven't
1:31:04
Speaker 1: fully surveyed, dense, star-forming regions obscured by dust, other galaxies
1:31:11
Speaker 1: where individual stars blend together at our current resolution. Stevenson
1:31:17
Speaker 1: 2-18 holds the record not because we've checked every star
1:31:21
Speaker 1: in the universe, but because we've checked the stars we
1:31:24
Speaker 1: can detect and measure. There might be larger ones out there,
1:31:29
Speaker 1: we just haven't found them yet. Or maybe we have.
1:31:33
Speaker 1: Maybe the measurements are wrong in the other direction. Maybe
1:31:37
Speaker 1: Stevenson 2-18 is actually smaller than we think. and one
1:31:42
Speaker 1: of the revised estimates for UY Scuti or VY Canis
1:31:46
Speaker 1: Majoris will turn out to be correct after all. The
1:31:50
Speaker 1: uncertainty cuts both ways. What we can say with confidence
1:31:55
Speaker 1: is that red supergiants and hypergiants represent the upper limit
1:32:00
Speaker 1: of stellar size. No star gets much bigger than this category.
1:32:05
Speaker 1: The physics won't allow it. go too large and radiation
1:32:10
Speaker 1: pressure overwhelms gravity. The star becomes unstable, shedding mass faster
1:32:17
Speaker 1: than it can maintain its bloated envelope. The Eddington limit
1:32:21
Speaker 1: defines this boundary. It's the point where outward radiation pressure
1:32:27
Speaker 1: equals inward gravitational force. For the most massive luminous stars,
1:32:33
Speaker 1: the actual luminosity approaches the Eddington limit closely. Push beyond it,
1:32:39
Speaker 1: even temporarily, and the star ejects material in violent outbursts.
1:32:46
Speaker 1: Luminous blue variables experience this regularly. They're massive stars that
1:32:51
Speaker 1: undergo episodic eruptions, sometimes brightening by several magnitudes in a
1:32:57
Speaker 1: matter of weeks. During these outbursts, they exceed their Eddington
1:33:02
Speaker 1: limit and blow off entire solar masses of material. Eta
1:33:08
Speaker 1: Carinae famously did this in the 1840s, ejecting more than
1:33:13
Speaker 1: 10 solar masses in a single event. Red supergiants are
1:33:17
Speaker 1: more stable than LBVs, but they still hover near their
1:33:21
Speaker 1: Eddington limits. That's why their mass loss rates are so high.
1:33:26
Speaker 1: The radiation pressure is almost strong enough to levitate the photosphere.
1:33:32
Speaker 1: Material in the outer layers feels an outward force nearly
1:33:36
Speaker 1: equal to the inward pull of gravity. It doesn't take
1:33:39
Speaker 1: much to tip the balance and drive mass loss. For
1:33:43
Speaker 1: Stevenson 218, with its luminosity of 440,000 suns, the radiation
1:33:50
Speaker 1: pressure is immense. The star is constantly fighting against its
1:33:56
Speaker 1: own light. Every photon generated in the core eventually makes
1:34:01
Speaker 1: its way to the surface and each one carries momentum.
1:34:06
Speaker 1: Billions upon billions of photons per second all pushing outward.
1:34:11
Speaker 1: At the photosphere, where the star becomes transparent and light
1:34:16
Speaker 1: escapes freely, that outward push is at its maximum. Atoms
1:34:21
Speaker 1: and molecules in the outermost layers feel the full force
1:34:25
Speaker 1: of the radiation field. For heavy elements, especially metals like iron.
1:34:32
Speaker 1: The cross section for absorbing photons is large. Those atoms
1:34:37
Speaker 1: get pushed more than lighter elements like hydrogen and helium.
1:34:41
Speaker 1: This selective pressure creates a wind. Material streams away from
1:34:46
Speaker 1: the star, accelerating as it moves outward. Near the photosphere,
1:34:52
Speaker 1: the wind might be moving at a few kilometers per second.
1:34:56
Speaker 1: Farther out, It reaches tens of kilometers per second. The
1:35:01
Speaker 1: material carries away mass, angular momentum, and energy. Over time,
1:35:07
Speaker 1: this wind reshapes the star. The envelope slowly shrinks. The
1:35:12
Speaker 1: radius decreases. The surface temperature might rise slightly as deeper,
1:35:17
Speaker 1: hotter layers become exposed. The star evolves, driven as much
1:35:23
Speaker 1: by mass loss as by nuclear burning in the core.
1:35:27
Speaker 1: How long can this continue? For red supergiants in general,
1:35:32
Speaker 1: the mass loss phase might last a few hundred thousand years.
1:35:35
Speaker 1: For a star as large and luminous as Stevenson 2-18,
1:35:40
Speaker 1: it could be shorter. Maybe only tens of thousands of
1:35:43
Speaker 1: years before the envelope is depleted, or the core reaches
1:35:47
Speaker 1: a critical state. And then what? Most likely, core collapse.
1:35:53
Speaker 1: The iron core grows until it exceeds the Chandrasekhar limit.
1:35:58
Speaker 1: roughly 1.4 solar masses. At that point, electron degeneracy pressure
1:36:04
Speaker 1: can no longer support it. The core implodes in less
1:36:08
Speaker 1: than a second, reaching nuclear densities. Protons and electrons merge
1:36:15
Speaker 1: into neutrons. The collapse halts abruptly when the core becomes
1:36:20
Speaker 1: a solid ball of neutrons, incompressible at those densities. The
1:36:25
Speaker 1: rebound launches a shockwave outward through the star. That shockwave
1:36:30
Speaker 1: doesn't carry enough energy by itself to blow apart the envelope.
1:36:34
Speaker 1: It stalls a few hundred kilometers from the core. But neutrinos,
1:36:39
Speaker 1: streaming outward by the trillions, deposit a fraction of their
1:36:44
Speaker 1: energy into the stalled shock. That's enough to revive it.
1:36:49
Speaker 1: The shock accelerates outward. tearing through the star's outer layers
1:36:55
Speaker 1: in a matter of hours. The supernova becomes visible when
1:37:00
Speaker 1: the shock reaches the photosphere. For Stevenson 218, with its
1:37:06
Speaker 1: enormous radius that could take weeks or even months, light
1:37:11
Speaker 1: from the explosion would take that long just to cross
1:37:16
Speaker 1: the star's diameter. Once the shock breaks through, the star
1:37:20
Speaker 1: brightens dramatically. At peak, a supernova can outshine an entire galaxy.
1:37:26
Speaker 1: For a few weeks, it's the most luminous object in
1:37:30
Speaker 1: its cosmic neighborhood, radiating energy at a prodigious rate. Then
1:37:35
Speaker 1: it fades. The expanding debris cools. Heavy elements synthesized in
1:37:41
Speaker 1: the explosion or in the final moments of core collapse
1:37:44
Speaker 1: spread into space. That material will eventually become part of
1:37:48
Speaker 1: new stars, new planets, Maybe even new life. But there's
1:37:54
Speaker 1: another possibility. Maybe Stevenson 218 won't explode as a supernova.
1:38:00
Speaker 1: The red supergiant problem suggests that the most massive red
1:38:04
Speaker 1: supergiants might not produce visible explosions. Observations show that stars
1:38:11
Speaker 1: above a certain mass and luminosity threshold simply disappear. They're
1:38:17
Speaker 1: there one year, gone the next. with no supernova detected
1:38:22
Speaker 1: in between. One explanation is that these stars undergo direct
1:38:26
Speaker 1: core collapse to a black hole. The shockwave stalls and
1:38:31
Speaker 1: never revives. The entire star, envelope and all, falls into
1:38:36
Speaker 1: the newly formed black hole. No explosion, no ejected material,
1:38:42
Speaker 1: just a black hole where a star used to be.
1:38:45
Speaker 1: Another explanation involves thick dust shells. If the star is
1:38:50
Speaker 1: surrounded by enough dust, the supernova light might be absorbed
1:38:54
Speaker 1: before it can escape. The explosion happens, but we don't
1:38:59
Speaker 1: see it. Infrared observations could detect the dust heating up,
1:39:05
Speaker 1: but in optical wavelengths, the event would be invisible. Recent
1:39:11
Speaker 1: discoveries support this second scenario. A few supernovae have been
1:39:16
Speaker 1: found where the progenitor star was hidden by dust. The
1:39:20
Speaker 1: explosion itself was detected, but only because instruments sensitive to
1:39:25
Speaker 1: infrared or radio wavelengths were observing at the right time.
1:39:31
Speaker 1: If Stevenson 2-18 is surrounded by a dense dust shell,
1:39:36
Speaker 1: and if it explodes, we might miss the event entirely.
1:39:41
Speaker 1: Or we might see a faint, reddened outburst. that doesn't
1:39:45
Speaker 1: look like a typical supernova. The dust would absorb the
1:39:50
Speaker 1: optical light, re-radiate it in the infrared, and create a
1:39:55
Speaker 1: confusing signal that could be misinterpreted. This matters for understanding
1:40:00
Speaker 1: stellar evolution and for predicting what we should see when
1:40:04
Speaker 1: these giants die. If we're missing supernovae because dust hides them,
1:40:10
Speaker 1: our models need to account for that If stars collapse
1:40:15
Speaker 1: directly to black holes, that has implications for black hole
1:40:19
Speaker 1: formation rates and the distribution of black hole masses. Stevenson
1:40:25
Speaker 1: 2-18 might provide answers, or it might deepen the mystery.
1:40:31
Speaker 1: When it dies, if we're lucky enough to observe the event,
1:40:36
Speaker 1: we'll learn something fundamental. If it disappears quietly, that's also information.
1:40:43
Speaker 1: absence of evidence becomes evidence of absence, or at least
1:40:48
Speaker 1: evidence of thick dust. For now, it sits in Stevenson 2,
1:40:53
Speaker 1: blowing off mass, pulsating gently, radiating energy into space. How
1:40:59
Speaker 1: much longer it will continue is unknown. Could be tomorrow.
1:41:03
Speaker 1: Could be a hundred thousand years from now. The uncertainty
1:41:08
Speaker 1: is frustrating, but also oddly fitting. These stars exist at
1:41:13
Speaker 1: the edge of what's possible. Their extreme properties push our
1:41:17
Speaker 1: models to the breaking point. They're rare enough that we
1:41:20
Speaker 1: have limited data, far enough away that measurements are difficult,
1:41:26
Speaker 1: variable enough that nothing stays constant for long. In a sense,
1:41:32
Speaker 1: the fact that we can study them at all is remarkable.
1:41:37
Speaker 1: We're talking about objects 18,900 light-years away, glowing so brightly
1:41:43
Speaker 1: they're visible across that distance, large enough that their properties
1:41:48
Speaker 1: fundamentally challenge our understanding of stellar physics. Every observation adds
1:41:54
Speaker 1: a piece to the puzzle. Every revision to the measurements
1:41:57
Speaker 1: refines our picture. Every new discovery of a similar star
1:42:02
Speaker 1: gives us more data points to constrain the models. Stevenson
1:42:07
Speaker 1: 218 is the current record holder, the largest star we
1:42:11
Speaker 1: know of, the benchmark against which all other giants are measured.
1:42:16
Speaker 1: Whether it remains the champion or gets dethroned by future
1:42:20
Speaker 1: discoveries almost doesn't matter. What matters is what it represents.
1:42:26
Speaker 1: The absolute limit of stellar size. the endpoint of massive
1:42:31
Speaker 1: star evolution. A cosmic laboratory where gravity, radiation, and nuclear
1:42:37
Speaker 1: fusion battle for supremacy. And we get to watch from
1:42:41
Speaker 1: a safe distance, learning as we go. Beyond Stevenson 218,
1:42:47
Speaker 1: there's a wider question. What determines the absolute upper limit
1:42:51
Speaker 1: for stellar size? Physics doesn't care about titles or rankings.
1:42:57
Speaker 1: There's some maximum radius a star can achieve before something breaks,
1:43:02
Speaker 1: before gravity wins, or radiation pressure tears it apart, or
1:43:08
Speaker 1: mass loss strips away the outer layers faster than fusion
1:43:12
Speaker 1: can sustain them. The Eddington limit defines one boundary. When
1:43:17
Speaker 1: radiation pressure pushing outward equals gravitational force pulling inward, you've
1:43:23
Speaker 1: hit equilibrium. Push past that. and the star starts ejecting
1:43:28
Speaker 1: its own photosphere. Light becomes powerful enough to lift material
1:43:33
Speaker 1: off the surface and blow it into space. For hypergiants
1:43:38
Speaker 1: operating near this threshold, stability is fragile. Small fluctuations in
1:43:43
Speaker 1: luminosity can trigger mass ejection events. The star literally sheds
1:43:49
Speaker 1: pieces of itself to avoid crossing the line. But there's
1:43:54
Speaker 1: another constraint. In convection, red supergiants have massive convective envelopes.
1:44:00
Speaker 1: Hot plasma rises from deep inside, cools at the surface,
1:44:05
Speaker 1: then sinks back down. This cycle transports energy and prevents
1:44:10
Speaker 1: the core from overheating. As the star expands, these convective
1:44:15
Speaker 1: cells grow enormous, hundreds of solar radii across. The timescale
1:44:21
Speaker 1: for material to complete one cycle stretches to years or decades.
1:44:27
Speaker 1: At some point, convection becomes inefficient. Energy transport slows. The
1:44:33
Speaker 1: core heats up. Instabilities develop. Models suggest there's a maximum
1:44:39
Speaker 1: effective radius around 2,000 to 2,500 solar radii where convection
1:44:45
Speaker 1: starts failing. Beyond that, the star can't maintain equilibrium. It
1:44:51
Speaker 1: either contracts ejects mass, or undergoes some catastrophic restructuring. Stevenson
1:44:59
Speaker 1: 218 sits right at this edge. If the 2150 radius
1:45:05
Speaker 1: estimate is accurate, it's operating at the theoretical limit of
1:45:10
Speaker 1: what a red supergiant can sustain. This implies something provocative.
1:45:16
Speaker 1: We might have already found the largest possible star, Not
1:45:21
Speaker 1: just the largest currently known, but the largest that can
1:45:24
Speaker 1: physically exist given the laws of stellar physics. Future discoveries
1:45:30
Speaker 1: might refine measurements, might find competitors with similar sizes, but
1:45:36
Speaker 1: they probably won't find anything dramatically larger. Not unless our
1:45:41
Speaker 1: models are fundamentally wrong. Of course, there's always the possibility
1:45:45
Speaker 1: of outliers. Extreme mass, unusual composition, temporary states during rapid evolution.
1:45:54
Speaker 1: VY Canis Majoris demonstrated how complex these objects can be.
1:46:00
Speaker 1: Its surrounding nebula contains knots and arcs of ejected material,
1:46:05
Speaker 1: some moving at different velocities, suggesting multiple ejection events over time.
1:46:11
Speaker 1: The structure isn't symmetric. Different regions show different compositions. Some
1:46:19
Speaker 1: areas are oxygen-rich, others carbon-rich, which shouldn't happen in a
1:46:24
Speaker 1: single star unless it's undergone significant internal mixing or captured
1:46:29
Speaker 1: material from a companion. That raises another wrinkle. Binary evolution.
1:46:37
Speaker 1: Many massive stars exist in binary systems, two stars orbiting
1:46:42
Speaker 1: each other close enough that they can exchange mass. One
1:46:46
Speaker 1: star evolves faster swells into a giant and starts dumping
1:46:51
Speaker 1: material onto its companion. The companion gains mass, the giant
1:46:57
Speaker 1: loses it, and both stars end up in configurations they
1:47:01
Speaker 1: wouldn't achieve alone. Could Stevenson 2-18 have a hidden companion?
1:47:08
Speaker 1: Could mass transfer explain some of its properties? There's no
1:47:13
Speaker 1: evidence for it, no orbital motion detected. no signs of
1:47:18
Speaker 1: a second star. But at 18,900 light years, resolving a
1:47:24
Speaker 1: companion would be nearly impossible unless it's also extremely luminous.
1:47:31
Speaker 1: A neutron star or black hole companion would be invisible.
1:47:36
Speaker 1: A main sequence star would be drowned out by Stevenson
1:47:40
Speaker 1: 2-18's glare, so we can't rule it out. If a
1:47:44
Speaker 1: companion exists, it might affect the ultimate fate. Binary systems
1:47:50
Speaker 1: produce different supernova outcomes than single stars. The companion can
1:47:55
Speaker 1: strip away the hydrogen envelope before the primary explodes, creating
1:48:00
Speaker 1: a type 1b or 1c supernova instead of a type 2.
1:48:06
Speaker 1: Or the companion could accrete enough mass to eventually explode itself. Or,
1:48:12
Speaker 1: in rare cases, the two stars could merge, creating something
1:48:18
Speaker 1: even more extreme. Merger scenarios are speculative, but worth considering.
1:48:24
Speaker 1: If two massive stars collide and coalesce during their red
1:48:29
Speaker 1: supergiant phase, the result would be a single object with
1:48:33
Speaker 1: combined mass and angular momentum. Depending on how the merger proceeds,
1:48:39
Speaker 1: You might get a temporarily inflated envelope, higher luminosity, and
1:48:45
Speaker 1: unusual surface composition. The merged star wouldn't be stable long term.
1:48:52
Speaker 1: It would readjust over thousands of years, possibly ejecting mass,
1:48:56
Speaker 1: possibly collapsing inward. But during the transient phase, it could
1:49:01
Speaker 1: appear extraordinarily large. No evidence suggests Stevenson 2-18 is a
1:49:07
Speaker 1: merger product. It's just one possibility among many. The point
1:49:12
Speaker 1: is that stellar evolution at these extremes involves mechanisms we're
1:49:17
Speaker 1: still working to understand. Every assumption carries uncertainty. Every model
1:49:23
Speaker 1: has limitations. Distance remains the most fundamental problem. Stevenson 218's
1:49:31
Speaker 1: size estimate depends critically on its assumed distance of 5.8 kiloparsecs.
1:49:39
Speaker 1: That number comes from cluster membership. Stevenson 2 is a young,
1:49:44
Speaker 1: massive cluster, and kinematic studies suggest it's about 18,900 light-years away.
1:49:52
Speaker 1: But what if Stevenson 218 isn't actually in the cluster?
1:49:57
Speaker 1: What if it's a foreground or background object that just
1:50:00
Speaker 1: happens to appear in the same region of sky? Unlikely,
1:50:04
Speaker 1: but not impossible. The star's radial velocity matches the cluster.
1:50:10
Speaker 1: Its reddening matches. Its age seems consistent with other cluster members.
1:50:16
Speaker 1: Everything points toward physical association. But until we get a
1:50:21
Speaker 1: direct parallax measurement, there's always residual doubt. GAIA, the European
1:50:28
Speaker 1: Space Agency's astrometry mission, has been measuring parallaxes for billions
1:50:33
Speaker 1: of stars. but Stephenson 2-18 is faint, distant, and embedded
1:50:39
Speaker 1: in a crowded field. Early Gaia data releases didn't include it.
1:50:45
Speaker 1: Future releases might. If they do, we'll have a direct
1:50:50
Speaker 1: distance measurement independent of cluster assumptions. That could revise the
1:50:55
Speaker 1: size estimate up or down by significant margins. UY Scuti
1:51:00
Speaker 1: went through exactly this process. For years, it was cited
1:51:05
Speaker 1: as the largest known star with a radius around 1,700
1:51:10
Speaker 1: solar radii. Then Gaia measured its parallax. The star turned
1:51:15
Speaker 1: out to be closer than assumed. Closer distance means lower luminosity.
1:51:22
Speaker 1: Lower luminosity means smaller radius. The updated estimate dropped to
1:51:27
Speaker 1: around 755 solar radii. Still huge, but less than half
1:51:34
Speaker 1: the previous value. The largest star became just another large star.
1:51:40
Speaker 1: Could the same happen to Stevenson 2-18? Absolutely. If Gaia
1:51:45
Speaker 1: reveals it's closer than we think, the radius shrinks proportionally.
1:51:49
Speaker 1: If it's farther, the radius grows. The uncertainty cuts both ways.
1:51:55
Speaker 1: We won't know until the data arrives. offers an interesting counterpoint.
1:52:04
Speaker 1: It resides in the Large Magellanic Cloud, a satellite galaxy
1:52:09
Speaker 1: orbiting the Milky Way. Distance to the LMC is well-established
1:52:14
Speaker 1: at about 160,000 light-years. No ambiguity there. 64's properties are
1:52:24
Speaker 1: therefore better constrained than galactic stars at comparable distances. Its
1:52:30
Speaker 1: radius is estimated at 1,540 solar radii, with relatively tight
1:52:36
Speaker 1: error bars. Recent observations detected dramatic changes. The star dimmed
1:52:43
Speaker 1: significantly between 2014 and later observations. Spectral analysis suggests it
1:52:50
Speaker 1: transitioned from a red supergiant to a yellow hypergiant. If confirmed,
1:52:57
Speaker 1: that's extraordinary. Stars don't usually change spectral types on human timescales.
1:53:04
Speaker 1: The transformation might signal an impending supernova. Yellow hypergiants represent
1:53:10
Speaker 1: a brief transitional phase between red supergiants and final collapse.
1:53:16
Speaker 1: The star is contracting, heating up, shedding mass violently. This
1:53:22
Speaker 1: phase lasts perhaps a few thousand years, a blink in
1:53:26
Speaker 1: cosmic terms. Catching a star mid-transition is rare. If WOHG-64
1:53:33
Speaker 1: explodes soon, we'll witness a supernova in a neighbouring galaxy
1:53:38
Speaker 1: with a well-documented progenitor. That would be invaluable data. But soon,
1:53:44
Speaker 1: in astronomy, means something different than in everyday life. Soon
1:53:50
Speaker 1: could be next year. Could be a thousand years. We
1:53:53
Speaker 1: don't know. The star could stabilize, expand back into a
1:53:58
Speaker 1: red supergiant, and continue for another 100,000 years. Or it
1:54:03
Speaker 1: could explode tomorrow. The uncertainty is maddening, but unavoidable. All
1:54:10
Speaker 1: these stars, Stevenson 2-18, VY Canis Majoris, WOHG 64, UY Scuti,
1:54:19
Speaker 1: exist in the same evolutionary neighborhood. Massive. Evolved. Unstable. They're
1:54:25
Speaker 1: all approaching the end. They're all grappling with the same physics.
1:54:30
Speaker 1: Radiation pressure, mass loss, convection, nuclear fusion. The same fundamental
1:54:37
Speaker 1: forces govern them all, yet each behaves slightly differently. Each
1:54:42
Speaker 1: has unique quirks, unusual properties, individual histories. That diversity matters.
1:54:50
Speaker 1: It tells us stellar evolution isn't a simple linear process.
1:54:55
Speaker 1: There's no single path from main sequence to supernova. Stars
1:55:00
Speaker 1: of similar mass can evolve differently depending on rotation, composition,
1:55:06
Speaker 1: mass loss history, magnetic fields, binary interactions. The final state
1:55:13
Speaker 1: depends on the journey. Stevenson 218's extreme size might be temporary,
1:55:19
Speaker 1: a phase it's passing through before contracting or exploding. Or
1:55:23
Speaker 1: it might represent the star's equilibrium state for this stage
1:55:27
Speaker 1: of evolution, maintained for tens of thousands of years. We
1:55:31
Speaker 1: can't tell from a single snapshot. We need long-term monitoring.
1:55:36
Speaker 1: Observations spanning decades or centuries to track changes in luminosity, radius,
1:55:43
Speaker 1: mass loss rate, surface composition. Only then can we reconstruct
1:55:48
Speaker 1: the full story. Some of this monitoring is already happening.
1:55:53
Speaker 1: Amateur astronomers track red supergiants photometrically, recording brightness variations over years.
1:56:02
Speaker 1: Professional surveys revisit these objects periodically, checking for changes. Space
1:56:08
Speaker 1: telescopes image them in infrared, penetrating the dust to see
1:56:13
Speaker 1: the central star. Radio observations detect masa emission from surrounding gas.
1:56:21
Speaker 1: tracing outflows and magnetic fields. Each technique adds another piece
1:56:27
Speaker 1: of information. Eventually, the accumulated data will paint a clearer picture.
1:56:33
Speaker 1: We'll understand how these stars evolve, what triggers their instabilities,
1:56:38
Speaker 1: how they lose mass, when they explode. We'll refine the
1:56:42
Speaker 1: theoretical models until they match observations. We'll push the boundaries
1:56:47
Speaker 1: of measurement until uncertainties shrink to acceptable levels. But that
1:56:53
Speaker 1: takes time. Decades, probably. Maybe longer. For now, Stevenson 218
1:57:01
Speaker 1: holds the title. Largest known star. A behemoth at the
1:57:06
Speaker 1: edge of possibility, defying theoretical predictions, challenging our understanding, and
1:57:13
Speaker 1: reminding us how much we still don't know. It's a
1:57:18
Speaker 1: placeholder in some sense, a temporary answer to a question
1:57:22
Speaker 1: that will keep evolving as technology improves and data accumulates.
1:57:27
Speaker 1: But it's also a real object, a physical star shining
1:57:31
Speaker 1: in the night sky, visible if you know where to
1:57:34
Speaker 1: look and have the right equipment. Its photons travel 18,900
1:57:41
Speaker 1: years to reach Earth, carrying information encoded in wavelengths of light.
1:57:47
Speaker 1: That information tells a story about stellar physics, about nuclear fusion,
1:57:52
Speaker 1: about the life cycle of matter in the universe. The
1:57:56
Speaker 1: story is incomplete, full of gaps and uncertainties. But it's
1:58:01
Speaker 1: being written, one observation at a time. And every observation
1:58:07
Speaker 1: refines our understanding of how these monsters behave. What makes
1:58:12
Speaker 1: them expand? What drives their mass loss? What determines their
1:58:17
Speaker 1: final fate? Think about the sheer scale of uncertainty involved. Stevenson,
1:58:23
Speaker 1: 2-18's radius could be off by hundreds of solar radii.
1:58:28
Speaker 1: Maybe it's 2000. Maybe 2300. The measurement techniques have margins
1:58:35
Speaker 1: wide enough to swallow multiple solar systems. That's not a criticism.
1:58:41
Speaker 1: It's just reality. These stars are incredibly distant. Their atmospheres
1:58:46
Speaker 1: are fuzzy, irregular, pulsating. They're surrounded by thick shells of
1:58:52
Speaker 1: ejected material. Getting precise measurements is genuinely difficult, Distance errors
1:59:00
Speaker 1: compound everything. If Stevenson 2 to 18 is actually closer
1:59:05
Speaker 1: or farther than 18,900 light years, all the derived properties change.
1:59:13
Speaker 1: Luminosity scales with distance squared. Get the distance wrong by
1:59:18
Speaker 1: 10% and luminosity shifts by 21%. That changes the inferred radius.
1:59:27
Speaker 1: Changes the evolutionary interpretation. Changes everything. Parallax measurements help. Gaia
1:59:34
Speaker 1: revolutionized distance estimates for many stars. But Stevenson 2-18 is
1:59:41
Speaker 1: too distant for reliable parallax. We're stuck using cluster membership,
1:59:46
Speaker 1: spectroscopic distances, other indirect methods. Each has assumptions. Each introduces uncertainty.
1:59:55
Speaker 1: Future missions might improve this, better infrared telescopes, interferometry with
2:00:01
Speaker 1: longer baselines, direct imaging of stellar surfaces, technology advances, measurements improve.
2:00:09
Speaker 1: What seems impossible today becomes routine tomorrow. But some limitations
2:00:15
Speaker 1: are fundamental. Light takes time to travel. We're always seeing
2:00:20
Speaker 1: these stars as they were thousands of years ago. By
2:00:24
Speaker 1: the time we observe them, they've already moved on, changed,
2:00:29
Speaker 1: evolved into something different. We're studying history, not the present.
2:00:35
Speaker 1: That temporal disconnect matters. If Stevenson 218 exploded today, we
2:00:42
Speaker 1: wouldn't know for 18,900 years. The star could be gone,
2:00:48
Speaker 1: dispersed into a nebula, and we'd still be analyzing it
2:00:52
Speaker 1: as if it existed. cataloging its properties, calling it the
2:00:57
Speaker 1: largest star. Meanwhile, reality has already moved forward. This happens
2:01:03
Speaker 1: constantly in astronomy. Every observation is archaeology. Every measurement is
2:01:09
Speaker 1: a fossil record. We piece together the past and extrapolate
2:01:14
Speaker 1: into the future. But the present remains forever out of reach.
2:01:19
Speaker 1: The philosophical implications are strange. When we ask what is
2:01:24
Speaker 1: the largest star, we're really asking what was the largest
2:01:27
Speaker 1: star thousands of years ago. The current title holder might
2:01:31
Speaker 1: not exist anymore, might have already detonated, might have contracted
2:01:36
Speaker 1: back down. We'll never know until the light reaches us.
2:01:40
Speaker 1: That's frustrating but unavoidable. The speed of light is finite.
2:01:46
Speaker 1: Distances are vast. Information propagates slowly. We work with what
2:01:52
Speaker 1: we have. And what we have is remarkable. Despite all
2:01:56
Speaker 1: the uncertainties, despite all the limitations, we've identified dozens of
2:02:02
Speaker 1: stellar behemoths. We've measured their properties within reasonable margins. We've
2:02:08
Speaker 1: constructed theories that explain their existence. We've predicted their futures.
2:02:14
Speaker 1: Stevenson 218 will likely explode. The physics demands it. Stars
2:02:21
Speaker 1: this massive, don't fade quietly. They don't shrink into white dwarfs.
2:02:28
Speaker 1: They reach a critical point where the core collapses, rebounds,
2:02:32
Speaker 1: and blows the entire star apart. Supernova. When? That's the
2:02:39
Speaker 1: impossible question. Could be tomorrow. Could be a million years.
2:02:44
Speaker 1: The timescales involved are so short compared to most stellar
2:02:48
Speaker 1: evolution that predicting the exact moment is hopeless. We can
2:02:53
Speaker 1: say soon on cosmic timescales, but that's not particularly useful.
2:02:59
Speaker 1: What we can say is this. When it happens, it'll
2:03:04
Speaker 1: be spectacular. A supernova this distant won't be visible to
2:03:08
Speaker 1: the naked eye, but telescopes will catch it. The light
2:03:13
Speaker 1: curve will tell us about the explosion mechanism. The spectrum
2:03:17
Speaker 1: will reveal what elements were created in the blast. The
2:03:21
Speaker 1: remnant will show us what's left behind. That data will
2:03:25
Speaker 1: be invaluable. Every supernova observation refines our models, tells us
2:03:31
Speaker 1: more about nuclear physics, shock waves, neutrino emission, gravitational collapse.
2:03:38
Speaker 1: These events are laboratories for extreme physics, conditions we can't
2:03:43
Speaker 1: replicate on Earth, pressures, temperatures, densities that exist nowhere else.
2:03:51
Speaker 1: Red supergiants produce Type IIp supernovae. The P stands for plateau.
2:03:57
Speaker 1: After the initial brightening, the light curve flattens for weeks
2:04:01
Speaker 1: or months before declining. This happens because the star's extended
2:04:07
Speaker 1: hydrogen envelope takes time to recombine and cool. The plateau
2:04:12
Speaker 1: phase is diagnostic, tells us about the progenitor's structure. Stevenson
2:04:19
Speaker 1: 218's supernovae. will likely fit this pattern. Massive hydrogen envelope.
2:04:25
Speaker 1: Gradual decline. Long plateau. But there could be surprises. Unusual
2:04:31
Speaker 1: mass loss history might create unexpected features. Circumstellar interaction could
2:04:37
Speaker 1: brighten the explosion. Binary effects might complicate things. We won't
2:04:43
Speaker 1: know until it happens. And when it does, astronomers worldwide
2:04:48
Speaker 1: will mobilize Telescopes will swing toward it. Spectroscopy will begin immediately.
2:04:55
Speaker 1: Monitoring will continue for years. Every photon will be analyzed.
2:05:01
Speaker 1: The remnant will persist for millennia. A expanding shell of
2:05:05
Speaker 1: gas enriched with heavy elements. Oxygen, silicon, iron, all the
2:05:12
Speaker 1: products of nuclear synthesis scattered into space. That material will
2:05:17
Speaker 1: eventually form new stars, new planets, new possibilities. This is
2:05:23
Speaker 1: how the universe recycles itself. Massive stars forge heavy elements.
2:05:28
Speaker 1: Supernovae distribute them. New generations of stars incorporate that enriched material.
2:05:35
Speaker 1: The process continues, generation after generation, increasing the metallicity of
2:05:41
Speaker 1: the galaxy. Our solar system contains elements from supernovae. The
2:05:47
Speaker 1: iron in your blood was created in a stellar core
2:05:51
Speaker 1: and expelled in an explosion billions of years ago. You're
2:05:55
Speaker 1: made of star stuff, literally. The atoms in your body
2:06:00
Speaker 1: were forged in nuclear furnaces and scattered across space. Stevenson
2:06:07
Speaker 1: 218 will contribute to that cycle. When it explodes, Its
2:06:11
Speaker 1: enriched material will spread outward at thousands of kilometers per second.
2:06:17
Speaker 1: Some of it will encounter dense molecular clouds. Compression from
2:06:22
Speaker 1: the shock wave might trigger new star formation, and those
2:06:27
Speaker 1: new stars will contain atoms from Stevenson 218's interior. In
2:06:33
Speaker 1: a sense, the star will live on. Not as a
2:06:36
Speaker 1: coherent object, but as dispersed material incorporated into countless other systems.
2:06:43
Speaker 1: A legacy written in atomic nuclei across the galaxy. That's
2:06:47
Speaker 1: the ultimate fate of all massive stars. They don't persist.
2:06:52
Speaker 1: They transform. They become something else. Nebulae, neutron stars, black holes,
2:06:59
Speaker 1: but also future generations of stars and planets. The cycle
2:07:03
Speaker 1: is continuous. Stevenson 2-18 is just one participant in this
2:07:09
Speaker 1: grand process. Temporary, transient, doomed, but significant nonetheless. It represents
2:07:17
Speaker 1: an extreme case, a boundary condition, a test of theoretical limits.
2:07:23
Speaker 1: Its existence proves that stars can reach these sizes under
2:07:27
Speaker 1: the right conditions. That stellar evolution allows for such extremes.
2:07:32
Speaker 1: that the Eddington limit can be approached but not exceeded
2:07:36
Speaker 1: for extended periods. It also raises questions. Why aren't there
2:07:42
Speaker 1: even larger stars? What prevents them from growing indefinitely? The
2:07:48
Speaker 1: answer involves radiation pressure, mass loss, and instability. Beyond a
2:07:54
Speaker 1: certain point, the star can't hold itself together. Radiation blows
2:07:59
Speaker 1: off the outer layers faster than they can accumulate. The
2:08:03
Speaker 1: star reaches equilibrium or exceeds it and self-destructs. Stevenson 2-18
2:08:10
Speaker 1: might be near that limit. Its extreme mass loss suggests
2:08:14
Speaker 1: it's struggling to maintain stability. The radiation pressure is enormous
2:08:20
Speaker 1: The gravity barely contains it. One wrong move, one internal fluctuation,
2:08:26
Speaker 1: and the balance could tip. If it tips toward instability,
2:08:31
Speaker 1: mass loss accelerates. The star sheds its envelope rapidly. What
2:08:36
Speaker 1: remains might contract, heat up, evolve into a different type.
2:08:41
Speaker 1: Yellow hypergiant, luminous blue variable, wolf ray at star. Each
2:08:46
Speaker 1: stage has different characteristics. different appearances, different fates. Or it
2:08:52
Speaker 1: might tip toward collapse. The core reaches critical density. Fusion stops.
2:08:59
Speaker 1: Gravity wins. Collapse begins. In seconds, the core compresses into
2:09:04
Speaker 1: a neutron star or black hole. The outer layers fall inward,
2:09:09
Speaker 1: hit the rebounding core, and explode outward. Supernova. Both paths
2:09:16
Speaker 1: are possible. Both have precedent. WOHG 64's recent transformation toward
2:09:23
Speaker 1: yellow hypergiant status suggests the first path can happen. But
2:09:28
Speaker 1: most red supergiants probably follow the second. Explode rather than evolve.
2:09:34
Speaker 1: Statistical studies help. By observing many red supergiants, astronomers can
2:09:40
Speaker 1: determine which outcomes are more common. What percentage explode directly?
2:09:46
Speaker 1: What percentage evolve into other types first? The data is
2:09:50
Speaker 1: still incomplete, but patterns are emerging. Most red supergiants below
2:09:56
Speaker 1: a certain mass explode. Above that mass, evolution might intervene.
2:10:02
Speaker 1: The star might shed enough mass to avoid immediate collapse,
2:10:06
Speaker 1: might transform into something else, might delay the inevitable for
2:10:11
Speaker 1: thousands of years. Stevenson 218's fate depends on its current mass.
2:10:17
Speaker 1: If it's retained most of its original mass, it'll likely
2:10:20
Speaker 1: explode soon. If extreme mass loss has reduced it significantly,
2:10:26
Speaker 1: it might evolve instead. We don't know which scenario applies.
2:10:32
Speaker 1: Estimating current mass is difficult. Initial mass can be inferred
2:10:37
Speaker 1: from evolutionary models. But how much has been lost? The
2:10:41
Speaker 1: mass loss rate gives a clue. Multiply by the time
2:10:45
Speaker 1: spent in this phase. But we don't know that time precisely.
2:10:50
Speaker 1: Could be ten thousand years. Could be a hundred thousand.
2:10:55
Speaker 1: This uncertainty pervades everything. Every question about Stevenson 2-18 leads
2:11:02
Speaker 1: to more questions. Every answer comes with caveats. The star
2:11:08
Speaker 1: exists in a fog of unknowns. Yet that's what makes
2:11:12
Speaker 1: it interesting. If everything was certain, there'd be no mystery.
2:11:17
Speaker 1: no reason to keep observing, no discoveries waiting. The uncertainties
2:11:24
Speaker 1: are what drive research forward. Astronomers will keep watching Stevenson 2-18,
2:11:30
Speaker 1: monitoring its brightness, checking for changes, comparing it to models. Slowly,
2:11:38
Speaker 1: data will accumulate, understanding will deepen, and eventually, maybe decades
2:11:44
Speaker 1: from now, we'll have a clearer picture. Or it'll explode first.
2:11:49
Speaker 1: And then we'll learn everything in one dramatic event. Either way,
2:11:54
Speaker 1: the story continues. For now, it remains the largest known star.
2:12:00
Speaker 1: A record holder with an asterisk. A provisional answer to
2:12:05
Speaker 1: a question that might not have a permanent answer. Because
2:12:09
Speaker 1: the universe doesn't care about our categories. Doesn't care about
2:12:13
Speaker 1: records or rankings. It just is. Stars form, evolve, die.
2:12:20
Speaker 1: Some get big, some get enormous. A few push the
2:12:24
Speaker 1: absolute limits. Stevenson 218 is one of those few. A
2:12:29
Speaker 1: giant among giants. A behemoth at the edge of what's
2:12:33
Speaker 1: physically possible. And somewhere out there, beyond our current observational reach,
2:12:39
Speaker 1: there might be something even larger. A star we haven't
2:12:43
Speaker 1: discovered yet. one that dwarfs even Stevenson 218. It's possible.
2:12:50
Speaker 1: The universe is vast. Our surveys are incomplete. Future telescopes
2:12:56
Speaker 1: will find it if it exists. The James Webb Space
2:13:00
Speaker 1: Telescope is already revealing previously hidden objects. Next-generation ground-based telescopes
2:13:08
Speaker 1: will push further. Space-based infrared observatories will penetrate deeper into
2:13:13
Speaker 1: dust-shrouded regions. Each new capability brings new discoveries. The title
2:13:19
Speaker 1: of Largest Star might change hands multiple times in coming decades.
2:13:25
Speaker 1: New measurements will revise old estimates. New discoveries will challenge
2:13:30
Speaker 1: current records. That's how science works. Provisional answers replaced by
2:13:36
Speaker 1: better ones. But the fundamental question remains compelling. How large
2:13:42
Speaker 1: can a star actually get? What's the theoretical maximum? Where
2:13:48
Speaker 1: does physics draw the line? Models suggest an upper limit
2:13:52
Speaker 1: somewhere around 2,500 to 3,000 solar radii. Beyond that, instabilities dominate.
2:14:01
Speaker 1: Radiation pressure tears the star apart. Mass loss becomes catastrophic.
2:14:07
Speaker 1: The object can't maintain coherence. Stevenson 2-18 approaches that limit.
2:14:15
Speaker 1: It's in the neighborhood of maximum possible size. Whether it
2:14:19
Speaker 1: represents the true maximum, or just our current observational limit,
2:14:24
Speaker 1: remains to be seen. Either way, it's a spectacular object.
2:14:30
Speaker 1: A testament to the universe's ability to create extremes. a
2:14:34
Speaker 1: reminder that nature operates on scales that dwarf human comprehension.
2:14:39
Speaker 1: When you look up at the night sky, remember that
2:14:42
Speaker 1: up there, invisible to your eyes, are stars thousands of
2:14:47
Speaker 1: times larger than the sun, giants beyond imagining, objects so
2:14:53
Speaker 1: vast they redefine what large means. Stevenson 218 is one
2:14:59
Speaker 1: of them, and it's still out there, shining, evolving, approaching
2:15:05
Speaker 1: its inevitable end. So where does this leave us? Stevenson
2:15:11
Speaker 1: 218 sits nearly 19,000 light years away, radiating with the
2:15:16
Speaker 1: fury of 440,000 suns, stretching to a radius that would
2:15:22
Speaker 1: swallow Jupiter if it replaced our star. By current measurements,
2:15:27
Speaker 1: it's the largest known star in the universe. But that
2:15:31
Speaker 1: title comes with conditions. Distance uncertainties, measurement limitations, pulsation cycles,
2:15:40
Speaker 1: dust obscuration. Any of these could shift the numbers tomorrow.
2:15:45
Speaker 1: UY Scuti held the crown until better data cut it
2:15:49
Speaker 1: down to size. VY Canis Majoris dominated for decades before
2:15:55
Speaker 1: revisions knocked it from the throne. WOHG-64 might be larger
2:16:02
Speaker 1: if its recent transformation hasn't altered its structure too dramatically.
2:16:06
Speaker 1: The rankings shift with every new observation, every refined measurement,
2:16:12
Speaker 1: every satellite that improves our view of the cosmos. The
2:16:16
Speaker 1: real answer isn't a single star. It's a category. Red
2:16:21
Speaker 1: supergiants and hypergiants represent the absolute upper limit of stellar size.
2:16:27
Speaker 1: Physics won't allow much larger. The Eddington limit defines one boundary.
2:16:33
Speaker 1: Radiation pressure equals gravitational force. Cross that line and the
2:16:39
Speaker 1: star tears itself apart. Convection provides another constraint. Expand too
2:16:46
Speaker 1: far and energy transport breaks down. The envelope becomes unstable.
2:16:52
Speaker 1: Something has to give. Stevenson 2-18 exists right at these limits.
2:16:59
Speaker 1: It's operating in a regime where small changes trigger dramatic effects.
2:17:04
Speaker 1: Mass loss accelerates. Pulsations intensify. The star dances on the
2:17:10
Speaker 1: edge of catastrophe, held together by forces that barely balance.
2:17:15
Speaker 1: How long it maintains this precarious state is unknowable. Could
2:17:20
Speaker 1: be millennia. Could be tomorrow. When it ends, we'll learn
2:17:25
Speaker 1: something fundamental. If it explodes as a supernova, we'll measure
2:17:30
Speaker 1: the properties of one of the most massive stellar detonations
2:17:35
Speaker 1: in galactic history. If it collapses directly to a black hole,
2:17:39
Speaker 1: that tells us about the upper limits of supernova physics.
2:17:44
Speaker 1: If dust hides the event, we'll understand why so many
2:17:48
Speaker 1: massive stars seem to disappear without visible explosions. Each outcome
2:17:53
Speaker 1: provides data. Each observation refines models. The accumulated evidence from Stevenson,
2:18:03
Speaker 1: VY Canis Majoris, Betelgeuse, and dozens of others gradually paints
2:18:12
Speaker 1: a clearer picture of how the largest stars live and die.
2:18:16
Speaker 1: But here's what matters most. These aren't just astronomical curiosities.
2:18:23
Speaker 1: They're cosmic engines. They forge heavy elements in their cores.
2:18:28
Speaker 1: They scatter those elements across space when they explode. They
2:18:33
Speaker 1: trigger new generations of star formations. They shape the chemical
2:18:38
Speaker 1: evolution of galaxies. The iron in your blood came from
2:18:43
Speaker 1: a supernova. The oxygen you breathe was created in a
2:18:47
Speaker 1: massive star's core. The calcium in your bones, the carbon
2:18:52
Speaker 1: in your cells, all of it originated in stellar furnaces
2:18:57
Speaker 1: like Stevenson 218. When it explodes, it'll contribute to that cycle.
2:19:04
Speaker 1: Material from its interior will spread across light years, eventually
2:19:08
Speaker 1: incorporating into new stars, new planets, maybe even new life.
2:19:14
Speaker 1: You're made of star stuff. That's not poetic metaphor. It's
2:19:18
Speaker 1: literal fact. The atoms comprising your body were forged billions
2:19:23
Speaker 1: of years ago in stars that no longer exist. They
2:19:27
Speaker 1: were scattered by supernovae. They drifted through space. They accumulated
2:19:33
Speaker 1: in a molecular cloud that collapsed to form our solar system,
2:19:38
Speaker 1: and now they're temporarily arranged into the pattern you call yourself.
2:19:43
Speaker 1: Stevenson 218 will follow the same path. Its atoms will disperse.
2:19:49
Speaker 1: They'll become part of future systems. The star itself will vanish,
2:19:55
Speaker 1: but its material persists, recycled endlessly through cosmic generations. The
2:20:02
Speaker 1: question of whether it's truly the largest star becomes almost
2:20:05
Speaker 1: irrelevant in that context. What matters is what it teaches
2:20:09
Speaker 1: us about stellar physics, about the limits of size and luminosity,
2:20:14
Speaker 1: about how matter behaves under extreme conditions. What matters is
2:20:19
Speaker 1: the role it plays in the grand cycle of cosmic evolution.
2:20:24
Speaker 1: Future discoveries will probably find larger candidates. Better measurements will
2:20:30
Speaker 1: revise current estimates. The title will change hands again. That's expected.
2:20:36
Speaker 1: That's how science works. But the fundamental understanding we've gained
2:20:42
Speaker 1: from studying these objects endures. We know stars can reach
2:20:46
Speaker 1: truly staggering sizes under the right conditions. We know they
2:20:51
Speaker 1: can't grow indefinitely. We know they're unstable, short-lived, and violent.
2:20:59
Speaker 1: We know they're crucial for galactic chemical evolution. We know
2:21:03
Speaker 1: they represent the endpoints of massive stellar evolution. Stevenson 218
2:21:10
Speaker 1: embodies all of this. It's a laboratory for extreme physics,
2:21:15
Speaker 1: a natural experiment running at scales we can't replicate, a
2:21:20
Speaker 1: window into processes that shape the universe and made life possible.
2:21:26
Speaker 1: So yes, it's probably the largest star we currently know about.
2:21:31
Speaker 1: But more importantly, it's a reminder that the universe operates
2:21:36
Speaker 1: on scales that dwarf human comprehension. That nature creates extremes
2:21:43
Speaker 1: we're only beginning to understand. That every answer leads to
2:21:48
Speaker 1: new questions. Somewhere out there, beyond our current observational limits,
2:21:53
Speaker 1: there might be something even larger. A star so massive
2:21:57
Speaker 1: it makes Stevenson 218 look modest. We haven't found it yet.
2:22:02
Speaker 1: Maybe we never will. But the search continues, because that's
2:22:07
Speaker 1: what drives astronomy forward. Not the answers, but the questions.
2:22:13
Speaker 1: Not the certainty, but the mystery. Not knowing exactly where
2:22:17
Speaker 1: the boundaries lie, but pushing against them anyway, seeing how
2:22:22
Speaker 1: far we can go. Stevenson 2-18 represents one boundary, the
2:22:28
Speaker 1: edge of stellar size, the limit of what current physics predicts.
2:22:33
Speaker 1: Whether it's the true maximum, or just our current understanding
2:22:37
Speaker 1: remains to be seen. Either way, it's out there right now,
2:22:42
Speaker 1: shining across 18,900 light-years of space, shedding mass into the void,
2:22:50
Speaker 1: approaching its inevitable end. And we're fortunate enough to witness it,
2:22:55
Speaker 1: to measure it, to marvel at what the universe is
2:22:58
Speaker 1: capable of creating. The largest known star. A behemoth at
2:23:05
Speaker 1: the limits of possibility. A temporary answer to an eternal question.
2:23:11
Speaker 1: And a reminder that no matter how much we learn,
2:23:15
Speaker 1: there's always more to discover.