Latest / Boring Science / Behemoth _ Is This the Largest Star in the Universe

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