The Biggest Stars in the Universe: A Sun So Vast It Could Swallow Saturn's Orbit

The largest known stars are red supergiants and hypergiants like Stephenson 2-18 and UY Scuti, estimated at over 1,500 times the radius of our Sun. Place one where the Sun currently sits, and its surface would swallow the four inner planets and reach out somewhere between the orbits of Jupiter and Saturn. Yet these giants are surprisingly lightweight for their size, and that contradiction is the whole story: in space, the biggest star by volume is almost never the biggest star by mass, and the record for "biggest" keeps changing as astronomers get better at measuring stars that sit thousands of light-years away.

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The short answer: largest known stars by radius

The table below lists the stars most commonly cited as the largest known, ranked by estimated radius in multiples of the Sun's radius (one solar radius, symbol R☉, equals about 696,000 kilometers, or 432,000 miles). Every single value here should be read as an estimate with a real error bar attached, for reasons explained in the next section.

Table 1. The largest known stars by estimated radius, with their major caveats

Star	Radius (× Sun)	Type	Distance	Key caveat
Stephenson 2-18	~2,150 (disputed)	Red supergiant	~19,000 light-years	Estimate rests on an unusually large and uncertain distance figure; widely viewed as an overestimate
WOH G64	~2,000, historically	Red supergiant	~160,000 light-years (Large Magellanic Cloud)	First-ever direct interferometric image (2024) shows a dust cocoon that complicates the radius measurement
UY Scuti	~1,700 historically; ~755 in revised Gaia-based distance work	Red hypergiant	~9,500 light-years	The "record holder" figure predates a significant downward revision of its distance
VY Canis Majoris	~1,420 (down from earlier estimates of 1,800–2,100)	Red hypergiant	~3,840 light-years	Pulsates and sheds mass, so its apparent radius changes over time
Betelgeuse	~764 (range 702–880)	Red supergiant	~548 light-years	Best-studied of the group because it is nearby and naked-eye visible

Radii are drawn from published distance and angular-diameter measurements, which is exactly why they disagree with one another; see the next section for why.

Notice something in that table before moving on: the numbers most people quote for UY Scuti and Stephenson 2-18 are not settled facts. They are the products of specific measurement campaigns, and in UY Scuti's case, a later distance revision cut the widely repeated figure by more than half. This is the central lesson of "biggest star" rankings, and it is worth sitting with before looking at any individual star in detail.

UY Scuti is still described in most popular articles as roughly 1,700 times the Sun's radius. But a re-derived distance based on more precise parallax data would shrink that to around 755 solar radii — still enormous, but less than half the commonly cited figure. The "record" changes depending on which distance measurement you trust, not because the star itself changed size.

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Why measuring the biggest stars is so hard

Here is the problem in plain terms: to know how big a star actually is, you need two numbers, and both are difficult to pin down at interstellar distances.

The first number is the star's angular diameter, meaning how wide it appears to look from Earth, measured in tiny fractions of a degree called milliarcseconds. Even the largest stars are so far away that they appear as little more than a point of light to ordinary telescopes. Astronomers get around this with interferometry, a technique that combines light from multiple telescopes spaced apart to synthesize the resolving power of one enormous telescope. It is the same principle the Event Horizon Telescope used to image a black hole's shadow: instead of one dish being big enough to resolve the target, several smaller dishes are combined so their combined baseline does the job.

The second number is the star's distance, and this is where most of the uncertainty creeps in. A star that looks a certain angular size could be a smaller star that is closer, or a bigger star that is farther away — the two possibilities are impossible to tell apart from angular size alone. Distances to stars thousands of light-years away are typically derived from parallax (the tiny apparent shift in a star's position as Earth orbits the Sun), and the European Space Agency's Gaia mission has spent the last decade refining these measurements across the galaxy. When a Gaia-based distance estimate replaces an older one, the calculated radius can swing dramatically, even though nothing about the star itself has changed.

That is exactly what happened to UY Scuti. Early estimates that produced its famous 1,700-solar-radii figure relied on distance figures that later parallax work suggested were too large. Apply a shorter, more confident distance to the same angular diameter, and the calculated radius drops to roughly 755 solar radii, according to more recent work using Gaia data. The star did not shrink. Our confidence in how far away it is changed, and radius is a direct function of distance.

How do astronomers measure the size of a star?

Astronomers combine two measurements: the star's angular diameter (its apparent width in the sky, found through interferometry, which merges light from separated telescopes into one high-resolution image) and its distance (typically from parallax). Multiplying angular diameter by distance yields the physical radius. Because both quantities carry uncertainty, and distant supergiants also pulsate and shed gas that blurs their apparent edge, radius estimates for the largest stars often carry error bars of 20 percent or more.

This is also why a genuinely new kind of evidence made 2024 a milestone year for this topic. In November 2024, a team using the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer captured the first-ever resolved, interferometric image of a red supergiant outside the Milky Way: WOH G64, sitting roughly 160,000 light-years away in the Large Magellanic Cloud. Instead of inferring a disc shape from indirect measurements, the team could see the star's immediate surroundings directly, and what they found reframed the whole picture: an elongated, egg-shaped cocoon of dust and gas wrapped tightly around the star, likely material the star has been shedding as it nears the end of its life. That cocoon does not just look striking; it actively complicates the question of where the "star" ends and the "shed material" begins, which is precisely the kind of ambiguity that makes radius rankings for the biggest stars so slippery.

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How big is UY Scuti, really?

UY Scuti, a red hypergiant in the constellation Scutum roughly 9,500 light-years away, became the poster child for "biggest star" articles across the internet for a simple reason: at its long-cited size of about 1,700 solar radii, it made for an extraordinary visual. To grasp the scale, imagine swapping it in for our Sun. Its glowing surface would engulf Mercury, Venus, Earth, and Mars, then keep expanding outward, past the asteroid belt, past Jupiter, reaching somewhere in the neighborhood of Saturn's orbit depending on which radius estimate you use.

Light, which crosses our actual Sun's diameter in about 4.6 seconds, would take hours to cross a star that size. You could, in principle, fit well over a billion Suns inside a sphere that large by volume alone.

And yet — this is the twist that makes the story worth telling rather than just repeating a number — UY Scuti contains only around 7 to 10 times the Sun's mass, according to standard stellar models for hypergiants of its type. Compare that to its bloated radius, and the implication is almost the opposite of what "biggest star" suggests intuitively: this is one of the least dense large objects known to astronomy. Its outer layers are thinner than the best vacuum humans have ever produced in a laboratory on Earth. If you could somehow scoop up a cubic meter of UY Scuti's outer atmosphere and weigh it, it would register as essentially nothing.

That contradiction is worth sitting with, because it reframes what "biggest" even means. UY Scuti earns a spot among the size-record holders featured on our page about the most massive objects in the universe purely because of its radius, not because of what it weighs. Size and mass are different axes entirely, and confusing them is the single most common mistake people make when talking about the biggest things in space.

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Why the biggest stars are so puffed up and thin

Red supergiants and hypergiants earn their enormous radii by dying, not by growing in the way a living organism grows. A star like the Sun spends most of its life in a stable balance: gravity pulls inward, and the energy released by nuclear fusion in the core pushes outward, and for billions of years those two forces cancel out almost perfectly.

That balance breaks once a massive star exhausts the hydrogen fuel in its core. The core contracts and heats up, igniting fusion of heavier elements in shells around it, and this new burning dumps far more energy into the star's outer layers than the old, stable hydrogen-burning phase did. The outer envelope responds by expanding, sometimes by a factor of hundreds, while cooling dramatically as it spreads out over an enormously larger surface area. A cooler surface radiates redder light, which is why the biggest stars glow red or orange rather than the white-hot blue of a young, compact, massive star.

Think of it like a marshmallow held near a campfire. Heat it gently and it puffs outward, growing dramatically in size while what's inside becomes lighter and more porous, not denser. The mass hasn't gone anywhere; it's just spread across a much larger, much thinner volume. A red hypergiant works the same way at a cosmic scale: the same handful of solar masses that made up a compact blue supergiant gets redistributed across a volume that could swallow the inner solar system, and the average density plummets accordingly.

This single mechanism is why size and mass diverge so sharply among the biggest known stars:

• Biggest by radius: bloated, cooling, dying red supergiants and hypergiants such as Stephenson 2-18, WOH G64, and UY Scuti — objects that are enormous but comparatively light for their volume.
• Biggest by mass: compact, ferociously hot, young blue stars such as R136a1, which packs far more mass into a body only a few dozen times the Sun's radius.

R136a1 is the heavyweight champion. UY Scuti and its red hypergiant relatives are the size champions. These are different competitions with different winners, in much the same way that the tallest buildings in the world are not necessarily the ones that weigh the most; height and mass measure different things about a structure, and the same is true of stars.

Why are the biggest stars red instead of blue?

A star's color reflects its surface temperature: hotter surfaces glow blue-white, cooler surfaces glow red. When a massive star exhausts its core hydrogen, its outer envelope swells to hundreds or thousands of times its original size and, in the process, spreads its heat across a far larger area, cooling the surface. That cooling shifts the star's glow toward red, which is why nearly every record-holder for "biggest star" is a red supergiant or hypergiant rather than a blue one.

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Meet R136a1: the heaviest star, and why it isn't the biggest

R136a1 sits inside the R136 star cluster, itself embedded in the Tarantula Nebula in the Large Magellanic Cloud, roughly 160,000 light-years from Earth. It has held the title of most massive known star since a 2010 study led by astronomer Paul Crowther at the University of Sheffield, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, calculated an eye-popping initial mass as high as 320 times the Sun's mass for the star.

That original figure has since been revised down. A 2020 reanalysis by Bestenlehner and colleagues, incorporating updated stellar-wind models and opacities, pushed the plausible upper bound for the most massive stars closer to about 200 solar masses, and subsequent work using newer spectroscopic methods estimates R136a1's current mass at somewhere between roughly 170 and 230 times the Sun's mass. Even at the lower end of that range, R136a1 remains, by a wide margin, the most massive star astronomers have confidently identified.

What is notable is how small R136a1 looks next to the red hypergiants in Table 1. It is a blue hypergiant, hot and compact by comparison, with a radius only tens of times the Sun's, not thousands. Squeeze that much mass into that comparatively modest volume and the result is a star with a density and surface temperature that would tear apart if it tried to expand the way UY Scuti has. It sits, essentially, near the theoretical ceiling of how much mass a single star can hold onto and still function as a star, which brings up the next question directly.

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The limit on how big — and how heavy — a star can be

Stars cannot grow without limit, in either direction. There is an upper mass limit, and it comes down to a balance discovered by the British astrophysicist Arthur Eddington in the early twentieth century: the Eddington limit, the maximum luminosity, or light output, a star can sustain while gravity and outward radiation pressure stay in equilibrium. Eddington's original calculation put the ceiling for stability at around 60 solar masses. Later refinements, incorporating more realistic opacities (how efficiently stellar material absorbs and scatters radiation) and solar-like chemical compositions, raised that practical ceiling to somewhere around 120 to 150 solar masses.

Push a star's mass higher than that limit while it is forming, and the star's own light becomes so intense that radiation pressure physically blows the surrounding gas away before it can fall in and add to the star's mass. Effectively, the star's own brilliance caps its own growth. That is the leading explanation for why the observed mass record, R136a1's roughly 170 to 230 solar masses, sits close to, but not dramatically beyond, the Eddington limit's theoretical range: any star that tried to accumulate significantly more mass would have blown away its own building material long before finishing the job.

Size has its own limit, but it operates on a completely different principle. Red supergiants and hypergiants at the top of the radius charts are inherently unstable. Their outer layers are so loosely bound to the star's gravity that they pulsate, brighten, dim, and shed enormous amounts of gas and dust into space, the same kind of dusty cocoon the 2024 VLTI observations caught around WOH G64 directly. These giants live only a few million years in this bloated phase, a blink of an eye compared to the roughly 10-billion-year lifespan of a Sun-like star, before ending in a supernova explosion. Big and short-lived go together: the more extreme the radius, the more unstable and fleeting the phase, and the biggest stars in any given era of the universe are, almost by definition, rare and doomed.

When these giants finally collapse, some become neutron stars; the most massive can collapse directly into black holes, joining the objects that rank among the most massive objects in the universe on an entirely different scale of density.

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Betelgeuse: the biggest star most people can actually point to

Every star discussed so far is thousands to hundreds of thousands of light-years away, effectively invisible without a telescope. Betelgeuse is different. At roughly 548 light-years away, it is the bright reddish-orange star marking the shoulder of the constellation Orion, visible to the naked eye across most of the inhabited world, and it is one of the best-measured red supergiants precisely because it is close enough to study in detail.

Betelgeuse's radius is usually cited around 764 solar radii, with a plausible range of roughly 702 to 880 solar radii depending on the measurement method and the moment it is measured, since like other red supergiants it pulsates. That is smaller than UY Scuti's historic figure and far smaller than Stephenson 2-18's, but it is still large enough that, placed at the Sun's position, Betelgeuse's surface would reach out past the orbit of Jupiter.

Betelgeuse made global headlines in late 2019 and early 2020 during what astronomers call the "Great Dimming," when the star's brightness dropped noticeably, fueling speculation it might be about to explode as a supernova. Follow-up study attributed the dimming to a combination of a temperature dip and a dust cloud the star had ejected, partially blocking its light from Earth's vantage point, not an imminent explosion. It remains a useful cautionary tale: even the best-studied giant star in the sky, sitting relatively next door in cosmic terms, still produces surprises and revised numbers as new instruments come online.

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What people get wrong about the biggest stars

The single most common error, repeated across countless "biggest star" listicles, is treating radius rankings as permanent facts rather than snapshots tied to a specific distance measurement. UY Scuti's 1,700-solar-radii figure is still the number most search results return, even though more recent distance work suggests something closer to half that. Stephenson 2-18's widely cited 2,150-solar-radii figure rests on a distance estimate that many astronomers now consider likely too large, meaning the "current record holder" title is genuinely contested rather than settled.

The second common error is conflating "biggest" with "heaviest," treated as interchangeable in casual writing. They are not. A star's radius tells you how much space it occupies; its mass tells you how much material it contains. Because the biggest stars by radius are bloated, low-density giants near the end of their lives, and the heaviest stars by mass are compact, young, blue giants, the two records almost never land on the same object. If an article calls one star both "the biggest" and "the heaviest" without qualification, that is a signal to check its sourcing.

A third, subtler error is assuming that "biggest known star" means "biggest star that exists." These rankings only reflect stars astronomers have measured well enough to publish a radius for, almost all of which sit within our own galaxy or its nearest satellite galaxies, the Large and Small Magellanic Clouds. Most of the observable universe's stars are simply too far away, or too obscured by dust, for any current instrument to resolve their size directly. The record, in other words, describes the largest star we have successfully measured, not necessarily the largest star that has ever existed.

Finally, people often assume a bigger radius automatically means a brighter, more luminous star. That is usually true in a broad sense, since luminosity scales strongly with surface area, but a red giant's cooler surface partially offsets its enormous size. Compact, blazing-hot blue giants like R136a1 can outshine far larger, cooler red hypergiants per unit of surface area, which is part of why R136a1 also holds records for luminosity despite its comparatively modest radius.

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Frequently Asked Questions

Q: What is the biggest star in the universe? A: Among stars astronomers have actually measured, red supergiants and hypergiants such as Stephenson 2-18 and UY Scuti are usually cited as the largest by radius, at roughly 1,500 to 2,150 times the Sun's radius depending on which distance measurement is used. Placed at the Sun's position, one would extend out toward Jupiter's or Saturn's orbit. There is no single settled answer because measuring the exact size of a star thousands of light-years away carries substantial uncertainty, and UY Scuti's own figure has been revised down significantly in more recent Gaia-based distance work.

Q: How big is UY Scuti compared to the Sun? A: The commonly cited figure is about 1,700 times the Sun's radius, but a revised distance estimate using more recent parallax data puts it closer to 755 solar radii. Even at the lower figure, if UY Scuti replaced the Sun, its surface would swallow Mercury, Venus, Earth, and Mars. It contains only around 7 to 10 times the Sun's mass, because its outer layers are enormously bloated and extremely thin.

Q: Is the biggest star also the heaviest? A: No, and this is the most important thing to understand about these records. The largest stars by radius are bloated, low-density red supergiants and hypergiants like UY Scuti and Stephenson 2-18. The most massive known star, R136a1, is far smaller in radius but carries an estimated 170 to 230 times the Sun's mass. Radius and mass are separate physical properties, and among the record-holding stars, they never belong to the same object.

Q: How massive can a star be? A: Roughly 120 to 150 solar masses is the theoretical ceiling set by the Eddington limit, the point at which a forming star's own radiation pressure becomes strong enough to blow away additional infalling gas before it can add to the star's mass. R136a1, the most massive star known, sits at an estimated 170 to 230 solar masses, near the upper edge of what current models consider physically achievable.

Q: Why are the biggest stars red instead of blue or white? A: Because they are dying. As a massive star exhausts the hydrogen fuel in its core, its outer envelope swells hugely and, in spreading over a far larger surface area, cools down. Cooler surfaces glow red rather than the blue-white of a young, hot, compact star, which is why nearly every record-holder for stellar size is a red supergiant or hypergiant.

Q: How do astronomers measure the size of a star that far away? A: They combine two measurements: the star's angular diameter, found using interferometry, a technique that combines light from multiple telescopes to achieve much higher resolution than any single instrument, and the star's distance, usually derived from parallax data such as that provided by the European Space Agency's Gaia mission. Multiplying those together yields a physical radius, but both inputs carry uncertainty, which is why radius estimates for the biggest stars shift as measurements improve.

Q: What happened with the 2024 image of WOH G64? A: In November 2024, astronomers using the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer captured the first-ever resolved image of a red supergiant outside the Milky Way, imaging WOH G64 in the Large Magellanic Cloud roughly 160,000 light-years away. The image revealed an elongated, egg-shaped cocoon of dust closely surrounding the star, evidence of significant mass loss late in its life, which complicates efforts to pin down its exact radius.

Q: How long do the biggest stars live? A: The bloated red supergiant and hypergiant phase that produces the largest radii typically lasts only a few hundred thousand to a few million years, a tiny fraction of the roughly 10-billion-year lifespan of a Sun-like star. These giants are unstable, pulsating and shedding mass, before ending their lives in a supernova explosion, which is part of why they are both enormous and rare at any given moment.

Q: What happens when the biggest stars die? A: Stars in this size and mass range typically end in a supernova explosion, collapsing their core into either a neutron star or, for the most massive progenitors, a black hole. Some of the resulting black holes eventually grow into the supermassive objects that anchor galaxies, joining the ranks of the most massive objects in the universe.

Q: Could there be a bigger star out there that we haven't found yet? A: Almost certainly. Current radius records only cover stars astronomers have been able to measure directly, which is overwhelmingly limited to the Milky Way and its nearest satellite galaxies, such as the Large Magellanic Cloud. The vast majority of stars in the observable universe are too distant or too obscured by interstellar dust for any current instrument to resolve their size, so today's "biggest known star" describes the limits of measurement as much as it describes the universe itself.

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The takeaway

The biggest stars in the universe earn their size not by burning brighter or living longer, but by dying: a massive star exhausts its core hydrogen, its outer layers billow outward by a factor of hundreds, and the same handful of solar masses that once made a compact blue giant gets spread across a volume that could swallow the inner solar system. That is why the size record and the mass record point to completely different stars, UY Scuti's family of bloated red hypergiants against R136a1's dense blue heavyweight, and why neither record is as fixed as it looks in a headline. Every figure in this article rests on a distance measurement made from thousands or hundreds of thousands of light-years away, and better instruments, from Gaia's parallax data to 2024's first direct image of a supergiant outside our galaxy, keep revising those distances and, with them, the numbers themselves.

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Read next:

• The Most Massive Objects in the Universe
• The Most Extreme Planets Ever Discovered
• The Most Extreme Temperatures in the Universe

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Sources

• Astronomy & Astrophysics: "Imaging the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud" (2024)
• ESO: "VLTI Captures Detailed Image of Red Supergiant in Large Magellanic Cloud"
• Professor Paul Crowther (University of Sheffield): "R136a1 – FAQs"
• ESO: "Stars Just Got Bigger — A 300 Solar Mass Star Uncovered" (2010 R136a1 discovery announcement)
• NOIRLab: "Sharpest Image Ever of Universe's Most Massive Known Star" (Gemini Observatory)
• Britannica: "Eddington mass limit"
• arXiv: "Betelgeuse, the Prototypical Red Supergiant" (review)
• arXiv: "Betelgeuse: a Review"
• NASA and ESA public data on stellar radius, mass, and Gaia parallax measurements

Bottom line: UY Scuti, Stephenson 2-18, and their fellow hypergiants are the biggest stars by sheer size, vast enough to drown the inner solar system, but they're puffed-up lightweights compared to R136a1, a reminder that "biggest" and "heaviest" almost never describe the same star, and that even the size record itself keeps moving as our distance measurements improve.

Last verified: July 2026