Roughly five billion Suns could fit inside UY Scuti, one of the largest stars ever measured. That staggering number follows from the best available estimate of UY Scuti’s radius, about 1,708 times the Sun’s, and from the basic geometry of spheres, where volume grows with the cube of the radius. But the precision of that figure hides genuine scientific uncertainty, and the star itself turns out to be a far stranger and more fragile object than a simple size comparison lets on.
How the Number Works
The calculation behind “five billion Suns” is straightforward once you know the radius. The volume of any sphere scales with the cube of its radius. If UY Scuti’s radius is about 1,708 times the Sun’s, then its volume is roughly 1,708 × 1,708 × 1,708 times the Sun’s volume. That multiplication comes out to approximately 4.98 billion, which rounds neatly to about five billion. Each of those “Suns” in the thought experiment is a sphere the size of our actual Sun packed, marble-like, into the interior of UY Scuti’s bloated envelope.
The 1,708 figure comes from observations made with the Very Large Telescope Interferometer in Chile, published in a 2013 study that characterized the atmospheric structure and fundamental parameters of UY Scuti alongside two other red supergiants.1Astronomy & Astrophysics. The atmospheric structure and fundamental parameters of the red supergiants AH Scorpii, UY Scuti, and KW Sagittarii That measurement has become the go-to number in popular science, but it sits at the high end of plausible values. A different set of assumptions about the star’s temperature or distance could push the radius up or down by hundreds of solar radii, which would shift the “how many Suns” figure by billions in either direction. Five billion is the headline answer, but it carries a wide margin of error.
What 1,700 Solar Radii Looks Like
Numbers in the billions lose their meaning quickly, so it helps to drop UY Scuti into a more familiar setting. If you placed UY Scuti where the Sun is, its outer edge would swallow Mercury, Venus, Earth, Mars, and Jupiter, and extend most of the way to Saturn’s orbit. The light from its surface, already enormously bright, would bathe the remaining outer planets in a glow roughly 300,000 times more luminous than what the Sun produces. Saturn, Uranus, and Neptune would be the only planets left orbiting outside the star rather than inside it.
That mental image also helps clarify what “fitting Suns inside” really means. The five billion figure assumes you could somehow pack Sun-sized spheres into every corner of UY Scuti’s volume. In practice, spheres do not pack perfectly; there are always gaps. Random packing of identical spheres fills about 64 percent of the available space. So if you were literally rolling Sun-sized balls into UY Scuti’s interior, you’d fit closer to three billion before running out of room. The five-billion figure is the pure volume ratio, not a packing estimate, but it is the number that appears in almost every discussion of the topic.
Why the Radius Is So Hard to Measure
UY Scuti sits about 5,100 light-years away in the constellation Scutum, close to the plane of the Milky Way where dust and gas along the line of sight complicate observations. Measuring the radius of a distant star starts with measuring its angular diameter, the tiny angle the star’s disk subtends on the sky, and then combining that with a distance estimate to get a physical size. For UY Scuti, the angular diameter has been measured using optical interferometry, a technique that links multiple telescopes to achieve the resolution of a much larger instrument.2The Astronomical Journal. Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer Even small errors in either the angular diameter or the assumed distance compound into large uncertainties in the derived radius.
There is also a subtler problem. The “edge” of UY Scuti is not a sharp boundary. Unlike the Sun, whose visible surface is relatively well-defined, a red supergiant has an extended, diffuse atmosphere that fades gradually into space. The point where you declare the star ends depends on how you define the photosphere, and different wavelengths of light suggest different answers. At shorter wavelengths the star can appear measurably smaller than at longer ones, because the opacity of the atmosphere varies.3The Astrophysical Journal. The Temperatures of Red Supergiants This means the radius of UY Scuti is not a single fixed number but a range that depends on how and where you draw the line. The 1,708 figure represents one plausible measurement, not a settled consensus.
Distance uncertainty adds another layer. UY Scuti’s distance has been estimated using several methods, and revisions to the distance would directly rescale the radius. If the star turned out to be somewhat closer than currently assumed, its physical radius would shrink proportionally, and the “number of Suns” answer would drop by billions. This is not a hypothetical concern; distances to stars embedded in the dusty Galactic plane are among the most contested measurements in stellar astronomy.
Enormous but Astonishingly Light
Here is the part that surprises most people: UY Scuti’s mass is estimated at only about seven to ten times the mass of the Sun. A star with five billion times the Sun’s volume but only around ten times its mass has an average density that is almost incomprehensibly low. If you do the division, UY Scuti’s mean density works out to something on the order of a millionth of the Sun’s density, far thinner than the air you’re breathing. Large parts of UY Scuti’s envelope would qualify as a better vacuum than what most laboratories on Earth can produce.
This extreme fluffiness is a defining feature of red supergiants. As a massive star exhausts hydrogen in its core and begins fusing heavier elements, the core contracts while the outer layers expand dramatically. The envelope is held up in part by radiation pressure, the outward push of photons generated in the star’s energy-producing interior. In massive stars, this radiation support is significant enough to inflate the envelope to extraordinary dimensions without requiring much material to fill the space.4Monthly Notices of the Royal Astronomical Society. Common envelopes in massive stars: towards the role of radiation pressure and recombination energy in ejecting red supergiant envelopes The result is a star that is gigantic in radius but modest in mass, more like a luminous fog surrounding a dense core than a solid body scaled up from the Sun.
This matters for the “fitting Suns inside” thought experiment because those five billion Suns, if they were real objects with the Sun’s actual mass, would collectively weigh five billion solar masses. UY Scuti’s entire envelope weighs roughly ten. The interior of UY Scuti is overwhelmingly empty space, not star-stuff. Saying five billion Suns “fit” inside is geometrically true but physically misleading if it conjures an image of dense, roiling plasma throughout. Most of that volume is barely there.
A Star That Is Slowly Disassembling Itself
Red supergiants like UY Scuti do not hold onto their outermost layers very well. The combination of low surface gravity, high luminosity, and cool surface temperatures creates conditions for vigorous mass loss. The outer atmosphere is cool enough for dust grains to form, and radiation pressure from the star’s intense luminosity pushes those grains outward. Gas dragged along with the dust streams away into space as a stellar wind.5Understanding Stellar Evolution. Stellar Winds and Mass Loss Pulsations in the star’s atmosphere, which cause the star’s brightness to vary over months or years, help loft material to altitudes where dust can condense, feeding the process.
UY Scuti is classified as a semiregular variable, meaning its brightness fluctuates in a somewhat periodic pattern. These brightness changes reflect physical pulsations in the star’s outer layers, and they play a direct role in driving mass off the surface. Over the course of its remaining lifetime, UY Scuti will shed a substantial fraction of its envelope, gradually shrinking in radius. The material it ejects forms a circumstellar shell of gas and dust that can extend far beyond the star itself, adding to the interstellar medium. In a sense, the star you see today is smaller than it was a million years ago and larger than it will be a million years from now.
Mass-loss rates for extreme red supergiants can reach the equivalent of a Sun’s worth of material ejected every few tens of thousands of years. That may sound slow in human terms, but on stellar timescales it is rapid enough to strip the envelope down to a much smaller, hotter core. When that happens, the star may briefly appear as a different type of object, perhaps a blue supergiant or a Wolf-Rayet star, before ending its life as a supernova. The bloated red supergiant phase is temporary, which means UY Scuti’s current size is a snapshot of a fleeting moment in its evolution, not a permanent state.
What the Surface of a Red Supergiant Looks Like
If you could somehow get close enough to see UY Scuti’s surface in detail, it would look nothing like the Sun. The surface of a red supergiant is dominated by enormous convection cells, regions where hot gas rises from deeper layers, radiates energy, and sinks back down. On the Sun, these convection cells (called granules) are roughly 1,000 kilometers across, small enough that millions of them tile the visible disk. On a red supergiant, the same physical process operates at a vastly larger scale. Imaging studies of other red supergiants using interferometric techniques have revealed surface features with characteristic sizes around half to two-thirds of the star’s radius, with large structures persisting for over a year while smaller bright spots change on timescales of months.6arXiv. Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg
On UY Scuti, this would mean individual convection cells potentially spanning hundreds of solar radii, each one a churning plume of gas larger than the entire orbit of Mars. These features are not merely academic curiosities. They affect the star’s apparent brightness, shape, and even its measured size. A giant bright spot on one side of the star can shift the measured center of light, making the star appear to wobble or change position slightly from year to year. The surface is so dynamic and irregular that calling UY Scuti a “sphere” is already a simplification. At any given moment, it is a lumpy, roiling, partially transparent envelope of gas with no well-defined edge, which circles back to the difficulty of assigning it a single precise radius.
Is UY Scuti Actually the Largest Known Star?
UY Scuti earned its fame in the mid-2010s as popular science content crowned it the biggest star ever discovered. That claim was always more fragile than the headlines suggested. The 1,708-solar-radii measurement is one estimate from one study, and it depends on assumptions about UY Scuti’s distance and effective temperature that other researchers have questioned. Some reanalyses of the same data, using updated distance estimates or different temperature models, produce a smaller radius, potentially around 750 to 1,000 solar radii. That would still make UY Scuti enormous, but it would drop it from the top of the largest-star lists.
Meanwhile, other red supergiants and hypergiants have emerged as contenders. Several stars in the Milky Way and the Large Magellanic Cloud have estimated radii in the range of 1,400 to 2,100 solar radii, depending on which measurements and assumptions you trust. The uncertainty bars on all of these stars overlap substantially, which means the question of which single star is “the biggest” may not have a meaningful answer with current data. It is a bit like trying to determine which cloud in the sky is the largest when all of them are shifting shape and you are measuring them from a great distance through fog.
For the purposes of the “how many Suns” question, this matters a lot. If UY Scuti’s true radius is closer to 1,000 solar radii rather than 1,700, then the volume drops to about one billion solar volumes instead of five billion. The cube relationship between radius and volume means that even a modest downward revision in the radius has a dramatic effect on the volume calculation. The honest answer to “how many Suns fit inside UY Scuti” is somewhere between one billion and five billion, with the commonly quoted five billion representing the upper end of current estimates.
How Red Supergiants Will Eventually End
UY Scuti will not stay this large forever. Stars in its mass range are expected to end their lives as core-collapse supernovae, events in which the iron core of the star can no longer support itself against gravity and collapses in a fraction of a second. The resulting explosion blows the remaining envelope into space at thousands of kilometers per second, creating an expanding nebula of enriched gas and leaving behind either a neutron star or, if the core is massive enough, a black hole.
Whether UY Scuti will explode as a red supergiant or will have shed enough of its envelope to appear as something smaller and hotter at the moment of collapse is an open question. Some massive stars seem to undergo dramatic eruptions in the centuries or decades before they explode, losing large amounts of mass in short bursts. Others quietly puff away their envelopes through steady winds. The final appearance of UY Scuti at the moment of its death depends on the balance between these mass-loss processes and the speed at which its core evolves, a race that current models cannot predict with confidence for any individual star.
When UY Scuti does go supernova, the explosion will be visible from Earth despite the star’s distance of over 5,000 light-years. It would appear as a bright new star in the constellation Scutum, potentially visible in daylight for days or weeks. The enriched material scattered by the explosion, including heavy elements forged in the star’s core and in the explosion itself, will eventually mix into the interstellar medium and contribute raw material for future generations of stars and planets. The atoms in your body were processed through similar cycles of stellar birth, swelling, and explosive death, though in stars far older than UY Scuti.
Why Supergiant Sizes Are So Persistently Uncertain
Given that astronomers can measure the distance to galaxies billions of light-years away, it might seem odd that the radius of a single star in our own galaxy is still debated. The difficulty is specific to red supergiants and comes from several compounding factors. First, these stars are rare. Only a handful of truly extreme red supergiants exist in the Milky Way at any given time, so the sample size for calibrating measurement techniques is tiny. Second, they sit preferentially in the dusty plane of the Galaxy, where extinction and reddening distort their observed properties. Third, their extended, semi-transparent atmospheres blur the boundary between star and space, as discussed earlier.3The Astrophysical Journal. The Temperatures of Red Supergiants
Interferometric techniques continue to improve, and the Gaia space mission has delivered better distance estimates for many Galactic stars, including some supergiants. Future observations with next-generation instruments may narrow the error bars enough to settle whether UY Scuti truly deserves its reputation or whether another star holds the record. For now, the honest scientific position is that UY Scuti is among the largest stars known but not definitively the largest, and its volume could plausibly range from roughly one billion to five billion times the Sun’s. Either end of that range is extraordinary by any human standard. Whether the answer is one billion Suns or five billion, the number has long since stopped fitting in your intuition.