How Many Times Bigger Is Stephenson 2-18 Than the Sun?

Stephenson 2-18 has an estimated radius roughly 2,150 times that of the Sun, making it one of the largest stars ever identified. That figure comes with significant uncertainty, though, because measuring the size of a distant, dust-shrouded red supergiant is far harder than it sounds. The star sits in a heavily obscured cluster near the galactic plane, and small shifts in the assumed distance or temperature can change the radius estimate by hundreds of solar radii in either direction.

Putting 2,150 Solar Radii in Perspective

The Sun has a radius of about 696,000 kilometers. Multiply that by 2,150 and you get a star roughly 1.5 billion kilometers across. If you placed Stephenson 2-18 at the center of our solar system, its outer layers would swallow Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The photosphere would extend to somewhere around Saturn’s orbit, give or take, depending on which radius estimate you use. Light itself would need roughly eight hours to travel from one side of the star to the other.

Volume scales even more dramatically than radius. Because volume grows with the cube of the radius, Stephenson 2-18 encloses a space roughly ten billion times the volume of the Sun. Yet its mass is probably only 30 to 40 times the Sun’s mass. That means the star’s outer envelope is extraordinarily thin, a near-vacuum by any earthly standard. You could fly a spacecraft through the outer layers and barely notice you were inside a star, aside from the heat.

How Astronomers Measure a Star This Large

Nobody points a ruler at Stephenson 2-18. The radius is calculated indirectly, and the method depends on two quantities: how much total energy the star emits (its luminosity) and how hot its surface is. The basic physics is straightforward: a star’s luminosity equals its surface area multiplied by the energy radiated per unit area, which depends on temperature. If you know how bright the star truly is and how hot its surface is, you can solve for its radius.

For nearby red supergiants, astronomers can sometimes measure angular diameters directly using interferometry, where multiple telescopes combine their signals to resolve the tiny disk of a star. That angular size, combined with a known distance, gives a physical radius. But Stephenson 2-18 is too distant and too heavily obscured by interstellar dust for this technique to work cleanly. Instead, its radius is inferred from photometric data: measuring the star’s brightness across multiple wavelengths, fitting that to a model spectrum, and extracting a temperature and luminosity.¹

Each step in this chain introduces uncertainty. The temperature of a red supergiant’s photosphere is tricky to pin down because molecular bands in the spectrum complicate the analysis, and the atmosphere is not a sharp boundary but a diffuse, extended structure. A shift of just 100 or 200 kelvins in the assumed temperature can change the derived radius considerably, because cooler temperatures demand a larger surface to produce the same luminosity.

The Distance Problem

The biggest source of uncertainty for Stephenson 2-18’s size is how far away it is. The star belongs to the Stephenson 2 cluster, a group of red supergiants deep in the Milky Way’s disk near the Scutum-Crux arm. Measuring distances to objects embedded in the galactic plane is notoriously difficult because thick dust clouds block and scatter the light, and the extinction (the dimming caused by that dust) varies patchily across even small areas of sky.

Studies of the cluster have placed it at a distance of roughly 5 to 6 kiloparsecs from Earth, with one careful analysis of the interstellar extinction suggesting a distance near the low end of that range, around 5.3 kiloparsecs. That study also noted that because the dust absorption is determined over a broad area and the absorption is clearly patchy across the field, the distance should not be taken as highly precise.²

Why does this matter so much for the star’s radius? Luminosity depends on the square of the distance. If the star is ten percent farther away than assumed, it is about twenty percent more luminous, and its derived radius grows accordingly. If it is closer, the radius shrinks. The commonly quoted figure of 2,150 solar radii assumes a particular distance and temperature combination. Shift either one and you can land on estimates anywhere from roughly 1,500 to over 2,500 solar radii. The 2,150 number is a best estimate, not a hard measurement.

What Kind of Star Produces Such Extreme Sizes

Stephenson 2-18 is a red supergiant, which is the final major evolutionary stage for massive stars before they die. Stars born with perhaps 20 to 40 times the Sun’s mass burn through their hydrogen fuel relatively quickly, in just a few million years. Once the core hydrogen runs out and the star begins fusing heavier elements, the outer layers expand enormously. The surface cools as it stretches, dropping to around 3,000 to 3,500 kelvins, which is why these stars appear red. But the total energy output remains enormous because the surface area is so vast.

Not every red supergiant reaches the extreme size of Stephenson 2-18. Betelgeuse, the most famous red supergiant, has a radius of roughly 750 to 900 solar radii depending on the measurement. UY Scuti, another frequently cited giant, has been estimated at around 1,700 solar radii, though that number too carries large error bars. The record for largest known star has changed hands several times over the past couple of decades as measurements have been refined, and Stephenson 2-18 currently sits at or very near the top of the list.

Why Stars Cannot Just Keep Getting Bigger

You might wonder whether there could be a star out there ten thousand or a hundred thousand times the Sun’s radius. There are physical limits. As a star becomes extremely luminous, the outward push of radiation on its outer layers approaches and can exceed the inward pull of gravity. This imposes a ceiling on how luminous and extended a star can become before it starts blowing itself apart.

This boundary is related to what astronomers call the Humphreys-Davidson limit, an observed upper boundary in the luminosity and temperature space that the most massive stars seem unable to cross without becoming violently unstable. Research into this limit has identified mode-coupling instabilities in massive stars that could explain why luminous blue variables erupt and why there appears to be a hard ceiling on the luminosity of cool supergiants.³

Red supergiants approaching this limit lose mass furiously from their surfaces, and that mass loss itself acts as a brake on further expansion. A star shedding its envelope fast enough simply cannot maintain an arbitrarily large photosphere. So while 2,150 solar radii is staggering, it is probably not far from the practical maximum that any star can reach under the physics we understand.

Mass Loss and the Thinning Envelope

Red supergiants are not stable, sharply bounded objects like the Sun. Their outer layers are continuously leaking into space. The mass-loss rate for red supergiants spans an enormous range across different stars and different measurement methods, from negligible trickles to rates that shed a meaningful fraction of a solar mass every few thousand years.⁴

For the very largest red supergiants like Stephenson 2-18, the mass loss is substantial. Dust forms in the cooling outflow, creating a circumstellar envelope that makes the star even harder to observe at visible wavelengths. Much of what we know about these stars comes from infrared observations that can peer through the dust. The mass the star sheds enriches the surrounding interstellar medium with heavy elements that will eventually be incorporated into new stars and planets.

This mass loss has direct consequences for the star’s measured size. Because the outer atmosphere is so extended and diffuse, the “surface” of the star is not a well-defined boundary. The radius astronomers quote typically corresponds to the layer where the atmosphere becomes opaque at a particular wavelength, but that layer shifts depending on which wavelength you observe. In the infrared, a red supergiant can appear meaningfully larger than it does at shorter wavelengths, which adds another layer of ambiguity to any single radius number.⁵

The Roiling Surface of a Red Supergiant

If you could somehow see the surface of Stephenson 2-18 up close, it would look nothing like the relatively smooth face of the Sun. Red supergiants have enormous convection cells, far larger relative to the star than the granulation pattern on the Sun. Simulations of red supergiant atmospheres show just a handful of giant convective upwellings covering the entire stellar surface, with lifetimes measured in years rather than the minutes typical of solar granules.

These convective motions matter for the star’s size because they drive waves outward into the atmosphere. The fast-moving downdrafts on the surface generate acoustic waves through turbulent stresses. When those waves reach the thin, cool outer atmosphere, they steepen into shocks. The shocks deposit energy that puffs up the atmosphere well beyond where it would sit in a quiet, non-convecting star. The resulting dynamical pressure exceeds the thermal gas pressure in the outer layers, significantly increasing the scale height of the atmosphere and making the star appear larger than a static model would predict.⁶

This means that some fraction of Stephenson 2-18’s enormous radius is not the star’s “body” in the traditional sense but rather a dynamically supported outer atmosphere held aloft by wave energy from the convective interior. It is genuinely part of the star, it contains stellar material and radiates light, but it behaves more like a perpetually shocked, billowing extension than a stable surface.

Competing Claims for the Largest Star

Stephenson 2-18 regularly appears in popular lists as the single largest known star, but the competition is closer and messier than most lists suggest. Several other red supergiants have been measured at radii that overlap with Stephenson 2-18’s range once you account for error bars. UY Scuti was long cited as the record holder at around 1,700 solar radii, but more recent temperature and distance revisions have shifted its estimated radius. WOH G64 in the Large Magellanic Cloud, VY Canis Majoris in our own galaxy, and a handful of other red supergiants have all traded places on these rankings as measurements improve.

The honest answer is that no astronomer would bet their career on Stephenson 2-18 being definitively, unambiguously the largest star we know of. The error bars on these measurements are large enough that several candidates could plausibly hold the title. What is fair to say is that Stephenson 2-18 is among the very largest, consistently appearing in the top handful of candidates with a radius estimate that is extreme by any standard.

Why the Ranking Keeps Shifting

Every few years, a new study revises the temperature scale for red supergiants or improves the distance measurement to a particular cluster, and the radius rankings shuffle. This is not sloppy science but rather the reality of working at the limits of observational astronomy. These stars are rare (only a few hundred red supergiants are known in the Milky Way), distant, and embedded in dusty environments that complicate every measurement. The instruments used to study them have improved dramatically over the past two decades, particularly in the infrared, and each new generation of data prompts revisions.

The Gaia spacecraft has delivered precise parallax measurements for billions of stars, but even Gaia struggles with the most distant, heavily reddened objects near the galactic plane. Stephenson 2-18 sits in exactly the kind of environment where Gaia’s distance measurements carry large uncertainties or fail entirely. Future infrared survey telescopes and longer Gaia baselines may tighten the constraints, but for now, calling Stephenson 2-18 “about 2,150 times the Sun’s radius” is the best we can do, with the implicit understanding that the true number could reasonably be anywhere from 1,500 to 2,500.

What Happens When a Star This Big Dies

Stephenson 2-18 will not last long in cosmic terms. Red supergiants of this size and luminosity are in the final stages of their lives, with perhaps tens of thousands to a few hundred thousand years remaining before core collapse. When the core exhausts its fuel and can no longer support itself against gravity, the star will almost certainly explode as a core-collapse supernova. The explosion will briefly outshine its entire host galaxy and scatter the star’s enriched outer layers across interstellar space.

Whether the remnant left behind will be a neutron star or a black hole depends on the core mass at the time of collapse, which in turn depends on how much mass the star has shed during its red supergiant phase. Stars that lose enough mass through winds may leave behind a neutron star; those that retain more mass in the core are more likely to form a black hole. Given the intense mass loss that red supergiants of this size experience, predicting the outcome for Stephenson 2-18 specifically is not straightforward.

When the supernova does happen, it will be visible from Earth despite the intervening dust, at least in infrared and possibly at other wavelengths. At a distance of about five kiloparsecs, it would be one of the closer supernovae in modern observational history, though not close enough to pose any danger. For astronomers, it would be an extraordinary event: the death of one of the largest stars ever catalogued, observed with instruments far more capable than anything available the last time a supernova went off this close to us.