The largest known star is so vast that light takes hours to cross it

The Sun is enormous by human standards — more than a hundred Earths would fit across its face — yet it is a dwarf beside the largest stars known. Among the biggest ever measured is UY Scuti, a bloated red supergiant so vast that a beam of light, the fastest thing in the universe, would need hours to travel from one side of it to the other. Where light crosses the Sun’s width in about four and a half seconds, the same trip across a star like UY Scuti stretches into a span better measured against a work shift than a heartbeat.

A red supergiant in the constellation Scutum

UY Scuti sits several thousand light-years away in the small southern constellation Scutum, near the crowded star fields of the Milky Way’s central regions. It belongs to a rare class called red supergiants: aging, cool, deeply swollen stars in the final chapters of their lives. Such stars have exhausted the hydrogen in their cores and ballooned outward as their outer layers puffed up and cooled to a ruddy glow. Their surfaces are so distended and thin that the boundary of the star is more a gradual fade into space than a crisp edge, which is part of what makes measuring one so challenging. UY Scuti is also a variable star, its brightness pulsing over a span of roughly two to three years as its bloated outer layers swell and contract. All the while it sheds material in a powerful stellar wind, wrapping itself in an expanding shell of gas and dust that further blurs where the star ends and space begins.

About 1,700 times the width of the Sun

The figure that earned UY Scuti its fame comes from careful measurement of its apparent size on the sky. Using an interferometer that combines light from multiple telescopes, astronomers estimated its radius at roughly 1,708 times the radius of the Sun. Stretched to that scale, the star’s diameter runs to about 2.4 billion kilometers. If it replaced the Sun at the center of the solar system, its surface would engulf Mercury, Venus, Earth, and Mars and reach out past the orbit of Jupiter, swallowing the inner planets whole.

About two hours at the speed of light

Sheer numbers of kilometers are hard to picture, so light offers a better yardstick. Light travels roughly 300,000 kilometers every second, fast enough to lap the Earth more than seven times in the same interval. The Sun, at a diameter of about 1.39 million kilometers according to NASA’s planetary fact sheet, is crossed by a light beam in roughly 4.6 seconds. UY Scuti’s diameter is more than seventeen hundred times wider, so the same beam would need about two hours to make the crossing — the difference between a snap of the fingers and an entire afternoon errand. For scale, sunlight reaches Earth in roughly eight minutes after crossing 150 million kilometers of empty space, which means a single one of these supergiants is wide enough that light would spend far longer traveling across the star than it does covering the gap between the Sun and Earth.

Why the size carries large uncertainty

The celebrated figure is not as solid as it sounds. A star’s physical size is calculated from its apparent width on the sky combined with its distance, and the distance to UY Scuti is poorly known. A modest error in distance translates into a large error in radius, so the estimate carries an uncertainty of roughly 190 solar radii even under the assumptions used to derive it. Some later analyses, using refined distance data, have argued the star may be considerably smaller than the classic value. The diffuse, pulsating atmosphere of a red supergiant only compounds the problem, because the star has no single, sharply defined surface to measure against.

Bigger contenders and the limits of the record

UY Scuti is often listed among the largest known stars, but the ranking is contested and keeps changing as measurements improve. Several other red supergiants and hypergiants — including Stephenson 2-18, estimated at around 2,150 solar radii, along with WOH G64 and others — have been proposed as equal or larger, according to compiled catalogs of the largest known stars. For a body the size of Stephenson 2-18, the light-crossing time climbs toward three hours. Because every one of these estimates depends on uncertain distances and fuzzy stellar edges, no single star can be crowned the definitive champion; the leaderboard shifts with each new round of data.

Vast in size, modest in mass

For all its bulk, a star like UY Scuti holds surprisingly little material for its volume. Its mass is thought to be only on the order of ten times that of the Sun, spread across a volume billions of times larger, which makes its outer layers thinner than the best vacuum achievable in a laboratory. That extreme puffiness is a symptom of a star nearing the end of its life. Red supergiants burn through their fuel quickly and live only a few million years before collapsing and exploding as supernovae, seeding the surrounding space with the heavy elements forged in their cores. The contrast is striking: although light needs hours to cross such a star, its core will collapse in a matter of seconds when its fuel finally runs out, triggering an explosion that can briefly rival the combined glow of billions of ordinary stars. The very qualities that make these giants so enormous are the same ones that mark them as short-lived and doomed.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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