What Is the Biggest Star in the Universe?

June 18, 2026 · 5 min read

cosmos physics

Meet UY Scuti and Stephenson 2-18 — hypergiant stars so vast that light would take hours to travel around them. Here's how they compare to our Sun.

Our Sun Is Average

The Sun feels enormous — you could fit about 1.3 million Earths inside it. But among stars, the Sun is thoroughly ordinary, a middling yellow dwarf. The true giants of the universe make it look like a speck.

The Record Holders

The title of "largest known star" shifts as measurements improve, but two names dominate the conversation: UY Scuti and Stephenson 2-18. Both are red hypergiants — dying stars that have swollen to colossal proportions near the end of their lives.

Stephenson 2-18 is often cited as the largest, with a radius estimated at about 2,150 times that of the Sun. If it replaced the Sun at the centre of our solar system, its surface would extend beyond the orbit of Saturn, swallowing Mercury, Venus, Earth, Mars, Jupiter, and Saturn entirely.

UY Scuti, another famous contender, has a radius around 1,700 times the Sun's. Light, which crosses the Sun's diameter in under 5 seconds, would take about 7 hours to travel around a star this size.

Big in Size, Not in Mass

Here's the twist: these hypergiants are enormous in volume but not proportionally heavy. Stephenson 2-18 has expanded so dramatically that its outer layers are thinner than the best vacuum achievable in a laboratory on Earth. It might hold only a few dozen times the Sun's mass spread across billions of times its volume.

The genuinely most massive stars, like R136a1, are different beasts — packing over 150 to 200 solar masses into a comparatively "small" body, blazing millions of times brighter than the Sun.

Why Measuring Size Is So Hard

Stars are too distant to see as discs, so their sizes are inferred from brightness and temperature, combined with distance estimates. Small errors in distance ripple into large errors in size, which is why the "biggest star" title comes with big uncertainty bars — and why the record changes as telescopes improve.

To feel the gulf between the Sun and a hypergiant, drop them side by side on our scale explorer and watch the Sun shrink to a pinprick.