About the image. Drawn as a seething red supergiant. Its temperature sets the colour; the surface pattern is imagined. Even its size is uncertain, because the distance is.
Stephenson 2-18, also called Stephenson 2 DFK 1, is a red supergiant in the constellation Scutum, about 18 900 light-years away. It is one of the largest stars known.
How large?
The most recent estimate, from a 2026 study, puts its radius at about 1 840 times the Sun’s. An older analysis gave about 2 150. Put the star where the Sun is and its surface would lie far beyond the orbit of Jupiter; at the larger estimate, beyond Saturn’s.
Those numbers come with heavy caveats. A star’s radius is worked out from its brightness and temperature, and its brightness depends on its distance. For Stephenson 2-18 the distance is uncertain by thousands of light-years, so the size is too. The 2026 team also noted that none of their models reproduced the star’s spectrum well.
At the limit
Theory says stars should not be able to stay much cooler and brighter than a boundary known as the Humphreys–Davidson limit. Some earlier estimates placed Stephenson 2-18 beyond it, which would make it unstable. It is not even certain that it belongs to the star cluster it is named after: its speed through space differs from the cluster’s.
What is clear is that it is a cool, bloated giant near the end of its life, shining with hundreds of thousands of times the Sun’s power.
To scale · radius, Sun = 1Sun: 1.00 · Stephenson 2-18: 1 840At this scale the Sun is a single pixel.
How hot is it?
Earth (average)288 K · 15 °C
Water boils373 K · 100 °C
Venus surface737 K · 464 °C
Iron melts1 811 K · 1 540 °C
Sun's surface5 772 K · 5 500 °C
Stephenson 2-183 200 K · 2 930 °C
Stephenson 2-18: surface temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
Stephenson 2-18 is about 18 900 light-years away.
Light299 792 km/s18 900 years
Breakthrough Starshotplanned light sail, 20% of light speed94 500 years
Parker Solar Probefastest spacecraft so far, 192 km/s29.5 million years
Voyager 1leaving the Solar System, 17 km/s333.3 million years
At Voyager 1's speed the trip would take about 4 166 227 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
Artist's impressions rendered from the same model as the view at the top of this page.
Timeline
2010Early luminosity estimate published.
2013An analysis implies a radius around 2 150 times the Sun's.
2026A new study revises it to about 1 840, with large uncertainty.
Questions people ask
What is the largest known star?
Stephenson 2-18 is one of the largest, at about 1 840 times the Sun's radius, but its size is uncertain because its distance is.
How big is Stephenson 2-18 compared to the Sun?
Placed where the Sun is, its surface would reach past the orbit of Jupiter.
How far away is Stephenson 2-18?
About 18 900 light-years, in the constellation Scutum.