Which Is the Largest Known Star?

Stephenson 2-18 is a red supergiant roughly 19,000 light-years away in the constellation Scutum, with an estimated radius of about 2,150 times the Sun's. If placed at the centre of our solar system, its surface could extend beyond the orbit of Saturn. Estimates for such stars are uncertain because their distances are hard to measure and their outer layers are diffuse. Other contenders have included UY Scuti and WOH G64, whose size estimates have been revised as data improved. The largest stars are not the most massive; the heaviest known star, R136a1, is far smaller but about 200 times the Sun's mass.