What is the largest star in the known universe?

Stephenson 2-18 (St2-18) is a red supergiant located in the constellation Scutum. Also catalogued as Stephenson 2-DFK 1 and RSGC2-18, it is the current record holder for the largest star known, with a size 2,150 times that of the Sun. The star is located at a distance of 18,900 light years from Earth. It is a member of the open cluster Stephenson 2.

Stephenson 2-18 is a red supergiant of the spectral type M6. It is also one of the most luminous red supergiants known. With a surface temperature of 3,200 K, it shines with about 440,000 solar luminosities.

If it replaced the Sun in our solar system, it would extend past the orbit of Saturn (1,940 – 2,169 R☉). The star’s size corresponds to a volume about 10 billion times greater than the Sun.

The open cluster Stephenson 2 is one of the most massive open clusters in the Milky Way. It was first noticed by American astronomer Charles Bruce Stephenson, who reported the discovery in June 1990. Stephenson discovered the cluster after a space-deep infrared objective-prism survey.

Scutum is one of the smallest and faintest constellations in the sky. It is the 84th constellation in size, stretching across only 109 square degrees of the southern sky. It does not contain any stars brighter than magnitude 3.00. None of its stars make the list of the 300 brightest stars in the sky.