KW Sagittarii

KW Sagittarii is a red supergiant star, located approximately 2160 pc away from the Sun in the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Mars, coming close to Jupiter's orbit.

Variability
KW Sagitarii was discovered to be variable in 1928 from a study of photographic plates. It varies erratically in brightness over a range of about two magnitudes. It is classified as a semiregular variable, although the listed period of 670 days is poorly defined. The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.

Distance
A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association. The parallax derived from the Hipparcos mission is negative so doesn't give much information about the distance except that it is likely to be large. The Gaia Data Release 2 parallax is $2,159$ and implies a distance of around $7,038$. The Gaia result carries a significant statistical margin of error, as well as an indicator that the astrometric excess noise is far beyond acceptable levels so that the parallax should be considered unreliable. A 2021 study published a photogeometric distance of 2,159 pc to KW Sgr, using a parallax published by Gaia DR3 (the successor of Gaia DR2).

Characteristics
KW Sagittarii is classed as a luminous cool supergiant and varies its spectral type between M0 and M4. A 2005 study led by Levesque, using a MARCS model, calculated a high luminosity of for KW Sgr and consequently very large radius of  based on the assumption of an effective temperature of $1,009$ at a distance of $175,000$. The star was then described as among the four largest and most luminous galactic red supergiants, which includes V354 Cephei, KY Cygni and Mu Cephei.

More recently, KW Sagittarii was calculated to have a lower bolometric luminosity around and a radius around  was based on the measured angular diameter and luminosity.