KW Sagittarii

KW Sagittarii is a red supergiant star, located approximately 2,420 parsecs (7,900 light-years) 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.

KW Sagittarii

Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	17h 52m 00.72695s[1]
Declination	−28° 01′ 20.5557″[1]
Apparent magnitude (V)	11.0[2]
Characteristics
Spectral type	M1.5Iab[3] (M0I - M4Ia[4])
Apparent magnitude (K)	1.43[2]
U−B color index	3.21[3]
B−V color index	2.47[3]
V−R color index	2.58[3]
J−K color index	1.56[3]
Variable type	SRc[4]
Astrometry
Radial velocity (Rv)	−7.40[5] km/s
Proper motion (μ)	RA: +0.843 mas/yr[1] Dec.: −0.946 mas/yr[1]
Parallax (π)	0.4355±0.0726 mas[1]
Distance	7,890 ly (2,420 pc)[6][a]
Absolute magnitude (MV)	−7.7[7]
Details
Mass	20 (or 20–40[b])[3] M☉
Radius	1,009±142[3] R☉
Luminosity (bolometric)	175,000±52,000[3] L☉
Surface gravity (log g)	0.0[3] cgs
Temperature	3,720±183[3] K
Other designations
KW Sgr, CD−27°12032, HD 316496, HIP 87433, AAVSO 1745-28, Gaia DR2 4063462206570029312, Gaia DR3 4063462206570029312
Database references
SIMBAD	data

Variability

AAVSO light curve of KW Sgr from 1 January 1990 to 24 November 2010. Up is brighter and down is fainter. Day numbers are Julian day.

In 1942, Henrietta Hill Swope listed KW Sagittarii as a variable star.^[8] It varies erratically in brightness over a range of about two magnitudes.^[9] It is classified as a semiregular variable,^[4] although the listed period of 670 days is poorly defined.^[9] The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.^[10]

Distance

A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association.^[6] 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.^[11] The Gaia Data Release 2 parallax is 0.5281±0.1392 mas and implies a distance of around 1,900 pc. 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.^[12] 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).^[13]

Characteristics

KW Sagittarii is classed as a luminous cool supergiant and varies its spectral type between M0 and M4.^[4] A 2005 study led by Levesque, using a MARCS model, calculated a high luminosity of 363,000 L_☉ for KW Sgr and consequently very large radius of 1,460 R_☉ based on the assumption of an effective temperature of 3,700 K at a distance of 3,000 pc. 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.^[14]

More recently, KW Sagittarii was calculated to have a lower bolometric luminosity around 200,000 L_☉ and a radius around 1,009 ± 142 R_☉ was based on the measured angular diameter and luminosity.^[3][15]

See also

• VX Sagittarii — another red supergiant (RSG) in the constellation Sagittarius
• UY Scuti
• AH Scorpii

Notes

1. Assumed as KW Sagittarii is assumed to be part of Sgr OB5 association
2. Assuming that KW Sagittarii doesn't rotate.