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.
| 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][lower-alpha 1] |
| Absolute magnitude (MV) | −7.7[7] |
| Details | |
| Mass | 20 (or 20–40[lower-alpha 2])[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 | |
| Database references | |
| SIMBAD | data |
Variability
KW Sagitarii was discovered to be variable in 1928 from a study of photographic plates.[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
- Assuming that KW Sagittarii doesn't rotate.
References
External links
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