| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 17h 52m 00.72665s [1] |
| Declination | −28° 01′ 20.5622″ [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: 0.39 mas/yr [1] Dec.: −1.62 mas/yr [1] |
| Parallax (π) | 0.436 ± 0.073 mas [6] |
| Distance | 7,038+1,506 −985 ly (2,159+462 −302 pc) [6] |
| Absolute magnitude (MV) | −7.7 [7] |
| Details | |
| Mass | 20 (or 20–40 [lower-alpha 1] ) [3] M☉ |
| Radius | 1,009±142 [3] R☉ |
| Luminosity (bolometric) | 173,800±9,600 [3] L☉ |
| Surface gravity (log g) | 0.0 [3] cgs |
| Temperature | 3,720±183 [3] K |
| Other designations | |
| Database references | |
| SIMBAD | data |
KW Sagittarii is a red supergiant star, located approximately 2,160 parsecs (7,000 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 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]
A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association. [11] 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. [1] 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). [6]
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. [13]
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] [14]
Mu Cephei, also known as Herschel's Garnet Star, Erakis, or HD 206936, is a red supergiant or hypergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. Since 1943, the spectrum of this star has served as a spectral standard by which other stars are classified.
W Sagittarii is a multiple star system star in the constellation Sagittarius, and a Cepheid variable star.
Tau Sagittarii is a star in the southern zodiac constellation of Sagittarius.
X Sagittarii is a variable star and candidate binary star system in the southern constellation of Sagittarius, near the western constellation boundary with Ophiuchus. It has a yellow-white hue and is visible to the naked eye with an apparent visual magnitude that fluctuates around 4.54. The star is located at a distance of approximately 950 light years from the Sun based on parallax, and is drifting closer with a radial velocity of −10 km/s. The star has an absolute magnitude of around −2.85.
V354 Cephei is a red supergiant star located within the Milky Way. It is an irregular variable located over 13,000 light-years away from the Sun. It has an estimated radius of 1,139 solar radii. If it were placed in the center of the Solar System, it would extend to between the orbits of Mars and Jupiter.
AG Carinae is a star in the constellation Carina. It is classified as a luminous blue variable (LBV) and is one of the most luminous stars in the Milky Way. The great distance and intervening dust mean that the star is not usually visible to the naked eye; its apparent brightness varies erratically between magnitude 5.7 and 9.0.
VX Sagittarii is an asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius. It is a pulsating variable star with an unusually large magnitude range. It is one of the largest stars discovered, with a radius varying between 1,350 and 1,940 solar radii (940,000,000 and 1.35×109 km; 6.3 and 9.0 au). It is the most luminous known AGB star, at bolometric magnitude –8.6, which is brighter than the theoretical limit at –8.0.
V4381 Sagittarii is a variable star in the constellation Sagittarius. A white supergiant of spectral type A2/A3Iab, it is an Alpha Cygni variable that varies between apparent photographic magnitudes 6.57 and 6.62. Its visual apparent magnitude is about 6.54.
V602 Carinae is a red supergiant and variable star of spectral type M3 in the constellation Carina. It is considered to be one of largest known stars, being around 1,000 times larger than the Sun.
HD 168607 is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and possible luminous blue variable, that can be seen at the south-east of M17, the Omega Nebula.
PZ Cassiopeiae is a red supergiant star located in the constellation of Cassiopeia, and a semi-regular variable star.
RY Sagittarii is a yellow supergiant and an R Coronae Borealis type variable star in the constellation Sagittarius. Although it ostensibly has the spectrum of a G-type star, it differs markedly from most in that it has almost no hydrogen and much carbon.
UY Scuti (BD-12°5055) is a red supergiant star, located 5,900 light-years away in the constellation Scutum. It is also a pulsating variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56, which is too dim for naked-eye visibility. It is considered to be one of the largest known stars, with a radius estimated at 909 solar radii, thus a volume of 750 million times that of the Sun. This estimate implies if it were placed at the center of the Solar System, its photosphere would extend past the orbit of Mars or even the asteroid belt.
AH Scorpii is a red supergiant variable star located in the constellation Scorpius. It is one of the largest stars known by radius and is also one of the most luminous red supergiant stars in the Milky Way.
BC Cygni is a red supergiant and pulsating variable star of spectral type M3.5Ia in the constellation Cygnus.
RW Cygni is a semiregular variable star in the constellation Cygnus, about a degree east of 2nd magnitude γ Cygni. Its apparent magnitude varies between 8.05 and 9.70 and its spectral type between M3 and M4.
CK Carinae is a variable star in the constellation Carina, the keel of Argo Navis. It is a member of the star association Carina OB1-D, at a distance of around 2,300 parsecs or 7,500 light years.
V528 Carinae is a variable star in the constellation Carina.
BO Carinae, also known as HD 93420, is an irregular variable star in the constellation Carina.
U Sagittarii is a variable star in the southern constellation of Sagittarius, abbreviated U Sgr. It is a classical Cepheid variable that ranges in brightness from an apparent visual magnitude of 6.28 down to 7.15, with a pulsation period of 6.745226 days. At its brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 2,080 light years based on parallax measurements, and it is drifting further away with a radial velocity of 2 km/s.
