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.
Variability
AAVSOlight 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 luminousgalactic 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
^ abDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
^Barbier-Brossat, M.; Petit, M.; Figon, P. (1994). "Third bibliographic catalogue of stellar radial velocities (Text in French)". Astronomy and Astrophysics Supplement Series. 108: 603. Bibcode:1994A&AS..108..603B.
^Swope, Henrietta Hill (1942). "Variable stars in MWF 189". Annals of Harvard College Observatory. 109: 1–10. Retrieved 13 December 2024.
^ abSwope, Henrietta Hill (1942). "Variable stars in MWF 189". Annals of Harvard College Observatory. 109: 1. Bibcode:1942AnHar.109....1S.
^Downes, Ronald A.; Webbink, Ronald F.; Shara, Michael M.; Ritter, Hans; Kolb, Ulrich; Duerbeck, Hilmar W. (2001). "A Catalog and Atlas of Cataclysmic Variables: The Living Edition". Publications of the Astronomical Society of the Pacific. 113 (784): 764. arXiv:astro-ph/0102302. Bibcode:2001PASP..113..764D. doi:10.1086/320802. S2CID16285959.
^Levesque, E. M.; Massey, P.; Olsen, K. A. G.; Plez, B.; Josselin, E.; Maeder, A.; Meynet, G. (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal. 628 (2): 973–985. arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID15109583.
