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Plaskett's star
	Location of Plaskett's star (circled)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Monoceros
Right ascension	06h 37m 24.04130s
Declination	+06° 08′ 07.3719″
Apparent magnitude (V)	6.06
Characteristics
Spectral type	O8I + O7.5III
U−B color index	–0.88
B−V color index	+0.05
Astrometry
Radial velocity (Rv)	+24.5 km/s
Proper motion (μ)	RA: –2.73 mas/yr ; Dec.: +0.31 mas/yr
Distance	5,245 ly ; (1,608 pc)
Details
A
Mass	54 M☉
Radius	14.2 R☉
Luminosity	224,000 L☉
Surface gravity (log g)	3.5 ± 0.1 cgs
Temperature	33,500 ± 2,000 K
Rotational velocity (v sin i)	75 km/s
B
Mass	56 M☉
Radius	10.8 R☉
Luminosity	123,000 L☉
Surface gravity (log g)	3.5 ± 0.1 cgs
Temperature	33,000 ± 2,000 K
Rotational velocity (v sin i)	300 km/s
Other designations
	V640 Monocerotis, HR 2422, BD+6°1309, GC 8631, HIP 31646 , HD 47129.
Database references
SIMBAD	data

Plaskett's Star /ˈplæskɪts/, also known as HR 2422 and V640 Monocerotis, is a spectroscopic binary at a distance of around 6600 light-years. It is one of the most massive binary stars known, with a total mass of around one hundred times that of the Sun.^[6] Indeed, it was long thought to be the most massive known binary system,^[7]^[8] but evidence collected between 1996 and 2005 demonstrated that Eta Carinae, which was previously thought to be a massive individual star, is a binary system.^[9]

This system is named after John Stanley Plaskett, the Canadian astronomer who discovered its binary nature in 1922. Plaskett was assisted in his observations by his son, Harry Hemley Plaskett. The pair of stars have a combined visual magnitude of 6.05, and are located in the constellation of Monoceros.

A light curve for V640 Monocerotis, plotted from *TESS* data^[10]

The orbital period for the pair is 14.39625±0.00095 d.^[6] The secondary is a rapid rotator with a projected rotational velocity of 300 km·s⁻¹,^[7] giving it a pronounced equatorial bulge.^[6]

The brightness varies irregularly from 6.0 to 6.1 on a timescale of a few hours, thought to be due to many factors including the binary orbit, hot spots in the colliding winds, and granulation.^[3] The luminosities of each component are much lower than expected for their spectral types. It has been suggested that the star may be twice as far away as assumed, not a member of the Monoceros OB2 association, and each component would be about four times as luminous as currently calculated.^{[citation needed]} The masses derived from the binary orbit are also somewhat higher than expected from the spectral types, but with considerable uncertainty due to assumptions about the inclination.^[3]

References

^ ^a ^b ^c ^d van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
^ ^a ^b ^c Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99): 99. Bibcode:1966CoLPL...4...99J.
^ ^a ^b ^c Mahy, L.; Gosset, E.; Baudin, F.; Rauw, G.; Godart, M.; Morel, T.; Degroote, P.; Aerts, C.; Blomme, R.; Cuypers, J.; Noels, A.; Michel, E.; Baglin, A.; Auvergne, M.; Catala, C.; Samadi, R. (2010). "Plaskett's star: Analysis of the CoRoT photometric data". Astronomy & Astrophysics. 525: A101. arXiv:1010.4959. Bibcode:2011A&A...525A.101M. doi:10.1051/0004-6361/201014777. S2CID 30687338.
^ Wilson, Ralph Elmer (1953). General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
^ Megier, A.; et al. (November 2009), "The interstellar Ca II distance scale", Astronomy and Astrophysics, 507 (2): 833–840, Bibcode:2009A&A...507..833M, doi:10.1051/0004-6361/20079144
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m Linder, N.; et al. (October 2008), "High-resolution optical spectroscopy of Plaskett's star", Astronomy and Astrophysics, 489 (2): 713–723, arXiv:0807.4823, Bibcode:2008A&A...489..713L, doi:10.1051/0004-6361:200810003, S2CID 118431215
^ ^a ^b ^c ^d Mahy, L.; et al. (January 2011), "Plaskett's star: analysis of the CoRoT photometric data", Astronomy and Astrophysics, 525: A101, arXiv:1010.4959, Bibcode:2011A&A...525A.101M, doi:10.1051/0004-6361/201014777, S2CID 30687338
^ Ley, Willy (August 1965). "The Galactic Giants". For Your Information. Galaxy Science Fiction. pp. 130–142.
^ Iping, Rosina C.; et al. (November 2005), "Detection of a Hot Binary Companion of η Carinae", The Astrophysical Journal, 633 (1): L37–L40, arXiv:astro-ph/0510581, Bibcode:2005ApJ...633L..37I, doi:10.1086/498268, S2CID 119350572
^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.