BM Scorpii

BM Scorpii (BM Sco) is a red supergiant and a semiregular variable star located in the constellation Scorpius. It lies approximately 490 parsecs (1,600 ly) away in the open cluster Messier 6, the Butterfly Cluster.

BM Scorpii

Location of BM Scorpii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius[1]
Right ascension	17h 40m 58.54640s[2]
Declination	−32° 12′ 52.0644″[2]
Apparent magnitude (V)	5.25 – 6.46[3]
Characteristics
Spectral type	K2Ib Hδ1[4]
Variable type	SRd[5]
Astrometry
Radial velocity (Rv)	−7.39[2] km/s
Proper motion (μ)	RA: −0.895[2] mas/yr Dec.: −6.073[2] mas/yr
Parallax (π)	2.0344±0.0681 mas[2]
Distance	1,600 ± 50 ly (490 ± 20 pc)
Absolute magnitude (MV)	−2.57[6]
Details
Mass	5-9[7] M☉
Radius	129[7] R☉
Luminosity	3,388[7] L☉
Surface gravity (log g)	1.1[7] cgs
Temperature	3,888[7] K
Metallicity [Fe/H]	−0.22[8] dex
Other designations
BM Sco, CD−32°13142, HD 160371, HIP 86527[9]
Database references
SIMBAD	data

Characteristics

The Butterfly Cluster Messier 6 with BM Scorpii prominent as the lone red supergiant

BM Scorpii is a red supergiant of spectral type K2Ib. While it is listed as a standard star for that spectral class, it is also considered to have somewhat stronger H_δ absorption lines than normal. Its luminosity and mass are considered to be borderline for a supergiant star, and its spectral class has been given as K3III, which would indicate a giant star.^[7]

BM Scorpii is a confirmed member of the open cluster Messier 6 (M6) and the brightest star in the cluster.^[10] The brightest other stars in the cluster are late B- and A-class main sequence stars. A search has been made for other cool supergiant members, but none were found.^[11]

With an effective temperature of 3,900 K and a radius of 129 R_☉, BM Scorpii has a bolometric luminosity of 3,388 L_☉. Its angular diameter has been measured at 2.5 mas.^[7]

Variability

A light curve for BM Scorpii, plotted from Hipparcos data^[12]

In 1920 Annie Cannon discovered that BM Scorpii is a variable star,^[13] and it was included in the first edition of the General Catalogue of Variable Stars.^[14] The brightness variations between apparent magnitudes 5.25 and 6.46 are not entirely regular, but were reported to show a primary period of 815 days.^[5] In Hipparcos satellite photometry, the dominant variation period is 4.6 days with an amplitude of 0.05 magnitudes.^[15] No secondary period longer than 815 days was detected.^[16]