BL Crucis

BL Crucis is a red giant and a semiregular variable in the constellation of Crux. A 5th magnitude star, it is visible to the naked eye under good observing conditions. It is 480±10 light-years distant from Earth.

A light curve for BL Crucis, adapted from Tabur et al. (2009)^[10]

BL Crucis

Location of BL Crucis (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Crux[1]
Right ascension	12h 27m 28.88s[2]
Declination	−58° 59′ 30.4″[2]
Apparent magnitude (V)	5.43[3]
Characteristics
Evolutionary stage	AGB[4]
Spectral type	M4/5III[5]
Variable type	SR[3]
Astrometry
Radial velocity (Rv)	71.7±0.9[6] km/s
Proper motion (μ)	RA: −19.294[2] mas/yr Dec.: +4.680[2] mas/yr
Parallax (π)	6.8429±0.1942 mas[2]
Distance	480 ± 10 ly (146 ± 4 pc)
Absolute magnitude (MV)	−0.28[1]
Details
Mass	3.6[7] M☉
Radius	106[8] R☉
Luminosity	1,234[8] L☉
Surface gravity (log g)	1.94[7] cgs
Temperature	3,320[8] K
Metallicity [Fe/H]	−0.33[7] dex
Other designations
CD−58°4560, HD 108396, HIP 60781, HR 4739, SAO 239960[9]
Database references
SIMBAD	data

In 1969, Olin J. Eggen announced that the star, then called HR 4739, is a variable star.^[11] It was given its variable star designation, BL Crucis, in 1977.^[12] Three periods have been detected in its light curve, 30.7, 42.3 and 43.6 days.^[10] The maximum amplitude of variation is 0.35 magnitudes.^[3]

An aging red giant, BL Crucis is on the asymptotic giant branch having exhaust both its core hydrogen and core helium. It now fuses hydrogen and helium in separate shells outside the core.^[4]