Westerlund 1-243

Westerlund 1-243 or Wd 1-243 is a luminous blue variable (LBV) star undergoing an eruptive phase located within the outskirts of the super star cluster Westerlund 1.^[7] Located about 13,400 ly (4,100 pc) from Earth,^[6] it has a luminosity of 0.73 million L_☉ making it one of the most luminous stars known.^[4]

Westerlund 1-243

Westerlund 1 super star cluster (north is towards top left). The location of Wd 1-243 is circled. Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Ara[1]
Right ascension	16h 47m 7.5041s[2]
Declination	−45° 52′ 29.123″[2]
Apparent magnitude (V)	15.807[3]
Characteristics
Spectral type	A3Ia+[4]
Apparent magnitude (J)	6.407[5]
Apparent magnitude (H)	5.271[5]
Apparent magnitude (K)	4.632[5]
Astrometry
Proper motion (μ)	RA: −1.574[2] mas/yr Dec.: −4.036[2] mas/yr
Parallax (π)	0.0119±0.0812 mas[2]
Distance	13,400 ly (4,120[6] pc)
Details[4]
Mass	40 M☉
Radius	376.9 R☉
Luminosity	730,000 L☉
Surface gravity (log g)	~0.65 cgs
Temperature	~8,500 K
Rotational velocity (v sin i)	10 km/s
Age	10.4+1.3 −1.2[6] Myr
Other designations
Westerlund 1 W 243, Westerlund 1 BKS G, 2MASS J16470749-4552290, Gaia DR3 5940105830990286208
Database references
SIMBAD	data

Observation

Westerlund 1-243 is the second brightest star in Westerlund 1, behind only Westerlund 1-4.^[3] It is one of several different hypergiant stars in Westerlund 1. It may also have a companion star, possibly an O-type supergiant.^[8]

Spectrum

Westerlund 1-243 displays a complex, time-varying spectrum with emission lines of hydrogen, helium and Lyman-α pumped metals, forbidden lines of nitrogen and iron, and a large number of absorption lines from neutral and singly-ionized metals. Many lines are complex emission/absorption blends, with significant spectral evolution occurring on timescales of just a few days.^[4]

Properties

Westerlund 1-243 has a temperature of ~8,500 K determined from modelling the absorption line spectrum. It has expanded to a radius of 376.9 R_☉, and a Rosseland radius of 450 R_☉. It is radiating at a luminosity of 730,000 L_☉. It is losing mass at a rate of 6.1×10⁻⁷ M_☉/yr.^[4]

Evolution

Westerlund 1-243 is believed to be either in an advanced pre-red supergiant LBV phase, or has evolved through the RSG phase and returned to the blue side of the HR diagram. In the future it is expected to evolve toward a WR phase. The K-band spectrum also implies a higher temperature than that of a typical yellow hypergiant and suggests that Westerlund 1-243 may be evolving back towards a hotter state.^[4]