R Centauri

For r Centauri, see HD 115310.

R Centauri (R Cen) is a Mira variable star in the constellation Centaurus. When it is near its maximum brightness, it is faintly visible to the naked eye under very good observing conditions.

R Centauri

The visual band light curve of R Centauri, from AAVSO data[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Centaurus
Right ascension	14h 16m 34.319s[2]
Declination	−59° 54′ 49.29″[2]
Apparent magnitude (V)	5.2 - 11.5[3]
Characteristics
Evolutionary stage	AGB[4]
Spectral type	M5IIe[5] (M4e-M9.5[3])
U−B color index	+1.24[6]
B−V color index	+2.04[6]
Variable type	Mira[3]
Astrometry
Radial velocity (Rv)	−19.8[7] km/s
Proper motion (μ)	RA: −9.077[2] mas/yr Dec.: −6.692[2] mas/yr
Parallax (π)	1.1082±0.2413 mas[2]
Distance	approx. 2,900 ly (approx. 900 pc)
Absolute magnitude (MV)	−1.49 (at maximum)[8]
Details
Radius	615[9] R☉
Luminosity	47,000[5] L☉
Surface gravity (log g)	−0.733[9] cgs
Temperature	3,450[5] K
Other designations
R Centauri, HR 5326, HD 124601, CP−59°5476, HIP 69754, SAO 241580, GC 19234, CCDM J14166-5955
Database references
SIMBAD	data

The distance to R Centauri as indicated by its Gaia Data Release 3 parallax is about 2,900 light years, but that is considered to be potentially unreliable.^[2] The Gaia Data Release 2 parallax was negative and relatively meaningless.^[10] The older Hipparcos parallax suggested a distance of about 1,300 light years, but with a wide margin of error.^[11] Estimates based on an assumed brightness for the star, adjusted for extinction, give distances as low as 750 light years.^[12]

The effective temperature of R Centauri's photosphere has been calculated by different methods to be 2,403 K^[9] or 3,450 K.^[5] Its luminosity is even more uncertain, depending on assumptions about the distance. At a distance of 384 pc, the bolometric luminosity would be 11,342 L_☉,^[9] while assuming a larger distance of 640 pc the luminosity would be over 47,000 L_☉.^[5] In either case, it is a very large star, over 600 R_☉.^[9]

Benjamin Apthorp Gould discovered R Centauri, in 1871.^[13] The star is a Mira variable and its brightness varies from magnitude +5.2 to +11.5 with a period of about 500 days. It used to have an unusual double-peaked light curve, but by 2001 this had reverted to an almost normal single-peaked curve. Prior to 1950 the period was about 550 days, but since then has decreased to about 500 days. A 2016 analysis of ASAS data derived a period of 498.84 days.^[14]

It is thought that the unusual behaviour of R Centauri is caused by a flash in the helium shell around its core, which occurs periodically in asymptotic giant branch (AGB) stars as the mass of the helium shell increases with helium from the outer hydrogen shell.^[4] It is also an H₂O maser source.^[15]