S Coronae Borealis

Not to be confused with Sigma Coronae Borealis.

S Coronae Borealis (S CrB) is a Mira variable star in the constellation Corona Borealis. Its apparent magnitude varies between 5.3 and 13.6, with a period of 360 days—just under a year. Within the constellation, it lies to the west of Theta Coronae Borealis, and around 1 degree southeast of the eclipsing binary star U Coronae Borealis.^[10] When it is near its maximum brightness, it can be seen with the naked eye, under good observing conditions.

S Coronae Borealis

Observation data Epoch J2000      Equinox J2000
Constellation	Corona Borealis
Right ascension	15h 21m 23.9561s[1]
Declination	+31° 22′ 02.573″[1]
Apparent magnitude (V)	5.3 – 13.6[2]
Characteristics
Spectral type	M7e[3]
U−B color index	0.36[1]
B−V color index	1.71[1]
Variable type	Mira[2]
Astrometry
Radial velocity (Rv)	-5.12[4] km/s
Proper motion (μ)	RA: -7.73[1] mas/yr Dec.: -13.03[1] mas/yr
Parallax (π)	2.39±0.17 mas[5]
Distance	418+21 −18[5] pc
Absolute magnitude (MV)	-0.8±0.3[6]
Details
Mass	1.34[7] M☉
Radius	308[7] (537–664)[8] R☉
Luminosity	5,897[9] L☉
Temperature	2,864[7] (2,350–2,600)[8] K
Other designations
S Coronae Borealis, HD 136753, BD+31°2725, HIP 75143, GC 20662, SAO 64652, GSC 02563-01338, DO 15223, AAVSO 1517+31
Database references
SIMBAD	data

Variability

Light curve for the Mira variable S Coronae Borealis over six years

S Coronae Borealis was discovered to vary in brightness by German amateur astronomer Karl Ludwig Hencke in 1860.^[11] It was classified as a long period variable star as other similar objects were discovered,^[12] and later as a Mira variable.^[2] The maximum range of variation is from magnitude 5.3 to 13.6 although individual maxima and minima can vary in brightness. The period of 360 days is fairly predictable.^[13]

Properties

S Coronae Borealis is a cool red giant on the asymptotic giant branch (AGB). It pulsates, which causes its radius and temperature to change. One calculation found a temperature range of 2,350 K to 2,600 K,^[8] although a more modern calculation gives a temperature of 2,864 K.^[7] Similarly a calculation of the varying radius gives 537 to 664 R_☉^[8] although a modern calculation of the radius gives 308 R_☉.^[7] The bolometric luminosity varies much less than the visual magnitude and is estimated to be 5,623 L_☉.^[7] Its parallax has been measured by very-long-baseline interferometry (VLBI), yielding a result of 2.39±0.17 mas, which converts to a distance of 1,300±100 light-years.^[5]

The masses of AGB stars are poorly known and cannot be calculated from their physical properties, but they can be estimated using asteroseismology. The pulsations of S Coronae Borealis lead to a mass estimate of 1.34 times that of the Sun.^[7]