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S 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]

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]

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 millarcseconds, which converts to a distance of 1300 ± 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]