R Coronae Borealis

R Coronae Borealis is a peculiar low-mass yellow supergiant star, and is the prototype of the rare RCB class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, in the constellation of Corona Borealis, but at intervals of several months to many years fades to as faint as magnitude 14. Over successive months it gradually returns to its normal brightness, giving it the nickname "Fade-Out star," or "Reverse Nova".

The variability of R CrB was discovered by the English astronomer Edward Pigott in 1795. It was known as Variabilis Coronae, "Variable of Corona". In 1935 it was the first star shown to have a different chemical composition than the sun via spectral analysis.

R Coronae Borealis is a variable star and the prototype of its class. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii. Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days. These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under one M_☉.

At irregular intervals a few years or decades apart R Coronae Borealis fades from its normal brightness near 6th magnitude for a period of months or sometimes years. There is no fixed minimum, but the star can become fainter than 15th magnitude in the visual range. The fading is less pronounced at longer wavelengths. Typically the star starts to return to maximum brightness almost immediately from its minimum, although occasionally this is interrupted by another fade. This irregular fading is the defining characteristic of the R Coronae Borealis variables and is thought to be due to the condensation of carbon into "soot" surrounding the star.