R Coronae Borealis stars
R Coronae Borealis stars(R CrB stars) A small group of variable stars in which the brightness decreases – typically by about four magnitudes – at irregular intervals and often quite rapidly, returning to its normal maximum often slowly and with considerable fluctuation. The frequency of occurrence of the decrease varies greatly between the stars. The prototype of the group is R Coronae Borealis, which drops from 6th magnitude usually down to 14th. R CrB stars are supergiant carbon stars that have an abnormally low proportion of hydrogen. The decrease in brightness apparently arises from the strong absorption of light by layers of carbon particles occasionally formed in the outer regions of the stellar atmosphere and gradually blown away by radiation pressure. This interpretation is supported by the observation that the infrared output remains nearly constant during the dramatic visual dimmings. Some R CrB stars, including the prototype, are also pulsating variables. See also HdC stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006