R Coronae Borealis star
Typical is the behavior of R Coronae Borealis (R CrB), the prototype of the class. R CrB is an F8 or G0 Ib supergiant whose variability was discovered nearly 200 years ago by Edward Pigott. Most of the time, R CrB is at its maximum of around magnitude 6 (see light curve above). But at irregular intervals it goes, over a period of several weeks, into a decline of up to 8 magnitudes. The star may remain faint for many months or have several recoveries and declines in succession. Often the final rise back to maximum light is slow, taking several months to a year.
The RCB phase is probably fairly short-lived, maybe on the order of 1,000 years, as evidenced by the fact that less than 50 of these stars are known. Their evolutionary status is uncertain, though there are two main theories: (1) the Double Degenerate (DD) model or (2) the Final Helium Shell Flash (FF) model. Both involve the expansion of white dwarfs to the supergiant phase. The DD model invokes the merging of two white dwarf stars, while the FF model assumes that a single white dwarf star expands to the supergiant stage by means of a final helium flash.
Related entry• variable stars
Related category• TYPES OF STAR
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