R CrB

R Coronae Borealis, the prototype star for a rare type of variable star...

A list of the few known R CrB stars!

These are among the strangest of all variable stars, and also peculiar in that their normal (i.e., non-variable) state is maximum, so while the dwarf novae have outbursts, these have fades. This is still a rather mixed bag of stars, and the class appears to include some stars which do not conform to the classical R Coronae pattern.
They are typically yellow stars, not too unlike the Sun, but with a very high proportion of Carbon in their atmospheres. At sufficiently low temperatures this carbon will condense out into vast clouds of soot, and cut off large amounts of light from the star, though a perusal of some of the following will show you that astronomers are by no means as agreed as this! If these clouds are in our line of sight, we will see the star fade, sometimes by us much as eight magnitudes in the case of R Coronae or RY Sagittarii. Naturally, these variations are completely random and unpredictable, both in style and duration, though typically the recovery to maximum is slower than the fall, and tends to be interrupted by irregular fluctuations.
Many of these stars, RY Sgr being the best example, show independent, quasi-periodic fluctuations obviously of some pulsatory character, and they appear to be related to the Cepheid variables in this respect, though those stars do not contain the abnormal proportion of carbon in their atmospheres as do the RCB stars.
The following interesting snippets are from the AAVSO's "Variable star of the Month" and includes links to several other useful astrophysical sites.

"The variable star R Coronae Borealis is a jewel worthy of a place in any crown. It is one of the most interesting and most peculiar of all variables, and is often called the 'ideal' irregular variable. Its times of minima are distributed absolutely at random, according to the laws of pure chance" (Margaret Mayall, The Review of Popular Astronomy, May/June 1962, p33).

R Coronae Borealis (R CrB) has been a favorite with observers ever since its discovery nearly 200 years ago by the English amateur, Edward Pigott. Located inside the bright circlet of stars that form the Northern Crown, R CrB is usually easy to find with binoculars or even the unaided eye at 6th magnitude.
The critical contribution that amateurs have made to astronomy by observing R CrB is articulated on the ISO (Infrared Space Observatory)/ESA (European Space Agency) 12 June, 1996 information note page and is quoted below. The ISO homepage may be located at. The ESA homepage may be found at www.esa.int/.
Shortly before ISO's launch, amateur astronomers reported that the star called R Coronae Borealis was fading from view. This elderly star is normally quite easy to see with binoculars, but intermittently it puffs off clouds of dust that almost hide it from view. Professional astronomers do not have the time to monitor irregularly variable stars, and rely on amateurs to alert them to such events like that in R Coronae Borealis. A few months later when the star could be seen only with powerful telescopes, ISO obtained an infrared spectrum of the star in just one minute, using the high-speed spectroscopic facility of the photometer ISOPHOT.
"We caught this star smoking," says Helen Walker of the Rutherford Appleton Laboratory in England, who was in charge of the observation. "The amateurs saw the star fade from view in visible light in October, but it remained bright in the infrared. The telltale wavelengths revealed sooty carbon compounds newly formed in the star's vicinity. Without ISO [and amateur astronomers] we could not hope to analyse such a striking event."
R CrB is currently recovering from a period of fadings that started in August of 1999. The AAVSO encourages observers to monitor this elusive star and report their observations to headquarters. For more information on submitting observations, visit their Observations page and Membership page.
The absolute magnitude of the RCBs in the LMC and the small number of Galactic RCBs implies that they are very rare stars. Together with statistics for related stars, the total number of hydrogen-deficient stars in the Galaxy is estimated to be about 1000 ([Warner 1967]). Whilst the luminosity of RCBs is similar to that of other low-mass stars as they evolve rapidly from the asymptotic giant branch to become white dwarfs, their frequency is much lower. They do not, it seems, represent a stage in the evolution of all normal low-mass stars. From the preceding estimates of mass and luminosity, it is seen that RCBs have very high luminosity-to-mass ratios ( ) and lie close to the Eddington limit for radiative stability. If they were any more luminous, their atmospheres would be expelled by radiation pressure.