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: http://star.arm.ac.uk/~csj/rcrb_rev/node3.html
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RCB stars are named after a star just visible to the naked eye in the constellation of Corona Borealis. RCrB was discovered by Edward Pigott, an English amateur astronomer, who had been observing a star in the `Northern Crown' for over a decade. In the spring of 1795 he noticed that it had disappeared, but over succeeding months it gradually reappeared and recovered its original brightness. Pigott estimated a period of 10.5 months for this phenomenon, but he was puzzled that the changes did not repeat themselves precisely.
In fact, unlike CEPHEID and other variable stars known at the time, RCrB's changes in brightness are irregular and unpredictable. This irregularity has prompted numerous astronomers to observe RCrB so regularly that its light curve has now been monitored continuously for almost two hundred years.
Since 1795, other RCB stars have been discovered and their properties have been carefully observed. The defining property is their characteristic light curve, marked by the irregular fadings first seen by Pigott. In addition, nearly all RCB stars pulsate with periods between 40 and 100 days. Their surfaces contain very little hydrogen, unlike the vast majority of stars which have about 90% hydrogen. Instead their surfaces are rich in helum and carbon, which must have been produced by nuclear reactions deep within the star.
It is established that the sudden fadings of RCBs are caused by clouds of carbon-rich dust passing in front of the star, but it is unclear what causes carbon gas in the stellar atmosphere to condense into dust. Other questions remain about the precise composition and origin of RCB surfaces.
Remarkable as their properties are and extensive as two centuries of observations have been, RCB stars continue to pose many problems. The irony is that the original question posed by Pigott's observations in 1795 remains essentially unsolved - ``what causes the sudden fadings of RCrB ?''