R CrB STARS C.S. JEFFERY
C. SIMON JEFFERY
The R Coronae Borealis stars (RCB) stars are one of the longest known classes of variable star. They fade dramatically and unpredictably by factors of up to one thousand within a few weeks. Over succeeding months, they gradually recover their original brightness. This spectacular fading is caused by the formation of sooty dust clouds above the surface of the star. The surfaces of RCB stars are unusually poor in hydrogen, and rich in carbon and nitrogen, which implies that they are the remnants of evolved stars. Practically all RCB stars pulsate, which may help to explain the dust formation episodes. However, many questions remian regarding their evolutionary origin and the physical mechanism of dust formation.
This article was written with the non-specialist astronomer in mind - particularly because RCB astronomy is one area where amateur observers continue to make an invaluable contribution. It reviews the historical context and the major properties of RCB stars, their physical characteristics and evolutionary status. It introduces a number of related objects and describes some of the unsolved problems still posed by RCB stars.