R Coronae Borealis stars, or RCB stars, are an important class of VARIABLE STAR. Stars vary in many different ways, often by changing the amount and colour of the light they emit. They can change regularly or irregularly. RCB stars, remarkably, do both.
RCB stars are best known because of their irregular `fades'. Unpredictably and every few years or so, their brightness suddenly drops by a huge amount. Then they get slowly brighter until they look the same as before the fade. The fades are caused by enormous clouds of dust which cover a large part of the star's surface. The dust is thought to be made of small particles of carbon - rather like ground-up pencil sharpenings.
Why the dust clouds form is harder to understand. As well as irregular fades, the light output from RCBs goes up and down by a few per cent every 40 days or so. This is due to the star swelling and shrinking - known as PULSATION. It seems that these pulsations may cause dust to form which is then thrown out from the star. Thus, indirectly, they may be responsible for the irregular fades.
Another remarkable thing about RCB stars is that their surfaces contain almost no hydrogen, the most common element in space. In fact, they are mostly made of helium and carbon. These elements are made by nuclear reactions deep inside stars. Since they are now seen on the surface, RCB stars must be very old.
This article introduces the history and basic properties of RCBs, before exploring them in more detail. Three brief sections address the origin of RCBs, the occurrence of RCB activity in other stars, and recent progress towards solving some of the outstanding problems.