Wednesday, 11 November 2020

Astrognome 100 Great Stars No 68 R Corona Borealis

R Corona Borealis

A pretty semi circle of stars Corona Borealis the Northern Crown graces the northern spring and early summer skies. Within it lies one of the rarest and oldest stars that can be seen with the naked eye. This is R Corona Borealis a variable star that was discovered by Edward Pigott in 1795. Pigot worked with the deaf astronomer John Goodricke in York between 1781 and 1786 and is one of the ‘Fathers of Variable Star Astronomy’



When at its brightest R CrB is about magnitude 6 which means that if you have a clear dark sky you can just about see it, when it fades down to as faint as magnitude 15 a large telescope is needed to find the star. It does this at irregular intervals sometimes separated by months or even years. It can sometimes fall in brightness slowly or very quickly and the rise back to maximum brightness almost as soon as minimum is reached. So what is going on?

R CrB is the prototype star for this small group of stars that have a cloud of soot form around then from the condensation of carbon rich dust. As the dust cloud forms the star is masked by this cloud of dust, the brightness of the star is affected and we see the star fade in brightness. Then when the cloud of soot clears R CrB returns to normal brightness.

R CrB is an old star and is only one of two stars in this class that can be seen with the naked eye the other is RY Sagittarius and is sometimes referred to as a reverse nova. The star will continues its long slide into white dwarf hood in the future.


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