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  • Writer's pictureKGB

R Corona Borealis.

Updated: Jul 1, 2023

Yet again, no clear skies means no new images. However, in this weeks blog I thought it might be interesting to focus on a star thats quite visible now, if we ever have a clear sky again. The star in question is R Corona Borealis. This star is located in the constellation of Corona Borealis (The Northern Crown), which is much more interesting than the crowning of idiots that's been taking place today (6th May 2023).

R Corona Borealis. Variable star in The Northern Crown.

R Coronae Borealis is a low-mass yellow supergiant eruptive variable star. It is the prototype of the R Cor Bor class of variable star, which fade by several magnitudes at irregular intervals. This incredible star normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades as faint as 15th magnitude! Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova", after the more common type of star which rapidly increase in brightness before fading.

R Coronae Borealis does not have a traditional name. Johann Bayer did not give it a Greek letter designation although it is marked on his map. John Flamsteed numbered all the Bayer stars but did not add any additional designations for fainter stars, so R Coronae Borealis does not appear in either of these two catalogues. The variable star designation R Coronae was introduced, as "Coronae R" by Friedrich Wilhelm Argelander in 1850. The variability of the star was discovered by English astronomer Edward Pigott in 1795.

The star R Corona Borealis as imaged by David Ritter.

The cause of the star's behaviour is believed to be a regular build-up of carbon dust in the star's atmosphere. The sudden drop in brightness may be caused by a rapid condensation of carbon-rich dust similar to soot (imagine this dropping down your chimney!), resulting in much of the star's light being blocked. The gradual return to normal brightness results from the dust being dispersed by radiation pressure. In August 2007, R Coronae Borealis began to fade to an unprecedented minimum. It fell to 14th magnitude in 33 days, then continued to fade slowly, dropping below 15th magnitude in June 2009. It then began an equally slow rise, not reaching 12th magnitude until late 2011.

This was an unusually deep and exceptional long minimum, longer even than a deep five year minimum which had occurred in 1962–7. The star then faded again to near 15th magnitude, and by August 2014 it had now been below 10th magnitude for 7 years! In late 2014, it brightened quickly to 7th magnitude but then began to fade again. By mid-2017, it had been below its "normal" brightness for an amazing ten years! It also reached a new record faintness at magnitude 15.2. The distance to R Corona is around 6,000 light years from Earth.

Artists impression of dust (soot) cloud around the star R Corona Borealis.

R Corona Borealis and indeed, The Northern Crown are rising now just to the East of the constellation of Bootes, which of course contains the star Arcturus. On a clear night the Northern Crown constellation is easily seen sandwiched between Bootes and Hercules. If you have a telescope this star is worth following, as no one knows what it might do next! Also, if the subject of variable star's interest you, there is a coal sack (soot, dust, get it, no, ok) of information available on the internet. I hope this blog has been interesting for you. As always any comments good or bad are very welcome. Stay safe and I will see you next time, metaphorically speaking of course...

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