Astronomical Society of Edinburgh
Journal 46

The Recent Fade of R Coronae Borealis

R Coronae Borealis (R CrB) is one of the best and most interesting variable stars for the amateur to observe, and may be found on almost any star map, including that in the BAA Handbook for 2003, available in our Library. These notes are taken from the description of the star therein, by John Toone (pp 93-94), and from the Webb Society Variable Star Handbook.

Light curve showing dimming and brightening of the star between January and July 2003

R CrB is a yellow, very luminous star of a type which is vary rare, indicating a short-lived phase in stellar evolution. They have probably passed through the Red Giant stage and having ejected their hydrogen-rich envelopes are about to evolve into white dwarfs. Their atmospheres are rich in carbon and other elements pumped up from the stellar interior by helium flashes. They exhibit sudden fades down to deep minima when clouds of carbon particles are ejected in our direction at irregular intervals, sometimes only once in many years, and may remain faint for any length of time up to several months or even years, sometimes with partial recoveries and dips. R CrB itself faded to about mag 8 then 10 from its usual 6 in late 1998, recovering in early 1999, and was observed by some of us. Unfortunately its "timing" wasn't very good because like the fades before that (1989, 1993) it happened in the autumn when the star was low in the western sky then we lost it until the spring. However, this present fade happened in early spring when the star was coming up in the east in the evening and with the remarkably fine weather we have enjoyed this year a lot of observations were possible. By a happy coincidence R CrB was chosen as the BAA's "Variable Star of the Year" for 2003.

Dave Gavine and Des Loughney made systematic estimates using binoculars and telescopes with a set of detailed BAA charts. The star became so faint at the end of February that Dave had to borrow Lorna's 203 mm Schmidt-Cassegrain but by the first week of March we lost it completely. Now the star has recovered to its maximum brightness but should be watched because it could fade again. The light curve shows the variation in magnitude from January to the start of July. Observations from the discovery by Piggott in 1795 suggest that the period of the star may be shortening.

Dave Gavine