Star of the Month
R Coronae Borealis
Position: 15 hrs 48 min 34.4 sec +28 degrees 09 min 24 sec
Due south at 23:13 (BST) on 15 June 20
R Coronae Borealis
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
The vast majority of stars are mostly hydrogen with some helium, corresponding initially to roughly the composition of the universe soon after the Big Bang. However as the star ages, some of the hydrogen is fused into helium, the process which fuels the star’s luminosity. This is a very slow process in a star about the mass of the Sun (fortunately for us), even now after nearly five billion years, the Sun still only has 25% of its total mass in the form of helium. But the yellow supergiant R Coronae Borealis is nearly 90% helium and 1% hydrogen although its mass is slightly less than that of the Sun, a so-called extreme helium star. Such stars are often formed by the merger of two white dwarfs and it is though R Coronae Borealis is a merger of a carbon-oxygen white dwarf and a helium white dwarf. But the presence of lithium in its atmosphere makes it possible that was created by a late helium flash in a very elderly (post-AGB) star whereby in a rapid runaway reaction, helium is converted into carbon.
R Coronae Borealis is the type star for a group of fairly rare variables called R Coronae Borealis variables. It was discovered by the English astronomer and early variable star observer Edward Pigott in York in 1795, just before he moved to Bath. Usually it is only slightly variable, with fluctuations of about 0.1 magnitude of periods of roughly 40 and 51 days as it is pulsating. However once in a while, it can undergo a major dimming thanks to clouds of carbon dust (i.e. soot) that it puffs out from time to time, in a similar manner to the dimming of Betelgeuse in early 2020. Hence the main interest for the variable star observer is to keep a look-out for these marked drops in luminosity. The star usually recovers fairly quickly after these dimmings although there can be further dips before it finally reaches its normal brightness. There was a dimming in 1962 to 1967, but this was surpassed by the great dimming of 2007-2018. In August 2007, it initially dropped to 14th magnitude in a month, then continued to dim until it reached 15th magnitude in June 2009. It then started to brighten again but only slowly. In 2014 it reached 7th magnitude but faded again. However it has now returned to its “normal” magnitude although perhaps a little dimmer than its historical magnitude of 5.7; it varied between 6.0 and 6.5 in March 2021 which suggests that it is fluctuating more than previously as well.
You will need binoculars or a telescope to see R Coronae Borealis, but it is fairly easy to locate. Look for a yellowish star within the “crown” which forms a triangle with Epsilon and Delta Coronae Borealis (or with Delta and Gamma Coronae Borealis) and lies roughly half-way in a line between Alpha Coronae Borealis (Gemma or Alphekka) and the rather dim Iota Coronae Borealis at the left-hand end of the “crown”.