Star of the Month
R Coronae Borealis – the sooty star that dims with no predictable pattern
Corona Borealis, the Northern Crown, is a semicircular pattern of stars located off the northeast shoulder of Boötes. It is prominent during spring but can be seen all year round if you are prepared to put in some early mornings. The shape encloses an area of sky in which you may, or may not, see a faint star on the threshold of naked eye visibility. This is R Coronae Borealis, a remarkable variable star with an irregular period.
At its brightest R Coronae Borealis is on the edge of naked-eye visibility at mag. +5.9, but it is known for its unpredictable drops in brightness, taking typically a year to recover. Back in 2007 it began to fade to an unprecedented minimum, reaching 15th magnitude by the summer of 2009. It remained dim before eventually exhibiting a slow brightening to mag. +12.0 by summer
2011. Approximately two years after it started to climb, R Coronae Borealis had dimmed again. This collapse took it back to 14th magnitude. It recovered to 12th-magnitude by spring 2012, peaking at mag. +10.5 in spring 2013, then plunged back to mag. +14.5 that autumn. Another brief peak occurred before it fell back to mag. +15.0 by the end of 2013. By the start of 2015, it had almost managed to
recover, reaching 7th-magnitude. This again was short-lived, but by the start of 2017 it was on the increase again.
The reasons for the variations are related to R Coronae Borealis being a carbon star. It sheds carbon rich clouds into space which cool to form soot. The soot blocks the star’s light causing it to dim but when the clouds disperse, it appears to brighten once more.
You can check the current brightness of R Coronae Borealis by comparing it to its neighbours