R Coronae Borealis (R CrB)
R Coronae Borealis (R CrB) is a star in the constellation Corona Borealis, magnitude 5.9.
The ‘reverse nova’: a soot-shrouded star that vanishes for months when it chokes on its own carbon.
Most of the time R Coronae Borealis sits quietly near the limit of naked-eye vision. Then, without warning, it fades — sometimes to below 15th magnitude, thousands of times fainter — and may stay hidden for months or years before it slowly recovers. It is a ‘reverse nova’, the prototype of a whole class of variable stars.
The culprit is soot. R CrB’s atmosphere is about 90% helium, with less than 1% hydrogen and most of the rest carbon. Every so often carbon condenses into soot-like dust above the star, blocking its light, until radiation pressure blows the cloud away. That odd make-up was a first: in 1935 it became the first star shown, from its spectrum, to have a different chemical composition from the Sun.
How such a star forms is a puzzle. A favourite idea is that two white dwarfs, one of carbon and oxygen and one of helium, merged into one; another is a very late helium flash in a dying star. Edward Pigott discovered its fades in 1795.
R Coronae Borealis (R CrB) is 4,600 light-years away: the light we see from it set off around 2580 BC, around the time the Great Pyramid of Giza was completed.
Did you know?
- A decade in the dark. In August 2007 R CrB began a fade to an unprecedented minimum. By mid-2017 it had spent ten years below its normal brightness, reaching a record faint magnitude of 15.2.
- One of only two. R Coronae Borealis and RY Sagittarii are the only stars of their class bright enough to be seen with the naked eye.
- Puffs of soot. Hubble images show clouds of dust around R CrB. Puffs condense close to the star and, seen to one side of it, look like little cometary knots.
Normally a 6th-magnitude yellowish star inside the curve of the Northern Crown, just at the edge of naked-eye visibility. Check it in binoculars every few weeks: without warning it can fade to 14th magnitude or below and stay hidden for months.
When and where to see it
It is best placed in the evening sky around June, when it crosses the meridian at about 10 pm local time (around midnight in May). It can be seen from everywhere north of 62°S and never sets north of 62°N.
At its highest it stands 68° up from 50°N, 83° up from 35°N, 62° up from the equator, 27° up from 35°S.
Facts
- Type
- Star
- Constellation
- Corona Borealis
- Designations
- R CrB, HR 5880, HD 141527, HIP 77442
- Magnitude
- 5.9
- Position
- RA 15h 48.6m, Dec +28° 9′ (J2000)
- Spectral type
- G0Iep
- Colour
- yellow (B−V 0.60)
- Brightness
- varies from magnitude 5.7 to 15.2
- Distance
- 4,600 light-years
- Discovered
- Edward Pigott, 1795, Discoverer of its variability
Open AstroPilot to see when R Coronae Borealis (R CrB) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.