page_title: R Coronae Borealis
section_title: Variability
hierarchical_section_title: R Coronae Borealis / Variability
caption_attribution_description: English: Light curve of R Coronae Borealis from 1990 to 2017, created using the AAVSO light curve generator tool.
context_page_description: R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Cor Bor class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to 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 increases in brightness before fading.
context_section_description: The variability of R Coronae Borealis was discovered by English astronomer Edward Pigott in 1795.  In 1935 it was the first star shown to have a different chemical composition to the Sun via spectral analysis.
  R Coronae Borealis is the prototype of the R Coronae Borealis class of variable stars. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii.  Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days.  These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under one M☉.
  At irregular intervals a few years or decades apart R Coronae Borealis fades from its normal brightness near 6th magnitude for a period of months or sometimes years.  There is no fixed minimum, but the star can become fainter than 15th magnitude in the visual range.  The fading is less pronounced at longer wavelengths.  Typically the star starts to return to maximum brightness almost immediately from its minimum, although occasionally this is interrupted by another fade.  The cause of this 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, resulting in much of the star's light being blocked.  The gradual restoration to normal brightness results from the dust being dispersed by radiation pressure.
  In August 2007, R Coronae Borealis began a 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.  It then faded again to near 15th magnitude, and by August 2014 it had 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 ten years. It also reached a new record faintest at magnitude 15.2.
caption: Light curve of R Coronae Borealis from 1990 to 2017, showing the unprecedented deep minimum
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