R Coronae Borealis Stars

R Coronae Borealis (RCB or R CrB) is the prototype star for this class of variables. RCB variables are rare low-mass supergiant stars whose surface atmospheres are deficient in hydrogen and rich in carbon. RCB stars undergo irregular, often abrupt fading events of between 1 and 9 magnitudes. Then, over a period of several months or years, they return to their typical brightness. Because of this pattern, RCB variables have been given the nickname "reverse novae".

Only about 150 RCB variables have been discovered within the Milky Way Galaxy, making them an exceptionally rare class of star. Astronomers suspect that RCB variables are formed by a merger between two white dwarf stars. The fading events happen when carbon molecules condense into dust and soot that blocks visible light from the star. The infrared light observed from the star remains virtually the same, because infrared waves can penetrate the dust.

R Coronae Borealis

R Coronae Borealis normally has an apparent magnitude of about 6, which means it is bright enough to be seen with the naked eye. During a fading event, the star becomes as dim as 15 magnitudes. Meaning that it becomes a million times fainter, and appears to vanish from the night sky!

Below is a sonified video of observations of R Coronae Borealis taken from 2002 to 2021. Because the time range spans nearly two decades, the video has been compressed so that each interval of time represents two weeks of time on Earth. The result is a minute long video that still exhibits most of the interesting behavior of an RCB variable. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). The observations begin at R CrB's typical brightness of about 6 magnitudes. The normal brightness is interrupted by a quick fading event to about 12 magnitudes. Then, in 2007, R Coronae Borealis began an unprecedented, long, drawn-out fade to about 15.2 magnitudes. During the next decade of observations, R CrB brightens and fades intermittently but doesn't reach its normal 6 magnitude level until early 2019. The unpredictable, enigmatic nature of R Coronae Borealis variables is on full display during this light curve video.

Next, the observed spectrum of R Coronae Borealis is shown below as a sonified video. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3800 to 7200 angstroms. R CrB is low-mass and cool, so the baseline of its spectrum is highest at longer wavelengths towards the end of the video.

The spectrum of R Coronae Borealis was provided by the FAST Spectral Archive.
The light curve is an optical V-band from AAVSO. The longevity and consistency of the AAVSO data gathered for R Coronae Borealis is a testament to the organization and the important contributions made by amateur astronomers.