R Coronae Borealis is a rare breed of star. Rare enough that it gives it’s name to it’s own class of variable stars. Around 30 R Coronae Borealis variables are known in the sky, classified because of a bizarre shared trait. R CrB stars periodically fade by several magnitudes (temporarily appearing a deep red colour) before gradually regaining their original brightness.
R Coronae Borealis itself was first dicovered in the 18th century by english astronomer, Edward Pigott — also the first person to note the star’s odd behaviour. R CrB itself fades periodically and unpredictably from it’s normal magnitude of +6 to as faint as +14.
The clue to what’s going on lies in the fact that R CrB doesn’t change brightness much in infrared. It’s actually an eruptive variable star. Periodically, it undergoes an outburst of carbon rich material which forms a dense shell of dust around the star considerably obscuring it’s light. Literally star soot, this dust is also responsible for the deep red colour observed while the star is “fading out”.
The stars themselves can be identified through their spectra. In fact, here’s the spectrum of R Coronae Borealis itself.
These stars have hydrogen deficient photospheres, meaning their balmer lines (H-alpha etc) are weak or absent. Most stellar spectra have a sharp H-alpha emission spike around 6563Å, and a H-beta line around 4861Å too. It does have C2 absorption lines around 5165Å and 4735Å though — characteristic of a yellow supergiant. Most R CrB stars are spectral class G or F.
R CrB stars are interesting to professionals and amateurs alike. Amateurs like to observe them, because they’re rather an interesting and unusual type of variable star to keep track of and perform photometry on. Professionals are interested in them, because no one fully understands the mechanism behind the fading or knows precisely how or why they evolve. Another interesting thing about R CrBs is that when they’re at their lowest magnitude (in other words, at full eruption), they they exhibit infrared emission bands just like certain nebulae. Infrared emission bands which most people attribute to PAHs. These stars probably make a dazzling array of organic molecules and carbon clusters!