R Coronae Borealis

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!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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8 Responses to R Coronae Borealis

  1. invaderxan says:

    Curious, isn’t it?
    It never seems to go any lower than 14, and to my knowledge, no one’s sure why.
    It seems most likely that when the star’s down to 14th magnitude, it’s giving off carbon at the highest rate it can. The gradual return to normal brightness would thus be attributed to the dust gradually dispersing due to stellar wind.
    That’s just speculation on my part though… ;)

  2. invaderxan says:

    Sometimes it’s refreshing — you can just say it’s arbitrary and leave it at that. ;)

  3. stargzr_htn says:

    Almost looks like 14 is the bottom limit. Is that so & if so why? Seems to me that it could just keep piling on more dust & keep getting dimmer.

  4. Very interesting plots!
    So, the scale is arbitrary…not very uncommon in the science world! Janskys… I must remember this next time I decide to putz around with one of the younger grad students in my group! :D

  5. invaderxan says:

    I’m not sure if any average would be characteristic. They tend to be rather unpredictable. I do have a couple of magnitude/time plots for R CrB though…

    So, looking at those, you could say there’s an average of one major eruption roughly every five years…
    And do you mean the intensity scale of that spectrum? To be honest, the actual units tend to be a bit arbitrary. Relative EM flux density like this one is fairly common (probably calibrated against a “standard star” of known luminosity). Uncalibrated, absolute flux isn’t especially helpful, as it’s only a tiny amount of light.
    It’s also not unusual for flux to be measured in obscure units like Janskys (I don’t like them, but they’re effectively W m-2 Hz-1).

  6. invaderxan says:

    Well ok, so it’s not the most exciting star to observe. ;)
    But… it’s at the lowest magnitude right now? Damn… I’d love to get hold of some of the IR spectra for that!

  7. What is the average period of time in which their magnitude of emitted light changes? Also, since I’m ignorant of this, how does the intensity scale relate to any other scales?

  8. stargzr_htn says:

    Ah yes, the boring star that I was complaining about a year ago. I have not seen it since! It has hit bottom at about 14, which is fainter than my urban sky limit.

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