The Mysterious Disappearing Act Of R Coronae Borealis

R Coronae Borealis is tucked inside the crown not far from the bright star Gemma (JEH-mah). Maps created with Stellarium

Stars are big and live long lives. You need patience to see for yourself what makes them tick. Since 1982 I’ve kept my eye on an obscure star barely visible to the naked eye in the constellation Corona Borealis. The star goes by the deceptively simple name of “R” for R Coronae Borealis.

R CrB burns with a steady light for years at a time as it did from 2003 to 2007. At 6th magnitude, I could see it from the countryside on any old night without optical aid. Then without warning on July 6, 2007 it began to fade from sight, tumbling headlong into darkness with each passing week. Six months later I needed a 10-inch telescope to see it.

Finding Corona Borealis is a snap. Face south around 10 p.m.-10:30 p.m. and draw an arc from Saturn and Spica up through Arcturus and eastward to Corona's semi-circle of stars.

English amateur Edward Pigott, who scanned the skies in the late 1700s, was the first to notice R CrB’s peculiar habit of fading away. Today we know that R is the prototype of a class of stars called R Coronae Borealis variables that fade away at random intervals only to return to full light weeks, months or even years later.

In 2007 I got a thrill watching it fade by a magnitude a week. How often do you get the chance to see a star change so rapidly? Most of them are the epitome of stability.

Once R CrB bottomed out at 15th magnitude (very faint!) in early 2009 it went into hiding for almost two years. Would the star ever recover, I wondered? Starting in late 2010 through early 2011  R ticked upward a full magnitude, and I looked forward to seeing it shoot to the top. It wasn’t to be. It promptly faded again, putting off a second recovery until the fall of 2011.

A light curve for R CrB based on observations by amateur astronomers shows its dramatic disappearing act and slow recovery. Credit and copyright: Gary Poyner

Some stars fade and re-brighten because they’re partially eclipsed by a companion star. Others are unstable, brightening and fading as they expand and contract in predictable cycles of days to months.

R CrB stars are yellow supergiants 10,000 times as bright as the sun that have lost their outer envelope of hydrogen gas. Their atmospheres are rich in helium and carbon instead.

Stars are element-makers. Inside their searing interiors, hydrogen atoms fuse to make helium. When the hydrogen’s used up, helium fuses to make carbon. Every second, our own sun fuses 700 million tons of hydrogen into 695 million tons of helium and 5 million tons of pure energy. The energy, in the form of gamma rays, takes a million years to reach the surface as the light and heat we cherish.

R Coronae Borealis stars loft carbon dust into space in big puffs. When a puff is aimed in our direction, we see the star eclipsed or blocked by the dust for a time. Credit: ESO

The much hotter and more massive R CrB expels a powerful wind of helium and carbon atoms into space at 100,000 times the rate of the sun’s solar wind. A small percentage of that carbon condenses into discrete “puffs” of soot. If one of these carbon dust clouds happens to be in our line of sight, it blocks the light of R, causing it to dim. Think of the soot that forms on the inner glass of an old-fashioned oil lamp and you get the picture. Only when the cloud has expanded and thinned – or is blown away by continued winds –  does the star return to its normal brightness.

R CrB at its faintest. Like to follow the star's ups and downs in your own telescope? Click on the photo to link to a finder chart with stars labeled with magnitudes (decimals omitted). Credit: Joe Brimacombe

Nobody knows exactly what triggers R CrB’s sooty outbursts, but every few years it belches out another cloud and goes into hiding again. This latest dimming saw the star reach a historical low in brightness; it’s also been the longest fade on record.

Since the fall of 2011, R has been ever-so-slowly climbing in brightness to its current level of 12.5 magnitude. A 6-inch telescope will show it easily enough.

Will it follow through and return to full brightness in the coming weeks or months? Nobody knows. In 30 years, R has shared a half dozen of its ups and downs, trying to tell us on a human time scale what it’s like to be a star. Like listening to a great teacher, I spend a few minutes by its side taking in the lesson every clear night.

13 Responses

  1. caralex

    Can you imagine the inhabitants of a hypothetical planet orbiting R CrB? “Oh, here we go, R’s in a black mood again!”

  2. Mary Munn

    I think it would depend o the nature of the Carbon puff.
    just for the sake of discussion…
    perhaps at an equivalent of our 1 AU, the effect may be minor, yet coalescing further in space to form the eclipsing “brown out”

    just a thought

    1. astrobob

      Hi Mary,
      Actually you might be right. The puffs leave the star’s surface but condense into soot clouds much further out. The effect on inner planets would be minor, but perhaps not for very distant ones. Of course, distant ones would be the only habitable ones because the star is so huge and hot. Lots of assumptions here regarding planets but fun to play it through.

  3. Doug

    Hey bob I know this is off topic but asteroid 2012 kp24 is expected to come as close as 0.1 LD. Any chance of this thing hitting us? Seems a little too close for comfort and also a little frightening.

    1. astrobob

      On May 28, 2012 KP will be 0.1 LD or some 24,000 miles away. It’s a very small asteroid – about 75 feet – and will miss Earth by three times our planet’s diameter. This is considerably farther than several other recent closer approaches by other asteroids.

  4. Giorgio Rizzarelli

    Thank you for this post! So this is why I couldn’t see this star months ago!!
    This fall I compiled a list of to-see Carbon stars (which as you know always have incredible colors in telescope), and also found this one by chance in Stellarium. But, although it was listed there as magnitude 6, I couldn’t see it at all with my C8 tube. I took note of this curious fact, suspecting an error in the Stellarium catalog, like wrong coordinates. Now thanx you I have the solution: the catalog wasn’t updated to the latest years magnitude. And I couldn’t expect such big variability in few months. This night I look forward to check if I can see the star now!
    When I found the star in Stellarium I was also astonished by its enourmous distance (6000ly it says, or better 75000ly as from the parallax listed in Simbad, that is on the other side of Milky Way!). And, since as you say when uncovered it was visible at 6th magnitude at naked eye (in country), it must be a really giant star.
    Nice to meet you and thank you for your astroblog! Clear skies!

    1. astrobob

      Hi Giorgio,
      Glad the blog helped clear up the question of R CrB’s brightness. You should have no problem seeing it in your C-8. You can use the AAVSO chart. I linked to one, but you’ll see several are available from wider field to close in. Just go to and enter R CrB and select “Create a finder chart”. If you then click on “Return and Replot” it takes you to a page where you can print several different scales and customize the charts. Good luck!

  5. randy

    What’s the status of R Coronae Borealis since this post was written? Has it continued to brighten?

    1. astrobob

      Hi Randy,
      It did for a while until reaching about 10th magnitude. It declined again this spring and now shines only around 13.5 magnitude.

  6. David Majors

    Hi Bob- enjoyed this article.
    I’ve been following this star now for about the same length of time you have. One of my favorites
    I have a couple of good stories about this star.
    A couple of years ago at one of our star parties I showed 2 6’th graders this star when I was talking about variable stars. They got interested and ended up doing a science fair project on photometry- won 1’st place.
    Last year at another star Party one of Dr. Russ Genet’s freshman students tried her hand at an estimate. She saw it as dimmer than it should have been. I verified her estimate. When I got home I found out we we were among the very first to spot the new decline last May. I let Dr. Genet know and he had her write it up.

    1. astrobob

      Great story. I bet she was excited to find out her observation was among the first. What a great bit of motivation for writing that paper! R CrB has certainly been faint this winter. I sometimes will catch it in the morning hours when I’m comet watching.

Comments are closed.