R Coronae Borealis Quits Smoking, Comes Clean

Corona Borealis or the Northern Crown is a little crown or horseshoe-shaped constellation visible in the western sky at nightfall in late August and September. Its most famous star is R Coronae Borealis, a variable star that can unexpectedly fade from sight. Bob King

Today’s topic is about a fascinating star called R Coronae Borealis (R CrB) in the constellation of the Northern Crown. Unlike most of the stars we see R CrB does not shine with a steady light night after night, year after year. Its brightness varies in a completely unpredictable way. Normally, the star shines at magnitude 6 — the naked-eye limit under dark skies — but every so often R fades away.

To find Corona Borealis, start with the Big Dipper and follow the arc of its handle to the bright orange star Arcturus, which stands about halfway up in the western sky at nightfall. Corona Borealis with its brightest star, Gemma, is located a fist directly above Arcturus. Once you’ve spotted the curlicue use the map below to pinpoint R CrB with binoculars. Stellarium

Within a week it becomes invisible without optical aid. And when faintest, around magnitude 15, it’s only visible in larger amateur telescopes. R CrB is more than 1,500 times fainter at minimum light compared to its normal bright self. This last happened starting in early July 2007. Prior to that date R had been humming along at peak brightness since about the year 2000. After bottoming out in 2009 it took the star 10 years to claw its way back to normality. R CrB now gleams at magnitude 6 again, the first time it’s done so in more than 18 years!

This looks all science-y but have no fear. It’s just a serpentine wriggle tracking the brightness of R CrB from its minimum light in 2009 to its return to maximum in recent months. Brightness in magnitude is shown at left and a timeline along the bottom. Notice that R experienced a few ups and downs before finally returning to its bright state (at right). Where it will go from here is anyone’s guess. That’s what keeps things interesting. AAVSO

That’s why I wanted to share the news with you. So you could go out to see it yourself … before the star disappears again. Rural observers will glimpse it with the naked eye, but any pair of binoculars will show R CrB, even from the suburbs. I saw it faintly without optical aid from my home two nights ago but had a much better look in my 8×40 binoculars. I estimated the star’s brightness at magnitude 6.1.

In this closer view I’ve marked the location of the variable star R CrB inside the crown. To find it, start at Arcturus in the constellation Boötes then slide your gaze to Izar and from there to Gemma. Point your binoculars at Gemma and then use two stars to its left to form a nifty little triangle with R CrB. Stellarium

Seeing R Coronae Borealis fade from view is like losing a familiar friend. Of course it doesn’t completely disappear, but it took a big telescope to see it back in early 2000s. So what’s the root of all the drama? Some stars fade and re-brighten because they’re partially eclipsed by a companion star. Others experience temporary spells of instability and expand and contract in predictable cycles lasting days or months.

R CrB is a yellow supergiant 10,000 times as bright as the sun and many times larger. The tremendous pressure exerted by its powerful radiation spews zillions of tons of material into space in high-speed stellar wind. Something similar but more docile happens at the sun and is responsible for sparking the aurora. R CrB stars also have atmospheres rich in helium and carbon cooked up deep inside their interiors that rise to the surface through convection, the same process that makes bubbles rise in a pan of boiling water.

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. ESO

R CrB expels a powerful stellar 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 puffs happens to fall along our line of sight, it blocks the light of the star, dimming it much the same way soot lining an old-fashioned oil lamp dims the flame. Only when the cloud has expanded and thinned — or is swept away by continued winds — does the star return to its normal brightness.

Want to estimate the brightness of R CrB? You can use this chart which shows the magnitudes of stars near R. The numbers omit the decimal point, so the star labeled 65 is magnitude 6.5. Click for a larger version. AAVSO

Nobody knows exactly what triggers R CrB’s sooty outbursts, but every few years it belches out another cloud and goes into hiding again like a smoker that just can’t quit. This latest episode saw the star reach a historical low in brightness; it’s also the longest fade on record.

That’s why I wanted you to see R CrB now — at its shiniest. Sure, it looks like any other star, but its not anonymous. No star is when you get to know it. Every stellar pinpoint tells a story, and R CrB’s is one of the most fascinating.

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