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.
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.
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.
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.
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.