R CRB (R Coronae Borealis). Some stars explode, causing them to brighten. Usually we see a "nova," a surface detonation on a white dwarf by matter drawn from a close companion. On rare occasion the whole star goes off in a grand supernova (one not seen in our Galaxy since 1604). We mostly understand them. But where was it writ that stars should DISAPPEAR? Not completely, of course, but at least to the eye, even to one looking through a small telescope. You can see such a star for yourself, the brightest and the prototype of a rare breed called R Coronae Borealis stars, of which a mere 50 or so are known but none understood. R CrB itself is a simple find, as it's tucked within the southern part of the curve that makes Corona Borealis, the Northern Crown. It was first noted more than 200 years ago. Normally, this impressive class G (G0) supergiant stands at a faint but accessible sixth magnitude (around 5.9). Then, on the average of every few years, but with stunning unpredictability, "R Cor Bor" (as it is affectionately known) goes into hiding, dropping to as faint as magnitude 12, even 15 (becoming as much as 4000 times fainter) and taking as long as a year to recover. Superimposed are small semi-periodic 3.5-year fluctuations.

Four thousand days -- more than a decade -- in the life of R Coronae Borealis. Every few years, sometimes more, sometimes less, sixth magnitude R CrB suddenly plummets in brightness, then more slowly recovers. The events, completely unpredictable, are thought to be caused by ejected puffs of carbon dust that lie in the line of sight and dramatically dim the star. (Courtesy of the American Association of Variable Star Observers.)

R CrB is much too far for direct parallax measure, but similar stars in the Large Magellanic Cloud (the largest of our nearby satellite galaxies, whose distance is well known) have luminosities around 10,000 times that of the Sun. If similar, "R" would have to be between 4500 and 5000 light years away. An estimated temperature of 5000- 5500 Kelvin would give it a radius of some 100 times solar, which is half the distance from Earth to Sun. Not an ordinary high-mass supergiant, R Cor Bor carries a weight of but 0.8 solar, its great luminosity coming from a highly advanced, but still mysterious, evolutionary state. The spectrum reveals only a trace of hydrogen, leading us to conclude that its outer layers are made largely of helium, the star's original hydrogen envelope long-since expelled. Supporting the idea, R CrB is especially rich in carbon, which is the result of helium fusion. The high luminosity drives heavy mass loss estimated at 100,000 times the rate in the solar wind. About one percent of the outbound gas condenses into carbon dust, but in discrete "puffs." if one comes off in the line of sight, the "soot star" then hides from our view. In support, we see an inner surrounding dust shell with a temperature of just under 900 Kelvin that starts 100 stellar radii from the star and that is encased in a huge, cold "fossil cloud" some 25 light years across. Why the star huffs and puffs is unknown. Moreover, the dust should be made where the gas sufficiently chills, at least 20 stellar radii out. However, the spectrum shows it to be made 10 times closer, perhaps as a result of outbound shock waves. The origins of such stars are unknown. One theory is that while developing into an ordinary white dwarf (the core of a once more-massive giant), the innards suddenly suffered a violent onset of helium fusion (to carbon) that expanded the outer layers to supergiant proportions. Another is that R Cor Bor stars are the result of mergers of two white dwarfs in orbiting binary systems, with the same effect. Friendly R CrB is clearly trying to tell us something. We just don't yet know what. But have fun waiting for its disappearing act.
Written by Jim Kaler 6/10/11. Return to STARS.