R Coronae Borealis is the prototype of its class of stars, the RCB, or 'R Cor Bor' stars, as most astronomers call them. These stars are hydrogen-deficient, carbon-rich F or G supergiants.
Their most interesting quality, is the fact that occasionally, without warning and at completely random intervals, these stars fade rapidly by as much as eight magnitudes. They will remain in this faint state for some time and then slowly recover back to their original maximum light, only to undergo this mysterious process again at some unpredictable time in the future.
Each of these episodes is different from the previous one. Sometimes the star will recover slightly, only to fade again and then rise to maximum later. Sometimes they fade dramatically and then steadily rise again. Some fadings are shallow and brief, some are deep and last for extended periods of time. In short, they are totally unpredictable. That's what makes them so interesting and fun to follow.
R CrB is usually found at maximum light around 6th magnitude. It can fade to as faint as 14th magnitude at minimum. Currently, R CrB is in what may be the most prolonged fade in history. The AAVSO light curve below is for the last 4600 days. The vertical axis is magnitude (fainter at the bottom, brighter at the top) and the horizontal axis is time in days. As you can see, this current fade has lasted considerably longer than the previous episodes.
So, what is going on here? Studies indicate that the star's light is being dimmed by clouds of carbon rich dust belched up from the star. The most likely explanation is that as mass is lost through the star's stellar wind, it reaches a certain distance from the star where it condenses to form dust, eclipsing the light from the star behind it. As radiation pressure pushes the dust away, thinning it out, the star becomes visible again.
Exactly how far from the star this dust actually forms and why is unknown. Even after nearly two centuries of observations these stars remain largely a mystery.
When will R CrB begin its rise to maximum? Only nightly observations will tell.
For more information on R CrB and her cousins see the Variable Star Of The Season at the AAVSO website.
There is also an excellent interview of Dr. Geoff Clayton, an astronomer who specializes in the study of these rare stars on Slacker Astronomy.