| Right Ascension | 15 : 48 : 34.3 (h:m:s) |
|---|---|
| Declination | +28 : 09 : 24 (deg:m:s) |
| Distance | (ly) |
| Visual brightness | 5.85 .. 14.8 (mag) |
| Spectra type | G0Iep |
This variable star has been discovered by Edward Pigott in 1795 (Pigott & Englefield, 1797).
R Coronae Borealis is the prototype star of the R Coronae Borealis (RCB) type variables. These stars are hydrogen-deficient and carbon-rich supergiants of spectral type F or G, which go into "outburst" not by brightening like many other variables, but by fading! R CrB is normally shining at maximum around magnitude 6 and therefore at the limit of naked-eye visibility. At irregular intervals, however, its brightness drops in a sharp decline by up to 8 magnitudes, down to 14 mag, within a few weeks. Then, the star uses to remain faint for an extended period of time, or has several recoveries and declines in succession, until it returns to its usual brightness after several months to a year. Recent significant minima ocurred in 1995-6 and 1999-2000.
A possible explanation is that carbon particles at the outer layers of the star are more or less periodically (chaotic ?) are build up and then blown away darkening the radiation of the star. The origin of the dust is still subject to theoretical research (G.C. Clayton 1996). According to the currently preferred model, the so-called Dust Puff Theory, the material comes from the RCB star itself. Mass is lost from the RCB star and then moves away from the star until it reaches the condensation temperature of carbon dust (at about 20 stellar radii). Once the carbon dust has formed, the photosphere of the star is eclipsed, the star goes into minimum, and emission lines appear. When the dust is blown away by radiation pressure the photosphere of the star may again be seen.
