There remains one last class of intrinsic variable, which is like no other. The R CORONAE BOREALIS stars are novae in reverse-suddenly and unpredictably they decrease in brightness by up to ten magnitudes, then slowly they return to normal . The two brightest members are the prototype (range 5.8 to 14.8 mag.) and RY Sagittarii (range 6.5 to 14.0mag.).

As early as 1935, astronomers had observed that the atmosphere of R Coronae Borealis was rich in carbon and almost devoid of hydrogen. This suggested that the decreases in brightness might be caused by the occasional formation of solid carbon (soot!) in the cooler parts of the atmosphere. The star, thus enveloped in soot, would gradually disappear until the outward pressure of radiation and matter could blow the soot away. This hypothesis, with only minor modifications, is still accepted today. Soon after the soot begins to form, the absorption spectrum of the surface of the star fades, and is replaced by an emission-line spectrum from the top of the atmosphere. Soon, this spectrum also fades, as its source of energy (the surface of the star) is blocked from view. The light-energy which is absorbed by the soot is re-emitted as infrared radiation, which can now be observed directly, using infrared detectors.

R Coronae Borealis stars are supergiant stars with effective sur¬face temperatures similar to those of Cepheid variables. Not surprisingly, then, some R Coronae Borealis stars turn out to be pulsating variables. The pulsational light variations can be observed during the star’s quiescent periods at maximum light. B Coronae Borealis itself has a period of 44 days and a small range; RY Sagittarii has a period of 38.6 days and a larger range.

The unusual chemical composition of these stars may arise in two possible ways: either they are the cores of stars that have previously ejected their outer, hydrogen-rich layers, or they are stars in which carbon, synthesized in the core, has been mixed outward to the surface by convection. In either case, the soot which is ejected by these stars may be an important source of replenishment of the interstellar dust.

The R Coronae Borealis stars aptly summarize the interest and importance of intrinsic variable stars. They are complex, a bit mysterious and never predictably the same from one day to the next. Most important, they represent an interesting and significant phase-in the evolution of a star.

Comments are closed.