EPS CRB (Epsilon Coronae Borealis) along with T CRB (T Coronae Borealis). If fourth magnitude (4.15) Epsilon Coronae Borealis were not a significant part of the curve of stars that makes Corona Borealis, the Northern Crown, it would probably be paid little attention. As it is, it's still one more cool class K (K2) hydrogen-fusing giant, but one with a companion and, equally important, a star that leads the eye to other celestial phenomena. At a well-determined distance of 221 light years (give or take three), Eps Crb, with temperature of 4353 Kelvin, shines with the light of 156 Suns, from which we find a radius of 22 times solar, or 0.10 Astronomical Units, a quarter the size of Mercury's orbit (in good agreement with that found from an indirectly-determined angular diameter). Quietly fusing their helium into carbon and oxygen, the masses of such stars are devilishly difficult to determine, as giants with a good range of masses all rather look alike. This one appears to come in around 2.5 solar (though with a significant uncertainty), the star then having begun life as a class B9 hydrogen-fusing dwarf some 600-700 million years ago. Slowly rotating, with a projected equatorial velocity of 1 kilometer per second, the star could take as long as 275 days to make a full turn. Epsilon Cor Bor (as it might be familiarly known) is listed with two companions. Twelfth magnitude Eps CrB C, more than a 1.5 minutes of arc away, is clearly moving too fast relative to much brighter "A" (Eps itself) to belong to it, and must be merely a line-of-sight coincidence. Close-in Eps B, however, just two seconds of arc away from Eps A, is clearly genuine company. Just over the line into thirteenth magnitude, it is listed as a K3 (dwarf), though its absolute brightness suggests more like K9. In any case, it's dramatically instructive to compare these two class K stars, which reveal the vast differences between dwarf and giants. Some orbital motion is seen. At least 135 Astronomical Units apart, from the masses (assuming 0.55 Msun for Eps B) and Kepler's Laws, "B" would take at least 900 years to make an orbit. Even at that distance, "B" would shine with the light of a full Moon upon Eps-A's planets: were it to have any.

As much as anything, Epsilon CrB acts as a gateway to two other celestial sights. With Delta CrB just to the southwest, it forms a nearly equilateral triangle with the famed "disappearing variable" R Coronae Borealis, which lies at the triangle's northwest apex. Perhaps more significantly, Eps CrB is a marker for the "recurrent nova" T Coronae Borealis, which lies just a degree to the south and is one of only eight known. Normally of tenth magnitude, T CrB consists of a class M3 giant in a 227-day orbit with a massive white dwarf. Tidally distorted by its companion (and thus variable as it presents different-sized cross-sections with different temperatures to us), the giant feeds matter onto the white dwarf (at a rate close to a million times that of the solar wind). When the layer of fresh hydrogen is sufficiently great, the heat of compression causes the layer to blow up like a hydrogen bomb. Normal novae are made of lower mass white dwarfs that feed off low mass ordinary dwarfs, and after the new layer is thick enough, should pop off every few tens or hundreds of thousands of years. But T's white dwarf is so massive that the intervals between explosions are short. In 1866 the star hit magnitude 2, and in 1946 mag 3, taking but a few days to drop back to invisibility. Are we due for another blowup around 2026? Nobody really knows, but keep your eye on Epsilon. The event could be far grander. The accreted matter could push the white dwarf over the fabled 1.4 solar mass limit, beyond which the WHOLE STAR would collapse and blow up in a grand (Type Ia) supernova that -- even at a distance of 2500 light years -- could become as bright as a crescent Moon!
Written by Jim Kaler 8/19/11. Return to STARS.