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.