TAU CrB (Tau Coronae Borealis). Well north of the semi-circle of stars that
defines Corona Borealis, the northern Crown (its
southern cognate, Corona Australis, the Southern
Crown, falling south of Sagittarius) lies fifth
magnitude (but at 4.76 almost fourth) Tau Coronae Borealis, a class K (K1)
giant-subgiant that's fairly close, 117 (give or take 5) light years away.
Why bother with yet another orange K giant when there are so many others in the
sky.? Part of the reason is just that, that that there ARE so many others
(Arcturus, Aldebaran, and a vast slew of others) and the
range of the class in luminosity it huge, so it takes a lot of stars to cover
that space. Tau CrB is among the brighter ones of the K giants. With a
temperature of 4860 Kelvin, it radiates at an impressive rate of 3840 times
that of the Sun, which gives the star a radius 87.6
solar radii, 0.41 Astronomical Units, which is just over the mean orbital
radius of the planet Mercury. If placed at the Sun, Tau would appear some
50 degrees across, around 1000 times the solar angular diameter. The theory
of stellar structure and evolution gives Tau CrB a mass seven times that of
the Sun with an age of about 350 million years. The star is most likely fusing
its core helium into carbon and oxygen in preparation for becoming a
carbon/oxygen white dwarf of
about one solar mass similar to sirius.html Sirius B. First though, it
must generate a powerful wind that will sweep away the residual exterior
hydrogen envelope and present us with an ephemeral planetary nebula with the old stellar core at its center.
Tau CrB has a couple other features to recommend it. It was at one time listed
as a spectra.html#double">spectroscopic binary, but that seems not to
have been confirmed. There is, however, a thirteenth magnitude visible companion now 2.2 seconds of arc away,
which from its brightness would be a low-mass red dwarf. From a minimum
physical separation of 79 Astronomical Units, it would take at least 250 years
to make a complete circuit of Tau CrB itself. But since the separation changed
by over a second of arc in 69 years, it's more likely that the little one
is just in the line of sight and not physically connected. More interesting,
the star is speeding along at some 60 kilometers per second relative to the
Sun, some four times the usual, suggesting it is a visitor to our part of
the Galaxy. . Nevertheless, its metal
content appears to be quite normal.
Written byJim Kaler 06/16/17. Return to STARS.