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.