0 Theta Arae THETA ARA (Theta Arae). Scorpius, the celestial Scorpion of the Zodiac, is so overwhelming a figure that one might not think to look below it. Most northerners in fact can't, as Scorpius's southern curve is not ever that far above the horizon. But from the south, the stars of one of the most southern of ancient constellations, Ara, the Altar, can easily be seen shining within the confines of the Milky Way. The whole region around Scorpius, Centaurus, Crux, Ara, and beyond is filled with hot, class B stars of high mass that were born more or less at the same time from the same cold interstellar cloud complex, and that now form gravitationally unbound OB associations and their various subassociations. Many, however, are the massive stars whose relatives are unknown. Their number includes fourth magnitude (but at 3.66 on the bright side) Theta Arae, a class B (B2) supergiant (though of the lesser kind; and see below) 813 light years (plus or minus 30) away. Were it not for 0.19 magnitudes of dimming by the Milky Way's interstellar dust, the star would make it into the realm of third magnitude. Though there is no physical relation (the three at different distances), Theta Arae fits in nicely with its constellation mates Alpha Arae (a B2 dwarf) and Gamma Arae (a B1 supergiant). With a temperature of 17,900 Kelvin (needed to find the amount of invisible ultraviolet radiation), Theta radiates at the rate of 9100 times that of the Sun. From the calculated radius of 10 times solar and a projected equatorial rotation speed of 108 kilometers per second (apparently not enough to give it a surrounding disk), the star's rotation period is less than 4.7 days. Theory applied to temperature and luminosity yields an impressive mass of 8.5 times that of the Sun (comparable to Alpha Arae, but not up to Gamma's level). Rather than being a full blown supergiant, the star seems more to be a subgiant that is beginning to turn itself into one. Theta's mass puts it right on the lower edge of the fuzzy limit above which stars explode as supernovae. If it escapes that fate, its core will contract into a massive white dwarf, perhaps one made of neon and oxygen rather than of the usual mix of carbon and oxygen. If Theta had a close companion, tidal flows might increase the eventual white dwarf's mass enough to put it over the white dwarf mass limit of 1.4 times that of the Sun. The white dwarf would then catastrophically collapse and make an even grander supernova. No neighbor, however, is evident, our star seemingly quite alone.

Written by Jim Kaler 9/13/13. Return to STARS.