THETA CRT (Theta Crateris). Wrapping a third of the way around the sky, Hydra, the Water Serpent, carries three constellations on its back. From west to east they are modern Sextans (the Sextant), dim Crater (the Cup), and Corvus (the Crow), the latter two descending to us from ancient times. Among the ancient constellations, Crater is also among the dimmest, its brightest star (Delta Crateris) but fourth magnitude. Nevertheless, in a dark sky the outline of the Cup is quite obvious. At the tip of the Cup's northern lip lies the most northerly of the Greek-lettered stars, fifth magnitude (4.70) Theta Crateris, the star's dimness belying its importance to astronomers. A class B (B9.5) hydrogen-fusing dwarf, Theta lies just over the blue-side border with the white stars of class A, its relative faintness the result of a distance of 280 light years (give or take but 6). From that and a temperature of 11,170 Kelvin (to account for a fair bit of ultraviolet light), we find a luminosity of 120 times that of the Sun and a radius of 3.0 times solar. With a projected equatorial velocity of 189 kilometers per second, Theta makes a full rotation in under 0.8 days. The rapid rotation keeps the atmospheric gases stirred up enough to prevent separation of elements (and to produce "metallic line stars" and similar beasts), so that the iron abundance of 55 percent solar is probably representative of the entire star, or at least of its outer layers. The spin, however, also distorts Theta Crt into an oval that causes the poles to be hotter than the equator, which then makes a singular temperature a bit problematic. Theory leads to a mass of 3.0 Suns and shows the star to be about two-thirds of the way through its 350 million year dwarf lifetime. After sloughing off its outer layers as an advanced evolving giant, Theta will die as a white dwarf of about 0.7 solar masses, stars always expiring with much less mass than they start with, the gases returned to interstellar space in a grand system of cosmic recycling. A companion with a 150 day orbit has been suggested but never confirmed. No other orbiting neighbors have ever been found. With its simple blue-white light, Theta Crateris is best known for its role as a "standard star," one with a carefully determined energy distribution across its rather clean B-star spectrum. Comparison of any kind of spectral observations with those from Theta Crt and stars like it then make for easy correction for distortions caused by absorption and scattering of starlight in the Earth's atmosphere.

Written by Jim Kaler 4/19/13. Return to STARS.