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THETA CRT (Theta Crateris). Wrapping a third of the way around thesky,Hydra, the Water Serpent, carriesthreeconstellations on its back. Fromwest to east they are modernSextans(the Sextant), dimCrater (the Cup), andCorvus (the Crow), the latter twodescending to us from ancient times. Among the ancientconstellations, Crater is also among the dimmest, its brighteststar (Delta Crateris) but fourthmagnitude. Nevertheless, in a dark sky the outline of the Cup isquite obvious. At the tip of the Cup's northern lip lies the mostnortherly of theGreek-lettered stars,fifth magnitude (4.70) Theta Crateris, the star's dimness belyingits importance to astronomers. Aclass B (B9.5) hydrogen-fusingdwarf, Theta lies just over the blue-side border with the whitestars of class A, its relative faintness the result of a distanceof 280 light years (give or take but 6). From that and atemperature of 11,170 Kelvin (to account for a fair bit ofultraviolet light), we find a luminosity of 120 times that of theSun and a radius of 3.0 times solar. Witha projected equatorial velocity of 189 kilometers per second, Thetamakes a full rotation in under 0.8 days. The rapid rotation keepsthe atmospheric gases stirred up enough to prevent separation ofelements (and to produce "metallic linestars" and similar beasts), so that the iron abundance of 55percent solar is probably representative of the entire star, or atleast of its outer layers. The spin, however, also distorts ThetaCrt into an oval that causes the poles to be hotter than theequator, which then makes a singular temperature a bit problematic. Theory leads to a mass of 3.0 Suns and shows the star to be abouttwo-thirds of the way through its 350 million year dwarf lifetime. After sloughing off its outer layers as anadvanced evolving giant, Thetawill die as awhite dwarfof about 0.7 solar masses, stars always expiring with much lessmass than they start with, the gases returned to interstellar spacein a grand system of cosmic recycling. Acompanion with a 150 day orbithas been suggested but never confirmed. No other orbitingneighbors have ever been found. With its simple blue-white light,Theta Crateris is best known for its role as a "standard star," onewith a carefully determined energy distribution across its ratherclean B-star spectrum. Comparison of any kind of spectralobservations with those from Theta Crt and stars like it then makefor easy correction for distortions caused by absorption andscattering of starlight in the Earth's atmosphere.
Written byJim Kaler 4/19/13. Return toSTARS.
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