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.