ALPHA MON (Alpha Monocerotis). By general rule and common wisdom,
Alpha stars are supposed to be the
brightest stars within their constellations. This Greek letter listing, invented by
Johannes Bayer for his Uranometria of 1602 is, however, more than
occasionally violated, as it is in Monoceros (the Unicorn), in which Beta Mon (no star in the constellation has
a proper name) is the luminary. Other rule violators abound: Aquarius, Cancer, Capricornus, Cetus, Corvus, Crater, Delphinus,
Draco, Gemini, Hercules, Libra, Orion, Pegasus, Pisces, Sagitta, Sagittarius, Triangulum, and Ursa Major;
and that's just the ancient constellations. But in Monoceros's
case, it's pretty forgivable, since it's almost impossible to
distinguish the brightest among the top three, such as they are
since no star tops fourth magnitude. In order the "Big Three" are
fourth magnitude (3.92) Beta Mon, Alpha Mon (3.93), and Gamma Mon
(3.98), a mere one percent separating Alpha from Beta. That Beta
comes out on top is a bit of a cheat, since through the telescope
it is seen to be triple, each one just fifth magnitude, notably
dimmer than Alpha. Luckily for our Alpha it also escaped even
having its very name stolen, as in some older star catalogues,
Alpha and Gamma were swapped for each other, which would have
lowered Alpha's status even more. Physically, Alpha Mon is yet
another yellow-orange class K (K0) "clump giant," a helium-fusing
giant star similar to a great
many others of its class (so-called as they clump together in a
graph of temperature vs. luminosity). At a distance of 144 light
years, it radiates at a rate 60 times that of the Sun from a coolish surface of 4815 Kelvin,
which together yield a radius 11 times solar and a mass of 2.5
solar. With a total age of about 1.4 billion years, the star ran
out of core hydrogen fuel some 250 million years ago. Alpha Mon's
metal content is about 20 percent lower than that of the Sun (which
is not very unusual). The rotation speed, hence period, is
unknown. While appearing rather ordinary, Alpha Mon's very
ordinariness is a distinct advantage. The star is known for its
spectral stability, and serves astronomers as a standard for
determining the velocities of other giant stars that are not quite
so well behaved. Like all of its breed, once the helium runs out,
it will lose its outer hydrogen envelope and retire as a dense white dwarf about the size
of Earth.
Written by Jim Kaler 3/23/07. Return to STARS.