MU AND (Mu Andromedae). More than anything else, what fourth
magnitude (3.87, not far from third) Mu Andromedae has going for it
is its role as a significant part of the "dotted line pattern" that
make the classical outline of Andromeda, the heroine of the myth that is centered
upon her. Four degrees to the northwest of Mirach (Beta And), along with Beta, Mu And
also serves as a "pointer"to the great Andromeda galaxy, Messier 31, which lies at about the
same angle to the northwest of Mu as Mu does from Beta. The
galaxy, the best-know feature of the constellation and a near-clone of our own
Milky Way Galaxy, is, at a distance of some 2.4 million light
years, the farthest thing that can normally be seen with the naked
eye. Vastly closer, Mu And, a typical class A (A5) hydrogen-fusing
dwarf, lies only 130 light years away (give or take just 5),
showing how one's perspective of astronomical distances changes
when confronted with a REALLY distant object and also vividly
revealing that the sky has the third dimension of depth. But back
to the star itself. With a temperature of 8090 Kelvin (any
correction for invisible ultraviolet or infrared
radiation non-existent), Mu And shines with the light of 35 Suns, which leads to a radius of 3.0 times
solar. Direct interferometric measure of angular size (0.00066
seconds of arc) gives a somewhat smaller radius of 2.8 times that
of the Sun, implying that the temperature may be off a bit or that
the star appears somewhat different in the infrared where the
measurement was made. A projected equatorial rotation speed of 74
kilometers per second, enough to keep the star's outer layers
sufficiently stirred to prevent an odd chemistry through separation
of elements, gives a rotation period of less than 2 days. From the
theory of stellar structure and evolution, the luminosity and
temperature then yield a mass of 2.3 Suns and show that the star is
about three-fourths of the way through its 710 million year
hydrogen-fusing lifetime, after which it will turn into a red giant
and begin to die. Like so many of its kind, Mu And seems to have
something of a debris disk that implies planets, though it is marginal and no actual
planet has ever been detected. A purported quadruple star,
clumping around Mu proper are three faint "companions": 13th magnitude Mu
And B, last seen 51 seconds of arc away; 11th mag Mu And C 28
seconds away; and 11th magnitude Mu And D way out at 260 seconds.
Alas, the motions of all of the trio are too large to be consistent
with genuine gravitationally-bound neighbors, rendering them just
line-of-sight coincidences and leaving Mu Andromedae itself all
alone.
Written by Jim Kaler 10/21/11. Return to STARS.