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.