MU AQR (Mu Aquarii). Fifth magnitude (4.73) Mu Aquarii has the rather lowly distinction of being the second-most westerly Greek- lettered star in Aquarius (the Water Bearer) after notably brighter fourth magnitude Epsilon Aqr (Albali), which lies just over a degree southwest of it. Mu and Epsilon Aquarii's position not far above central Capricornus makes them look almost as if they belong more to the Water Goat than they do to the Waterman (thus highlighting two of the three constellations of the "wet quarter" of the Zodiac, the other being Pisces, the Fishes). Mu Aqr is more distinctive as a class A (A3) "metallic star" (A3m) with odd chemical abundances brought about by the settling of some elements (like calcium) in a quiet atmosphere, while others (typically copper, zinc, and "rare earths" such as europium) are lofted upward by stellar radiation. The odd composition is reflected in the assessment of spectral class, different kinds of absorptions in the spectrum running it from as hot as A3 to as cool as F3. The variance is reflected in the star's color, which is more like that of one in the middle of the range, and in the temperature of 7090 Kelvin, which is that of an F2 star. Needless to say, such "chemically peculiar" stars are both difficult and fascinating to work with. The odd chemistry is consistent with a modest projected equatorial rotation speed of 51 kilometers per second, which would keep the star's outer layers from being stirred up too much. From a distance of 157 light years (give or take just 2, temperature showing that most of the radiation falls in the visible portion of the spectrum), we find a luminosity of 23 times that of the Sun and a radius 3.2 solar. The above rotation speed, which is probably close to the mark, gives a rotation period of under 3.2 days. Theory then tells of a star with a mass of just under two solar masses, the exact value (1.8 to 1.9) depending on the particular state of evolution. In any case, Mu Aqr is close to giving up its core hydrogen fusion to become a subgiant, which would make it a near a billion years old. Another distinction is that Mu Aqr is a spectroscopic double, the companion going around with a period of 4.88 years, which (assuming a low mass) gives it a separation from Mu proper of 3.6 Astronomical Units, a modest eccentricity taking it from as close as 2.7 AU to as far as 4.4 AU. There was a time not so long ago when stars fell into either a category of "visual" (separable by eye through the telescope) or detected only through spectroscopy and the Doppler effect (or through eclipses or both) with practically no overlap. All that changed with the development of sophisticated interferometry, which for Mu Aqr gives an angular separation that (with distance) yields consistent physical separations that range from 2.6 to 3.2 AU, close to that surmised above.

Written by Jim Kaler 11/02/12. Return to STARS.