ALPHA MEN (Alpha Mensae). Some constellations dominate the sky, Orion a case in point. Astride the celestial equator, the figure is visible from all over the globe. Even at the Earth's poles, half the Hunter can be seen. Not so with many others. The Bears (with their Dippers) are lost to those living in the far southern hemisphere, while the glory of the Southern Cross is invisible to far-northern-hemisphere dwellers. Other constellations are obscure for their dimness. Mensa, the Table, falls into both categories, all of it within 30 degrees of the southern celestial pole and thereby invisible to northerners and so faint that it is nearly invisible to southerners as well. Even its brightest star, otherwise un-named Alpha Mensae, is but mid fifth magnitude (5.09). The faintness of both the constellation and the star belie Alpha's significance as one of the few naked-eye stars similar to our Sun. This class G (G6) dwarf, with a temperature of 5560 Kelvin, is the second-least luminous of all the Alpha stars, and is beaten only by the secondary class K companion of Alpha Centauri (which by itself appears at first magnitude). At a distance of only 33.1 light years, Alpha Men is also one of the closer stars to Earth, where it shines at 80 percent the power of the Sun. Like the Sun, it is a slow rotator, observations of its activity showing that it has a rotation period of 32 days, a week longer than the solar period of 25 days. No planetary companion has been detected. Somewhat metal rich (an iron content about 25 percent greater than solar), it is used as a comparison star for those that DO have planets (as they tend to be metal-rich as well). The mass and age of Alpha Mensae are somewhat hard to judge. The best estimate is a mass of 0.93 times that of the Sun and an old age of nearly 10 billion years, which is consistent with the slow rotation (as stellar magnetism combined with stellar winds act to slow down cooler stars with time). Other authorities, however, give a mass as high as 1.1 solar and ages that range from four to seven billion years. Now moving away from us at 35 kilometers per second, a quarter of a million years ago Alpha Men made a close pass to us of only 11 light years, when it shone in the sky at almost at second magnitude.
Written by Jim Kaler. Return to STARS.