Alpha Mensae

At only magnitude 5.08, Alpha Mensae is the brightest star in Mensa, the Table. Mensa one of the most obscure constellations - it lies within 30 degrees of the south celestial pole and is thereby invisible to northerners. It is so faint that it is nearly invisible to southerners as well! In fact, Alpha Mensae is the dimmest luminary (brightest star) of any constellation.

Properties

Alpha Mensae is one of the few naked-eye stars similar to our Sun. It is of spectral class G6 V, with a surface temperature of 5560 K. At a distance of only 33.1 light years, α Men is also one of the closer stars to Earth, and shines with 80% of the Sun's luminosity. Like the Sun, α Men is a slow rotator, with a rotation period of 32 days - a week longer than the Sun's period of 25 days. The mass of α Men's is estimated at 93% of the Sun's, and its age at nearly 10 billion years. This old age is consistent with its slow rotation (as the star's magnetic field drags on its stellar wind to slow its rotation over time). Other authorities, however, give a mass as high as 1.1 suns, and ages that range from 4 - 7 billion years.

Alpha Mensae has a relatively high proper motion across the sky, and is now moving away from us at 35 km/sec. A quarter of a million years ago, α Men made a close pass by us at only 11 light years, when it shone in the sky at almost at second magnitude.

Somewhat metal rich, with an iron content about 25% greater than solar, α Men is used as a comparison star for those that have planets, as they tend to be metal-rich as well. No planetary companions have yet been discovered around α Men, but it has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]