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.