AL NAIR (Alpha Gruis). Directly south of Piscis Austrinus, the Southern Fish, is the modern constellation Grus, the Crane. Unlike the bright ancient constellations, the "moderns" were invented between around 1600 and 1800 either to name the figures of the deep southern sky that could not be seen from northern classical lands or to fill in the blanks among the brighter northern groups. If Grus, included by Bayer in his famed atlas, had been farther north, it clearly would have received a classical name. Indeed the Arabians readily added it to Piscis Austrinus as the "fish's tail." "Al Nair," reminding us of those times, from Arabic means "the bright one," the name coming from a long phrase that means "the bright one in the fish's tail." "Grus," however, is surely appropriate, as the quite-outstanding figure, highlighted by three bright stars, looks for all the world like a long-legged bird. Difficult to see from northern countries, Al Nair, marking the crane's southwestern foot, cannot be seen at all above around 42 degrees north latitude, which excludes all of Canada. From the mid-US, Grus appears to be running across the southern horizon. Appropriately, Al Nair, at bright second magnitude (1.74), and the 31st brightest star in the sky, is the Alpha star. It is a hot, blue (class B) subgiant with a surface temperature of 13,500 Kelvin, 2.3 times hotter than the Sun. A century of light years away (actually 101), Al Nair shines at us with a total luminosity (including a fair bit of invisible ultraviolet) of 380 Suns, which combined with temperature yields a radius 3.6 times solar. Though not a huge star by many standards, it is close enough to have had its angular diameter measured (at 0.001 seconds of arc), resulting in a direct diameter measure of 3.3 times solar, the two evaluations satisfyingly close. Like most stars of its kind it is also spinning rapidly, at least 236 kilometers per second at the equator, 120 times that of the Sun, giving it a rotational period of under a day. As a "subgiant," this four solar mass star is either close to the end of its normal hydrogen fusing lifetime or perhaps has even reached that point already. Otherwise it seems perfectly normal, and in fact is used as a prime example of its class. Al Nair's high temperature and very normality and temperature make it valuable. Its spectrum -- rainbow of colors -- is very simple, with relatively few absorptions produced by atoms in its atmosphere. As a result, the star is frequently used as a background with which to examine the nature of the local clumpy interstellar gas.
Written by Jim Kaler. Return to STARS.