Alpha Gruis - Alnair, Al Nair, Al Na'ir

Alpha Gruis is, at magnitude 1.74, the brightest star in the modern constellation Grus, the Crane, and also the 31st brightest star in the sky. Grus was invented around 1600 to name part of the deep southern sky that could not be seen from northern lands. From mid-US latitudes, Grus appears to be running across the southern horizon. Difficult to see from northern countries, Al Nair marks the Crane's southwestern foot.

Directly south of Piscis Austrinus, the Southern Fish, the Arabians added Alpha Gruis to Piscis Austrinus as the fish's tail. Alpha Gruis has the traditional name Alnair, derived from the Arabic "al-nayyir al-dhanab", meaning "The Bright One of the Tail of the Southern Whale". Confusingly, Alnair was also given as the proper name for Zeta Centauri by a navigator in the middle of the 20th century.

Properties and Evolution

Al Nair is a hot, blue class B subgiant with a surface temperature of 13,500 K - 2.3 times hotter than the Sun. From 101 light years away, Al Nair shines at us with a total luminosity of 380 Suns, which implies a radius 3.6 times solar. It is close enough to have had its angular diameter measured, resulting in a similar diameter measure of 3.3 times solar. Like most stars of its kind, it is also spinning rapidly, at least 236 km/sec at its equator, giving it a rotational period under a day.

As a subgiant, this four-solar-mass star is close to the end of its normal hydrogen fusing lifetime. 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 make it valuable. Its spectrum is very simple, with relatively few absorptions from 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.

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