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.