BETA GRU (Beta Gruis). Below the ancient Southern Fish, Piscis Austrinus, lies the striking figure of Grus, the Crane, that for all the world looks like a great bird stalking the sky. The figure is especially captivating from the mid-northern hemisphere, where the Crane looks to be walking the horizon. At the Crane's southern end, representing the bird's feet, are two bright second magnitude stars, Al Nair (the Alpha star) to the west, and Beta Gruis to the east. Some four centuries ago, the stars of Grus were taken from the Fish, Al Nair meaning "the bright one in the fish's tail," a name for Beta referring to "the Rear One at the end of the tail." At mid-second magnitude (2.10), Beta is not that much fainter than Al Nair. Ranking 59th in the sky, it is the second brightest star without a proper name. This rather rare kind of star, a cooler class M (M5) giant, lies 170 light years away, 70 percent farther than its constellation-mate Alpha. The physical parameters are not well studied and consequently not well-known, the disadvantage of being in the rather deep southern hemisphere where the more numerous northern telescopes could not reach it. Such giants should have cool temperatures of around 3400 Kelvin, which indicates a considerable amount of invisible infrared radiation that when accounted for gives a luminosity 3900 times that of the Sun and a radius of just over 0.8 Astronomical Units, larger than the orbit of Venus. The temperature and luminosity in turn suggest an initial mass just shy of three times that of the Sun. The cool class, however, strongly implies that the star is in an advanced state of evolution, and is losing mass and brightening with a dead carbon-oxygen core in preparation for sloughing its outer envelope. Indeed, that Beta Gru is bright at very long infrared wavelengths suggests a surrounding envelope of its own making. Beta Gru is also classed as an "Lc" ("irregular supergiant," though the star is truly a giant) variable that changes erratically between magnitudes 2.0 and 2.3. It is most likely to be on its way to becoming a Mira-type variable with much bigger variations, over the past 450 million years having evolved from being a much hotter (though dimmer) class B8 star. Long thought to be single, sophisticated interferometry suggests an unresolved companion less than 0.22 seconds of arc from the bright star, which at 170 light years corresponds to a distance of 11 Astronomical Units. Nothing about the little companion is known. Even its existence is uncertain.
Written by Jim Kaler. Return to STARS.