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.