EPS GRU (Epsilon Gruis) Northern autumn is marked best by the constellations of the Andromeda myth as well as by the
appearance of the 18th brightest star in
the sky, Fomalhaut, which represents
the mouth of Piscis Austrinus, the Southern
Fish. To the south-southwest of Fomalhaut lies a set of stars that
were in older times linked to the fish's tail, but around 1600 were
separated away as Grus, the Crane. The
figure was cemented in astronomical lore by its inclusion in Bayer's
Uranometria. The luminary, Al Nair, closes in on first magnitude, while
Epsilon at magnitude 3.49, fourth brightest star in the
constellation, just barely gets over the line into third magnitude.
Lying south-southeast of the main constellation (which really does
look like a giant stalking bird) where it is barely noticed, Epsilon
Gru is a white class A (A3) dwarf with a temperature of about 8600
Kelvin (the average of a fairly sizable spread). From temperature
(needed to account for a bit of ultraviolet radiation) and a
distance of 129 light years (give or take 2.5), we find a luminosity
of 50.6 Suns and a radius of 3.2 times solar.
The theory of stellar structure and evolution then yields a mass 2.4
times that of the Sun. The age is uncertain, but it seems to be more
than halfway through its hydrogen-fusing lifetime of 600 million
years (pretty short as compared to the solar lifetime of 10 BILLION
years, showing the stark inverse effect of mass on stellar duration),
after which the helium core will collapse, turning the star as a whole
into a much more luminous red giant. Epsilon Gru's singular
feature seems to be its high equatorial rotation speed, the observed
values averaging 247 kilometers per second, which gives the star a
rotation period of under 0.65 days. It's high enough to stir the
surface gases so as to prevent separation of the elements, some
falling under the effect of gravity, others lofted upward by
radiation. The high speed will also make the star somewhat oblate,
which then gives problems with temperature determination (the poles
hotter, the equator cooler). Since we do not know the axial tilt,
the speed may be even higher. But it does not look as if there is
anybody there to admire the star, as there is no evidence for the
debris disk that often
accompanies planets, and thus there is no
chance of life, which in any case would probably be precluded by the
star's short lifetime. Neither are there any known stellar companions, the star seemingly
quite alone.
Written byJim Kaler 9/05/14. Return to STARS.