DELTA GRU (Delta Gruis). Double stars abound in the sky, but how
easy it is to be fooled. A fine case in point lies in Grus, the Crane, (indeed, two cases in point)
with a pair of fourth magnitude stars both called "Delta," the
northern (and slightly eastern) one labelled Delta-1, the southern
Delta-2. Separated by a little over a quarter of a degree, they
make a fine naked-eye pair, one that is entirely a line-of-sight
coincidence. Oddly, the two are at almost the same distance.
Delta-1 is a rather ordinary class G (G7) giant (magnitude 3.97)
295 light years away, while Delta-2 is a red class M (M4.5) giant
(magnitude 4.11) at 325 light years. The errors on the measured
distances are large enough that the two could in fact be at the
same distance. They are, however, going through space in quite
different directions, and have nothing whatever to do with each
other. Though of nearly the same visual brightness, Delta-2 is by
far the more luminous. Cooler (estimated at 3400 Kelvin), it
radiates most of its 2200 solar luminosities in the infrared
(Delta-1, at 4900 Kelvin, nearly 10 times dimmer). A truly great
star, Delta-2 is 135 Suns across, 0.65 or so
Astronomical Units, almost as big as the orbit of Venus. Its mass
probably hovers around thrice solar, and within the ageing process,
it is most likely a "second ascent giant," one with a dead carbon-
oxygen core, its fate to lose a great deal of its mass and turn
into a white dwarf.
(Delta-1, on the other hand, is a helium-fusing giant that will
someday become like Delta-2.) Consistently, Delta-2 is a slight
"irregular" variable star with a range of about 0.11 magnitudes.
Upon careful examination, such irregular giants are commonly found
to be "semi-regular." Unfortunately, Delta-2 has been rather well
ignored by the observers. A look through the telescope shows it to
be double, with a ninth magnitude class M companion (probably a
hydrogen fusing dwarf) a minute of arc away. But here we are
fooled again, as over the past 175 years, the two are shifting away
from each other much too fast, showing the line-up again to be
coincidental. However, Delta-1 (perhaps as much as four solar
masses) may have a real twelfth magnitude class K companion over
500 Astronomical Units of arc away from it, the two taking over
5000 years to make a full circuit of each other. Taking the
distances of Delta-1 and Delta-2 as correct, each would be
wonderfully prominent in the others' sky, shining at magnitude -
1.1, almost as bright as Sirius seems to
us on Earth.