GACRUX (Gamma Crucis). In our "western" star
lore, the stars with proper names come from the set that could be
seen from the lands of the ancient middle east and Arabia, and do
not include those of the far southern hemisphere, which is largely
invisible below the horizon from northern lands. Yet those who
needed the stars to find their way, the old navigators, also needed
quick names for some of the stars and just made them up. "Gacrux"
is a prime example of such a star, its name deriving strictly from
its Greek letter name, Gamma Crucis, the third-brightest star in
the constellation Crux, the Southern
Cross. Acrux, Alpha Crucis, was named
in the same way. Tied with Shaula in
Scorpius for third-place in the second
magnitude rank (1.63), Gacrux is (with Shaula) the 24th brightest
star in the sky. For a star of its brilliance, however, it has not
received much individual attention, probably again because it is
not observable throughout much of the world where we find most of
the telescopes. And a pity too, as it is rather unusual among
naked-eye stars. Most of the brighter stars in the sky are white
class A or blue class B dwarfs, or even orange giants. Gacrux,
however, is a cool red class M (M 3.5) giant star, one that shines
nicely in contrast to the other three, blue, stars of the Cross.
And a true giant it is. From its rather nearby distance of 88
light years, and its temperature of 3400 Kelvin (from which we can
estimate the amount of invisible infrared radiation shining from
its cool surface), we calculate a luminosity of 1500 Suns, which
leads to a radius 113 solar. If placed at the Sun it would extend over halfway to Earth.
At first, Gacrux looks double, with a faint, nearly seventh
magnitude class A "companion" about two minutes of arc away from
it, but it is a mere line-of- sight coincidence: the white class A
star is actually about 4 times farther away, so there is no
physical relation between the two. (From the A star, it would look
as if Gacrux and the Sun were double.) Though usually taken as a
single star, however, Gacrux may in fact be double. It is a mild
"barium star" whose surface might have been contaminated by the
winds from a companion that evolved first and is now an invisible
white dwarf (rather like Alphard in
Hydra. More certain, Gacrux is
variable by a few tenths of a magnitude. Though little is known of
the nature of the variability, it has been classified as a "semi-
regular variable," one that is not quite predictable. It is also
blowing a fairly strong, and quite noticeably variable, wind.
Gacrux's evolutionary -- ageing -- status is uncertain. With a
mass perhaps three times solar or less, it may well have given up
not just hydrogen fusion in its core (which it must do to become a
giant in the first place), but may also have gone through its core
helium-fusion stage. If that is the case, Gacrux is in the process
of becoming a "second-ascent" giant, one brightening into the
ethereal realm of the giant stars for the second time, a
speculation reinforced by its variability. Perhaps one day it will
appear to us as a full-blown pulsating variable like Mira in Cetus.