ALGORAB (Delta Corvi). Many stars come in pairs. Some come in
pairs in different ways. Algorab lies at the upper left corner of
Corvus, the Crow (or Raven), and with
Gienah Corvi (Gamma Corvi) points eastward to
Spica in Virgo.
Even its name is paired with Gienah, as both come from an Arabic
phrase that means "the raven's wing," "Gienah" from the word for
"wing," "Algorab" from that for "raven." Unfortunately, it is not
all that tidy, as Algorab used to belong to the star now called
Gienah. Though the third brightest star in the constellation, it
was called Delta by Bayer. The two stars, however, are paired only
as the raven's wings and as pointers. Gienah is 165 light years
away, whereas Algorab is a mere 88, so the two really have nothing
to do with each other. But then look closely at Algorab, and it
itself is a pair, a beautiful and easy-to-see double star that
consists of a third magnitude (2.95) white class B star (B9.5, just on
the edge of class A) and an orange class K star just over the edge
of ninth magnitude (8.51). The contrast gives them interesting
colors (quoted ages ago as "yellowish and pale lilac," "pale yellow
and purple"). The brighter, Algorab-A, is a very normal 2.5 solar
mass dwarf (hydrogen-fusing) star with a temperature of almost
exactly 10,000 Kelvin (a nice reference point), and shines 48 times
more brightly than our
Sun, its radius about double solar. The
orange companion, Algorab-B, with a mass three-fourths solar and a
surface temperature of about 5000 Kelvin, radiates much more dimly,
producing only 0.3 times the energy of the Sun. The contrast
immediately shows how only a small change in stellar mass leads to
a huge difference in brightness, Algorab-A 160 times more luminous
than Algorab-B. At a minimum distance of 650 astronomical units
from A, little B takes at least 9400 years to orbit. From B, A
would shine with the light of 500 full Moons. The real focus,
however, is actually on Algorab-B. A high level of infrared
radiation shows that the system is involved with a great deal of
dust that is left over from the formation of the stars. A stars is
created by the collapse of gas within a cold dark, dusty cloud, the
dust blockin out heating background starlight. "T Tauri stars" are
very youthful stars that have just been created from the
interstellar gases, and as such are heavily surrounded by disks of
dust (from which planets may be born). Algorab-B is a "post-T
Tauri star" that has just passed through that phase and is now
settling down on the main sequence to fuse its hydrogen. It
appears to be less than 110 million years old (as therefore is
Algorab-A), and has yet to clear the dust out of its system.
Algorab-B therefore makes Algorab-A one of the more unusual stars
of the naked-eye night sky, acting not just as a pointer for Spica,
but also as a pointer for rather remarkable Algorab-B.