DELTA VIR (Delta Virginis). Delta Virginis belongs to a small
group to the west of Spica called the "awwa," which may represent
some kind of barking dogs (or it may not), composed of Zavijava (Beta Vir), Porrima (Gamma), Vindemiatrix (Epsilon), Zaniah (Eta), and our otherwise un-named
Delta. Delta Vir is one of the sky's rather few naked-eye class M
red giants, and a coolish one (class M3) at that. Appropriately
ranking fourth in brightness (though named "Delta" more for
position), the star shines toward the faint end of third magnitude
(3.38) from a distance of 202 light years. With a temperature of
only 3720 Kelvin, Delta radiates much of its light in the infrared
where it cannot be seen with the eye. When that is taken into
account, the star is calculated to be 630 times more luminous than
the Sun, which with temperature gives it a
radius 61 times solar, or a quarter of the radius of the Earth's
orbit. The star's measured angular diameter of 0.0098 seconds of
arc gives a similar value of 65 solar radii, showing that all the
measures are closely self-consistent. Though Delta Vir is
certainly evolved and dying, it is not possible to delineate its
exact nature. The star, with a mass near 1.5 to 2 times that of
the Sun, is near the tip of the so-called "red giant branch" or
stars (where lower mass stars reach their maximum brightness as
their dead helium cores turn on to fuse helium to carbon). It
could be brightening with a dead helium core, it could be starting
to dim some as it fuses its newly-awakened helium, or it could even
be brightening with a dead carbon core. Delta may or may not have
a companion. An 11th magnitude star lies 80 seconds of arc away,
but its distance and motion remain unmeasured. If it is a true
companion, its brightness suggests that it is a class K dwarf that
lies at least 5000 Astronomical Units from Delta proper and takes
over 200,000 years to orbit. If true neighbors, from Delta Vir,
the little one would shine over twice as bright as our Venus, while
from the companion, the reddish primary star would appear four
times as bright as our full Moon. On the other hand, the alignment
of the two may be mere coincidence. It has not been important
enough to anyone to find out.