ADHARA (Epsilon Canis Majoris). The names of all the first
magnitude stars ring clearly to us; even the names of those in the
southern hemisphere are well known. Adhara has escaped the fame it
deserves. Also known as Epsilon Canis
Majoris, it is actually the second brightest star in the
constellation (after Sirius), and helps
form the western leg of Canis Major, the larger dog. Look just
below Sirius to find an outstanding triangle of bright stars.
Adhara is at the lower right. The ancient Arabs referred to this
small three-star pattern as "The Virgins," to which the name
"Adhara" (the westernmost of the three) actually refers. No one
knows why the name was given or who they were. Adhara has an
apparent magnitude of 1.50, and therefore is sometimes referred to
as the last of the "first magnitude" stars or as the brightest of
the "second magnitude." The latter view and the star's rather
southern position in the sky has led to its being somewhat ignored.
In fact, Adhara, a class B (B2) bright giant, is quite the magnificent
star. Among the hotter of bright stars, Adhara shines with a
surface temperature of some 20,900 degrees Kelvin (above absolute
zero), which gives it a sparkling bluish cast. From its distance
of 430 light years we calculate a luminosity to the eye of 3700
times that of the Sun. If at the distance
of Sirius, which dominates Canis Major, Adhara would shine at
apparent magnitude -7, 8 times brighter than Venus at its most luminous. Because
of the star's high temperature it radiates a good portion of its
energy in the invisible ultraviolet. If that is taken into
account, Adhara is actually 21,700 times more luminous than the
Sun. Indeed, if you had ultraviolet eyes, Adhara would be the
brightest star in the sky. Adhara's angular diameter leads to a
physical radius 11.4 times solar. Combining radius and temperature
yields a very similar luminosity of 22,000 Suns, showing that the
measured parameters are all very nearly correct. Temperature and
luminosity then lead to a mass 10 to 12 times that of the Sun,
which is probably enough to make the star someday explode as a supernova. Having ceased
hydrogen fusion in its core, Adhara is actually in the beginning of
its death stages (cessation of core hydrogen fusion), and is now
more in its "subgiant" state. It is very popular among those who
study local interstellar matter, as its simple and bright spectrum
is used to examine the stuff that lies between it and the Sun.
Seven seconds of arc away lies an 8th or 9th magnitude (depending
on whom you ask) companion
that from its brightness is probably a mid-class A star. The two
are at least 920 Astronomical Units apart and take at least 7500
years to make a full orbit.
Written and updated by Jim Kaler 5/04/07.
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