MIRZAM (Beta Canis Majoris). Commonly also seen spelled "Murzim,"
Mirzam is the Beta star of Canis Major, the Larger Dog, the greater of Orion's
canine companions. Around 1600, Johannes Bayer gave Greek letter names to the brighter
stars, more or less in order of brightness but with many
exceptions, some constellations first broken into sections. The
Alpha (Sirius), Beta, and Gamma stars of
Canis Major, for example, are all in the Dog's head and foreparts,
yet Mirzam ranks fourth in brightness and Gamma (Muliphein), a fourth magnitude star, a
distant eighth. Delta, Epsilon (first
magnitude Adhara), and
Zeta are lower
down, marking the Dog's hindquarters and rear legs. Mirzam's name,
something of a mystery, suggests that the star is an "announcer" of
brilliant Sirius, as Mirzam rises first, but no one seems to know
the exact meaning of the term. Though appearing relatively dim
compared to Sirius, Mirzam is by far the greater of the two, its
mid-second magnitude (1.98) status the result of its 500-light-year
distance, 60 times greater than Sirius's. Mirzam is a very hot,
blue class B (B1) bright giant
star with a temperature around 25,800 Kelvin. Much of its
radiation is therefore produced in the ultraviolet, where the eye
cannot see, and if that is taken into account this magnificent star
shines with a luminosity 34,000 times that of the Sun and nearly 1300 times that even of Sirius.
If Mirzam were at Sirius's distance of 8.6 light years it would
shine in our sky almost 15 times more brightly than Venus at its
brightest. The temperature and luminosity combined indicate a mass
around 15 times that of the Sun and that the star is really more of
a "subgiant," one very close to the point of giving up core
hydrogen fusion. Mirzam is interesting from two points of view.
It is one of the local stars whose light is used as a background to
study the thin gas of interstellar space, and lies along a sort of
tunnel in which the gas is especially hot and thin. More
importantly, the star is variable. Though the variations are too
feeble to be seen with the naked eye (only about 10%), they are
extremely complex and have multiple periods, the three strongest
about a quarter of a day, that beat against each other like out-of-
tune guitar strings. Mirzam is the brightest of the "Beta Cephei variables" (after the first
discovered), the class also called "Beta Canis Majoris variables."
The instability is related to the cessation of hydrogen fusion and
changes in internal structure. While Sirius will die as a white dwarf, Mirzam seems to
have sufficient mass such that it will someday explode as a
spectacular supernova.
Written and updated by Jim Kaler 5/04/07.
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