MIRFAK (Alpha Persei). Perseus, the
ancient hero who rescued Andromeda,
climbs the northeastern sky in northern autumn evenings, bright
Mirfak marking his side amidst a lovely stream of stars that lies
within the northern-hemisphere autumn Milky Way. Also called
"Algenib," Arabic for "side" (a name used for other stars as well),
the name Mirfak has nothing to with the hero, and comes from a long
Arabic phrase that means "the elbow of the Pleiades." Quite bright, only a bit below first
magnitude (1.82), Mirfak is the brightest (and Alpha) star of its
constellation, just somewhat brighter than its more famed neighbor,
the eclipsing double star Algol. Indeed
it makes a fine comparison star with which to watch Algol's
periodic plunges to third magnitude. Mirfak's rather healthy
measured distance of 590 light years implies great brilliance, the
star a mid-temperature (6180 Kelvin) class F (F5) supergiant with
a luminosity 5000 times that of the Sun and
a radius 62 times solar. It is important in that it lies just at
the warm edge of the properties of the class of stars known as
Cepheid variables, pulsating stars whose periods are linked to
their luminosities, allowing them to be precise cosmic yardsticks
with which to measure the distances of galaxies (the prime example
Delta Cephei). Mirfak is thus
instrumental in defining the natures of such stars, and may be a
very modest pulsator itself. The luminosity and temperature tell
of a star of about 8 solar masses if the it is evolving toward
cooler temperatures with a dead helium core, or about 7 solar if it
is already fusing helium into carbon and oxygen. Only 30 to 50
million years ago (depending on the exact mass), Mirfak was a hot
blue star of class B. Unlike most constellations, many of the
stars of Perseus are physically associated, resulting in the
figure's great prominence. Mirfak is the brightest member of the
young "Alpha Persei Cluster," which
contains many of the fainter surrounding stars (most likely
including Delta Persei), the "elbow of
the Pleiades" having something of a "Pleiades" of its own. The
cluster is one of a handful easily visible to the naked eye, and is
a spectacular sight in a small telescope. The cluster's distance
as a whole is given as 575 light years, consistent with Mirfak's
individual distance and the associated errors. The cluster's age
of 50 million years fits as well.