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.
Written by Jim Kaler. Return to STARS.