DELTA PER (Delta Persei). Dominated as it is by its two brightest stars, Mirfak and Algol (Alpha and Beta Persei), the next four stars of glorious Perseus, the mythical rescuer of Andromeda, are often ignored. This quartet, which consists of Gamma, Delta, Epsilon, and Zeta, are all nearly the same mid-third magnitude brightness, and add great sparkle to the northern constellation. Of them, Zeta barely tops the list, followed by Epsilon, Gamma, and then our Delta (at magnitude 2.99 almost defining third magnitude itself). Delta Per is a hot, luminous mid-class B (B5) giant, that shines from a rather large distance of 530 light years. From its 13,800 Kelvin surface radiates the light of 3400 Suns, the radius some ten times solar. From these figures we derive a mass of 6.5 times that of the Sun. Only 50 million years old (and consistent with its "giant" status), it recently gave up hydrogen fusion in its now-helium core, and is making the transition to becoming a swollen red giant. Of the family of four third magnitude stars, Delta bears the closest relation to the constellation's luminary Mirfak, as it is most likely a member of the sprawling "Alpha Persei cluster," a gravitationally bound system that on the average lies at a distance of 540 light years (though the affiliation is still being argued, actual membership in a cluster often being a very difficult thing to define). Like most hot class B stars, it is madly rotating with an equatorial speed of at least 255 kilometers per second, giving it a "day" of at most two of our Earth days. Companionship is highly questionable. About 1.5 minutes of arc away lies a much fainter tenth (10.4) magnitude star that, if a real mate, is a dwarf much like our Sun. Given that the two belong to each other, they are at least 16,000 Astronomical Units apart and take at least 750,000 years to orbit, the gravitational tie probably too weak to allow them to stay together for long. From Delta proper, the companion would shine a couple times brighter than our Venus, while from the companion, Delta would appear as bright as five full Moons. The star has also been suspected of having a close-in companion, but nothing is known about it, or even if it really exists. Slightly variable (by a few hundredths of a magnitude), Delta has also been classified as a questionable magnetic variable in the mold of Cor Caroli, all this uncertainty perhaps surprising for such a bright star. Thanks to Jeff Bryan, who suggested this star.
Written by Jim Kaler. Return to STARS.