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.