ZETA PER (Zeta Persei). Perseus, the celestial hero, rescuer of Andromeda, is perhaps best known for the eclipsing binary Algol (the "Demon Star"). It is also home to a variety of other sights that include famed clusters and loose gangs of hot blue stars called "OB associations" (after the class O and B stars they contain) that, unlike gravitationally bound clusters, are quickly disintegrating, their stars moving away from each other. One such association is centered on the constellation's luminary, Mirfak (Alpha Persei). Another more distant one, the "Perseus OB2" association, so markedly features Zeta Persei that the group is also known as the "Zeta Per Association." And rightly so, since though the third magnitude (2.85 and slightly variable) star is sixth in order of the Greek alphabet, it is number three in Perseus's brightness parade (though to be fair to Bayer, Gamma, Delta, Epsilon, and Zeta are all close to the same magnitude). At a measured distance of 980 light years, with errors of measurement that allow anywhere from 800 to 1300 light years, the star fits very nicely into the measured distance of 960 light years determined for the Per OB2 association as a whole. Though the association is closely related to the compact cluster IC 348 (1000 light years distant), the star itself is not a member of the cluster. At the pinnacle of the association, Zeta Persei is an ultraluminous hot (23,000 Kelvin) class B (B1) supergiant that radiates 105,000 solar luminosities into space from a sphere 21 times the diameter of the Sun. If you could remove the effect of dimming interstellar dust, the star would shine nearly at first magnitude (1.79). As a 19 solar mass supergiant only nine million years old, it has begun to die, having just shut down the fusion of hydrogen in its core. Its only fate is to explode as a supernova. Two companions, Zeta Per B and E, now watch the action. (Two other "companions," C and D, are just line-of-sight coincidences.) The brighter real neighbor, a ninth magnitude (9.16) class B (B8) star (classed as a dying subgiant, but surely an actual hydrogen-fusing dwarf), orbits at least 3900 Astronomical Units away, and takes at least 50,000 years to make a full circuit. The fainter, a 10th magnitude (9.90) class A (A2) dwarf is at least 36,000 AU out and takes at least 1.5 million years to orbit. Even from this distance, Zeta Per A would shine with the light of 10 full Moons. "E" will almost certainly be ripped away by outside gravitational forces before too long, if it has not so suffered that fate already.
Written by Jim Kaler. Return to STARS.