PSI PER (Psi Persei). Famed are the Pleiades and Hyades of northern fall and winter. But who pays attention to another large and bright open cluster, the Alpha Persei cluster that surrounds the brightest member, Mirfak, a second magnitude supergiant? The problem is that the cluster is so spread out within the surrounding Milky Way that it does not quite look like a real assembly. Just 2.6 degrees southeast of Mirfak, we run into the brightest "main sequence" (hydrogen-fusing) member of the cluster, the fourth magnitude B5 dwarf Psi Persei, which lacks a proper name. Stellar membership is always the biggest problem in dealing with clusters. The question of whether Psi belongs or not has been off and on over the years, but the star's distance of 583 light years (give or take 23) is similar to that of the cluster's core of 579 light years. Moreover, Psi's motion relative to the Sun is consistent with the cluster's, so the consensus is one of belonging. The star also beautifully stands out by itself. After correction for 0.35 magnitudes of dimming by interstellar dust and for a lot of ultraviolet radiation from its 16,670 Kelvin surface, Psi Persei is found to radiate energy at a rate of 2800 Suns, less than the luminosity of nearby Alpha and Delta Per, but not bad either. Luminosity and temperature then combine to give a radius of 6.4 times that of the Sun, which agrees well with a value of 6.7 solar radii listed from indirect means. Theory gives a mass of 6.5 Suns and shows that the hydrogen "fire" is almost out and that the star will shortly begin evolving, its age around 50 million years, nicely close that of 50 million years found for the cluster at large. The really big thing about Psi Per is its great equatorial rotation speed of at least 367 kilometers per second (the axial tilt not known), which gives it a rotation period of under 0.87 days. No surprise then that Psi Per is one of the brighter "B-emission" stars in the sky, one that has created for itself a surrounding fat disk that changes with time, one similar to the "Be" icons Gamma Cas, Zeta Persei, and Delta Scorpii. The radiating disks are not well understood. Rotation is clearly a culprit, but so might be binary companions, magnetic fields, radiation-driven mass loss, and who knows what. The star may be spinning near its critical breakup velocity. Someday, when the internal fuel runs out, Psi Persei will eject its outer envelope and die as a white dwarf of around 0.94 solar masses, rather well up on the scale. There are no known companions to watch its progress. Whether the star will even continue as its cluster membership is unknown, as "open clusters" of this sort tend to fall apart with time.

Written byJim Kaler 1/02/15. Return to STARS.