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.