ETA PER (Eta Persei). In the twentieth century rather mysteriously
given the "Arabic" name "Miram," Eta Persei (of Perseus, the hero who rescued Andromeda from Cetus's
jaws) is listed as having no proper name in Allen's vast book, so
we go here simply with the Greek
letter name given by Bayer, Eta
Persei, which lies at the northern tip of the classical outline of
the constellation. It also lies within
the realm of magnificent massive stars, though with a large
distance of 1300 light years, it is unrelated to the Alpha Persei cluster (which is less than
half that distance) and other collections of neighboring massive
stars. While most everything about the star is uncertain
(including the distance), the collection of characteristics seems
to hang together with some kind of consistency. This fourth
magnitude (3.76) star was originally classified as an M3 supergiant, but most modern
listings have it as a class K (K3) supergiant, which is consistent
with a determined temperature of 4300 Kelvin. At its large
distance within the Milky Way, there has to
be some interstellar dust absorption. If class M3 it would be
about a magnitude, if K3, then zero, so for want of anything
better, we adopt half a magnitude. That, allowance for infrared
light, and distance gives us a magnificent luminosity of 13,000
times that of the Sun, which leads then to a very large radius of
210 solar, or nearly as big as the Earth's orbit. Direct measure
of angular size through interferometry gives a remarkably
consistent value of 220 solar, suggesting that all the parameters
are close to correct. A measured equatorial rotation velocity of
at least 5.8 kilometers per second gives an amazingly long rotation
period of under 5 years (which could, given the axial tilt, be much
less). There is no question about the great mass, however. If
still expanding with dead helium core, the star carries a mass of
11 times that of the Sun, while if it has already fired its
internal helium, it comes in at somewhat less, around 9 solar. The
star is right on the cusp. It could created the most massive of white dwarfs, near 1.4 solar
(the maximum allowable, beyond which white dwarfs collapse), it
could make a rare neon white dwarf (most are balls of carbon and
oxygen), or it could be just above the uncertain limit at which
stars explode as supernovae. Clumping around
the big one are a batch of six lesser so-called "companions" that are all likely
to be just line of sight coincidences. The most accepted is Eta-B,
a 9th magnitude B9 dwarf that lies 29 seconds of arc away. If a
true companion, it would be at least 11,500 Astronomical Units from
Eta proper, and take at least 350,000 years to make a circuit, such
numbers making the companionship even less unlikely.
Written by Jim Kaler 12/05/08. Return to STARS.