48 Per (48 Persei). Perseus, the home of
Algol, the Double Cluster, and the Alpha
Persei cluster (which marks the Perseus OB3 Association of
hot, massive O
and B stars), is (like so many other constellations of the Milky Way) so filled with interesting stellar
sights that that it's hard to choose among them. Many stars that
would be prominent in other constellations
here even have no Greek letter
names. Flamsteed's 48 Persei is one of them, a fourth
magnitude (4.04) B-emission (B3e) hydrogen-fusing dwarf that is
nearing the end of its hydrogen-fusing run. Such "Be stars" are
hot class B rapid rotators that are surrounded by gaseous
equatorial disks of their own making. The formation of the low-
density circumstellar disks is somehow related to the rotation, but
no one quite knows how. The disks are unstable and can come and
go, and are related to stellar variability. Classic examples are
Gamma Cassiopeiae and Dschubba (Delta Scorpii). Our 48 Per
varies by a tenth of a magnitude or so over no particular period,
hence also carries the variable star name MX Per. At a rough
distance of 550 light years and in a thick portion of the Milky
Way, the star's light is dimmed by nearly 0.6 magnitude by
interstellar dust. Were it not there, 48 Per would brighten to
third magnitude (3.46). The observed projected equatorial rotation
velocity of 190 kilometers per second could be boosted to as high
as 370 if an estimate of a disk-tilt of 31 degrees is allowed for.
Such high rotations turn stars into ellipsoids, with higher
temperatures toward the poles, making simple temperatures
problematic. Our best estimate is around 17,200 Kelvin (which is
low for a B3 dwarf). Distance, apparent brightness, and allowance
for a lot of ultraviolet light lead to a substantial luminosity
3980 times that of the Sun, a radius of 7.1
times solar, and a rotation period (using the larger value above)
of just a day. Structure theory then gives a mass of 7 times that
of the Sun, and shows that 48 Per, with an age of 40 million years,
is only a few million years (if that) from giving up core hydrogen
fusion, after which it will expand to become a more luminous red giant and finally die as a
massive 1.0 solar-mass white
dwarf similar to Sirius B. (In 48's
case, though, there is no companion to watch the action.) The
star seems to be losing mass at a rate of some 4 hundred-millionths
of a solar mass per year (millions of times the flow rate of the
solar wind). The lost mass somehow feeds a disk whose inner opaque
portion has a radius 7 times that of the star, and a thinner outer
portion 18 times the stellar radius. Though once listed as a
member of the Perseus OB3 association (which is about the same
distance as the star), 48 Per's motion shows that it is not
involved. Such stars can be fun to watch, as we are never quite
sure what they will be doing, so while admiring the famous Perseus
sites, check out 48 once in awhile.
Written by Jim Kaler 1/16/09. Return to STARS.