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.