ALGENIB (Gamma Pegasi). Though the brightest star of Pegasus is Enif, the Epsilon star, the stars of the Great Square are of such obvious note that Bayer gave them Alpha through Delta. Delta (Alpheratz), the brightest, actually belongs to Andromeda as Alpha Andromedae, leaving Alpha, Beta, and Gamma Pegasi (Markab, Scheat, and Algenib) ranking 3-2-4 in the constellation and 2-1-3 in the Square, Algenib the faintest of them. The star's name, from Arabic, means "the side," and originally belonged to Mirfak in Perseus (whose alternative name is still Algenib). Second magnitude (2.83) as viewed from Earth, Algenib is a brilliant hot blue class B (B2) star with a high temperature of 21,500 Kelvin. From its distance of 333 light years, and allowing for a lot of ultraviolet radiation, we find a luminosity 4000 times that of the Sun (one suggestion as high as 12,000, though that seems extreme) and a radius of 4.5 solar. The lower figure calls for a mass 7 times solar, the higher up to 10. The star is now beginning slowly to evolve. Though classed as a "subgiant," it is probably still fusing hydrogen into helium in its core. It will evolve into a massive carbon white dwarf rather like Sirius B, the upper mass limit suggesting a more advanced status as a neon-oxygen white dwarf. Algenib is measured by the Doppler effect to be an especially slow rotator, only 8 kilometers per second (4 times solar), unusual for hot class B stars, which are ordinarily high-speed spinners. Most likely, we are looking at the star almost along its axis (its pole pointed at us), so that we do not sense the real rotation. (If a star is exactly pole-on, the Doppler effect, which is sensitive only to line-of-sight motions, would give zero rotation; that is, the star would not appear to rotate at all.) Algenib is also among the collection of Beta Cephei stars (named after Alfirk, Beta Cephei, and which include such luminaries as Mirzam, Hadar, and several others). All hot class B stars, most beginning to evolve in some way, they chatter away with multiple short periods, varying by only a few hundredths of a percent. Algenib itself has a principal very short period (the only one so far found) of only 3.6 hours, during which it changes by some 0.07 magnitudes. Algenib also has some mystery companions. One, with a period of only 6.83 days and observable only via the spectrograph, is perhaps 0.15 astronomical units away from Algenib proper. Then, off in the distance, up to almost three minutes of arc away, are two dim stars of 11th and 12th magnitude. If actual companions, which is rather unlikely, they are both M dwarfs with huge orbital periods measured in hundreds of thousands of years. Nothing is known about any of them.
Written by Jim Kaler. Return to STARS.