Gamma Pegasi - Algenib

Gamma Pegasi is the fourth brightest star the constellation Pegasus, at magnitude 2.83. It appears at the southeast corner of the constellation's Great Square asterism. It has the traditional name Algenib, from Arabic, which means "the side". Confusingly, however, this name originally belonged to α Per (whose alternative name is still Algenib). γ Peg is known as the First Star of the Wall in Chinese.

Properties and Evolution

Algenib is a brilliant hot blue class B2 IV subgiant with a surface temperature of 21500 K. From its distance of 335 light years, and allowing for ultraviolet radiation, it has a luminosity of 4000 suns, and a radius 4.5 times solar. It has a mass of 7 - 10 suns. Algenib is measured by the Doppler effect to be an especially slow rotator, only 8 km/sec, unusual for hot class B stars. Most likely, we are looking at the star almost along its polar axis.

As a subgiant, the star is beginning to evolve off the hydrogen-fusing main sequence. It will eventually become a massive carbon white dwarf rather like Sirius B, or a rarer neon-oxygen white dwarf if at the upper end of the mass range above.

Variability and Companions

Gamma Pegasi is a β Cepheid variable star that varies between magnitude +2.78 and +2.89 with a period of 3.6 hours. All are hot class B stars that chatter away with multiple short periods, varying by only a few hundredths of a percent.

Algenib also has some mystery companions. One, with a period of only 6.83 days and observable only spectroscopically, is perhaps 0.15 AU away from Algenib proper. Then, almost 3' 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 of hundreds of thousands of years. Nothing is known about any of them.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]