Beta Cephei - Alfirk

Beta Cephei is the second-brightest star in the constellation Cepheus, with visual magnitude 3.23. It also has the traditional name Alfirk (in Arabic "al-firqah"), meaning "The Flock" (referring to a flock of sheep).

A giant blue star 595 light years from Earth, β Cep belongs to spectral class B2 IIIev, with a surface temperature of 26,700 K. It contains 12 solar masses, and has a total luminosity of 14,600 Suns. Alfirk is classed as a subgiant, as a star at or near the end of its hydrogen-fusing lifetime. Many such stars pulsate, their stability lost due to the valving of the heat flow far below their surfaces.

Variability

Beta Cephei is the prototype of the β Cep variable stars. (These stars should not be confused with these Cepheid variables, which are named after δ Cephei.) β Cep stars exhibit variations in their brightness due to pulsations of their surfaces. Their maximum brightness roughly corresponds to the maximum contraction of the star. Typically, β Cep variables change in brightness by 0.01 to 0.3 magnitudes with periods of 0.1 to 0.6 days.

Alfirk's chief period is 4.57 hours, during which it varies from magnitude 3.16 to 3.27. Like all β Cep stars, however, Alfirk pulsates with many periods at once. Smaller changes take place with a variety of other periods between four and five hours, in addition to 6- and 12-day rotational modulations. β Cep is also a Be star that sheds matter, and has a magnetic field about 100 times stronger than Earth's. Beta Cephei has a hot corona that radiates about 2,000 times more X-ray radiation than the Sun - activity thought to be related to the star's advanced stage of evolution.

Companions

Two smaller, dimmer A stars accompany β Cep. The inner is about 45 AU away with an orbital period of some 90 years. The outer, easily seen in a small telescope, is at least 2,400 AU away with a period of at least 30,000 years. It makes a gorgeous double of brilliant white and emerald stars set 13" apart.