Beta Cephei

Lore & Background

Beta Cephei bears the traditional name Alfirk, derived from the Arabic al-firqah 'the flock' (of sheep). With Alpha Cephei (Alderamin) and Eta Cephei (Alkidr), they were known as Al Kawākib al Firḳ, 'the stars of the flock', by Ulug Beg. In 2016, the IAU Working Group on Star Names approved the name Alfirk for the component Beta Cephei Aa. The system consists of a binary pair (Beta Cephei A) and a third companion (B). The binary's components are designated Beta Cephei Aa (Alfirk) and Ab. Beta Cephei Aa is a blue subgiant star with a stellar classification of B1 IV, though it has previously been classified with either a main sequence or giant luminosity class. Its radius and mass have been estimated, with other sources giving higher masses. It rotates slowly, taking approximately 51 days for one revolution. Beta Cephei Ab is a Be star in an 81-year orbit, resolved using speckle interferometry at a distance of 0.25" in 1972. Beta Cephei B is a magnitude 7.8 A2 main sequence star 13.6" distant.

Reader's Guide

Beta Cephei is the prototype of the Beta Cephei class of variable stars, which are hot main sequence and giant stars that pulsate analogously to Cepheid variables. However, the pulsations in Beta Cephei stars are driven by iron opacity rather than by helium. The star itself pulsates regularly every 4 hours 34 minutes, producing a variation in its visual brightness of 0.11 magnitudes. Its magnitude varies between +3.16 and +3.27 with a period of 0.19048 days. The system is a triple star comprising a spectroscopic binary with a magnitude 8 optical companion. Beta Cephei is also notable as a visible star located within 5° of the precessional path traced across the celestial sphere by Earth's North pole. During the same period, Iota Cephei will also be within 5° of the precessional path on the other side, so that both are in contention as pole stars, a title currently held unambiguously by Polaris. The star is circumpolar throughout all of Europe, northern Asia, and North American cities as far south as Guadalajara in west central Mexico. Because it is a faint third-magnitude star, it may be difficult to identify in most light-polluted cities, though in rural locations it should be easily observable.

Did You Know?

• Its traditional name Alfirk means 'the flock' in Arabic.
• Beta Cephei lies within 5° of the precessional path of Earth's North pole.