Page:Star Lore Of All Ages, 1911.pdf/199

Rh not always among the brightest constellations that the most interesting objects are found.

Its three brightest stars, α, β, and γ Cephei, gain a certain interest when it is known that by reason of the precession of the equinoxes these stars will one after the other take the place of the Pole Star of ages to come.

In 4500 γ Cephei will be Polaris. In 6000 β Cephei succeeds to the title, and 1500 years later α Cephei marks the Pole of the heavens. Only the last will be as near the true Pole as our present Pole star is now.

β Cephei is a beautiful double star, a fine object in a small telescope, and when observing it interest is added by the thought that the primary is also double, although too close to be seen visually, that wonderful instrument the spectroscope revealing its duplicity. This spectroscopic binary has an exceedingly rapid revolution, a complete circuit of the orbit taking less than five hours, which is the most rapid orbital revolution so far known.

The stars ξ and κ Cephei are also fine doubles for a small telescope. For the naked eye observer there is situated in this constellation an object of great interest, the variable star δ Cephei, a typical example of a certain class of variable stars of short period, which are now called the Cepheid variables. Its changes in brightness are perfectly regular and it is an accurate time-keeper, successive maxima following one another at intervals of 5 days, 8 hours, 47 minutes, and 39 seconds. Unlike the so-called Algol variables its light changes are continuous without any period when the brightness is constant. The remarkable behaviour of this star furnishes one of the most puzzling problems of astro-physics. δ Cephei is easily visible to the naked eye and any one can watch these interesting variations in its magnitude. The range is from 3.7 to 4.9.

α, β, and γ Cephei were known respectively by the Arab names "Alderamin," meaning the "right arm," "Alfirk," "a flock," and "Errai," the "shepherd."