Delta Cephei

Delta Cephei is the prototype of the Cepheid variable stars. Approximately 891 light-years away in the constellation of Cepheus, the King, it is among the closest stars of this type to the Sun. Only Polaris is closer. Delta Cephei conveniently sits between two bright stars that shine at either end of its range of variability: ζ Cep at magnitude 3.6, and ε Cep at magnitude of 4.2.

Variability

The variability of δ Cephei was discovered in 1784 by the young English astronomer John Goodriche, a deaf-mute who died at age 21. It was the second Cepheid variable discovered, following η Aql earlier the same year. δ Cep varies from magnitude 3.6 to 4.3, over a period of 5.36634 days; the rise to maximum brightness is quicker than the subsequent decline to minimum. Its spectral type also varies, from about F5 to G3, with the temperature changing from 6800 K at warmest to 5500 at coolest.

Unlike the eclipsing binary Algol, δ Cephei's variability is due to the physical pulsation of the star. Delta Cephei is a supergiant. The star's luminosity at maximum is around 4,000 suns, pouring from surface swollen to some 40 solar diameters. Stars of this type are believed to form with masses of 3-30 times that of our Sun, and then have passed through the main sequence as class B stars.

With all the hydrogen fused to helium in their cores, these unstable stars are now passing through later stages of nuclear evolution. Currently, over 1000 stars of Cepheid-type have been catalogued, while the count continues to grow. Other bright, well-known Cepheids include η Aql, ζ Gem, and the North Star, Polaris!

Period-Luminosity Relation

In 1912, Henrietta Swan Leavitt of Harvard College Observatory discovered a surprising fact about Cepheid variables: the longer their light period, the greater their luminosity. Since the period gives their luminosity, we can find their distance by measuring the apparent brightness, and comparing it to the absolute luminosity.

Cepheids are so luminous that they are easily seen in nearby galaxies. Using this period-luminosity relationship, the distances of these stars - and most importantly the distances of the clusters and galaxies in which they are located - can be estimated with reasonable accuracy. Edwin Hubble's discovery of a Cepheid in the Andromeda Galaxy (M 31) allowed the first firm estimate of its distance to be determined.

Knowing the distance of δ Cep and other stars in its class is fundamental to calibrating their period-luminosity relationship; unfortunately, these efforts were marred until recently by only fair parallax accuracy. In 2002, however, the Hubble Space Telescope was used to determine the distance to δ Cep (890 light-years) within 4%.

Companion

Delta Cephei has a magnitude 6.3 companion star 41" distant. Though the true separation between the stars is at least 13,000 AU, the companion is believed to be physically related to the variable. The secondary is a B7 star with a luminosity of some 250 suns.