Delta Cephei - Properties

Properties

Delta Cephei is among the closest stars of this type of variable to the Sun, with only Polaris being nearer. The variability of this star was discovered by John Goodricke in April of 1784, making it the second Cepheid variable to be discovered following Eta Aquilae earlier the same year. It was later found that there are two types of Cepheid variables, so Delta Cephei is now known as a type I (Classical) Cepheid. Delta Cephei's variability is caused by regular pulsation in the outer layers of the star. It varies from magnitude 3.48 to 4.37, and its stellar classification also varies, from about F5 to G3. The pulsation period is 5.366341 days, with a rise to maximum occurring quicker than the subsequent decline to minimum.

Because the period of this class of variable is dependent on the star's luminosity, Delta Cephei is of particular importance as a calibrator for the period-luminosity relationship since its distance is now among the most precisely established for a Cepheid. This accuracy is thanks in part to its membership in a star cluster and the availability of precise Hubble Space Telescope/Hipparcos parallaxes. Hence, in 2002, the Hubble Space Telescope was used to determine the distance to Delta Cephei within a 4% margin of error: 273 parsecs (890 light-years).

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 B-type stars. With the hydrogen consumed in their core region, these unstable stars are now passing through later stages of nuclear burning. The estimated mass of Delta Cephei, as derived from the color index, is 4.5 ± 0.3 times the mass of the Sun. By comparison, the mass derived from evolutionary models is 5.7 ± 0.5 times the Sun's mass. At this stage of its evolution, the outer layers of the star have expanded to an average of 44.5 times the girth of the Sun.

Delta Cephei is emitting around 2,000 times the Sun's luminosity from the outer atmosphere. This is producing a strong stellar wind, which, in combination with the pulsations and shocks in the star's atmosphere, is ejecting mass at the rate of (1.0 ± 0.8) × 10–6 solar masses per year, or the equivalent to the mass of the Sun roughly every million years. This matter is flowing outward at a velocity of about 35 km s–1. The result of this expelled gas is the formation of a nebula about one parsec across, centered on Delta Cephei, and containing 0.07–0.21 solar masses of neutral hydrogen. A bow shock is being formed where the stellar wind is colliding with the surrounding interstellar medium.

The peculiar velocity of Delta Cephei is 13.5 ± 2.9 km s–1 relative to its neighbors. It is a suspected member of the Cep OB6 cluster of stars and hence may be around the same age as the cluster; namely around 79 million years. At an angular separation of 40 arc seconds from Delta Cephei is a 7.5 magnitude companion star with the identifier HD 213317, which is visible in small telescopes. This itself is a binary star system with a combined stellar classification of B7-8 III-IV. It is heating the matter being ejected by the stellar wind of Delta Cephei, causing the surrounding circumstellar material to emit infrared radiation.