Delta Cephei
Binary star system in the constellation Cepheus
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Discovery
Delta Cephei was discovered to be variable by John Goodricke during 1784. He describes his first observation on October 19, 1784, followed by a regular series of observations most nights until December 28. Further observations were made during the first half of 1785, the variability was described in a letter dated June 28, 1785, and formally published on January 1, 1786. This was the second variable star of this type, with Eta Aquilae being discovered just a few weeks earlier, on September 10, 1784.
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Properties
As well as being the prototype of the Cepheid class of variable stars, Delta Cephei is among the closest stars of this type of variable to the Sun, with only Polaris being nearer. Its variability is caused by regular pulsations 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 5.366249 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 one of the most precisely established for a Cepheid. This accuracy is thanks in part to its membership of 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) . However, a re-analysis of Hipparcos data found a larger parallax than before, leading to a shorter distance of 244±10 pc, which is equivalent to 800 light-years.
Radial velocity measurements of Delta Cephei have revealed the presence of a small spectroscopic companion star on a 6-year orbit around Delta Cephei A. The mass of this companion is about one tenth of the mass of Delta Cephei and the two come to within 2 astronomical units at pericenter passage. The presence of this companion will have to be taken into account when Gaia measures Delta Cephei's parallax (distance).
Delta Cephei has a common proper motion companion Delta Cephei C (HD 213307, sometimes referred to as component B) is also a spectroscopic and astrometric binary. The faint star listed as component B in multiple star catalogues is an unrelated background object.
Stars of this type are believed to form with masses of 3–12 times that of the 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 star's mass has been determined spectroscopically at 5.3 times the mass of the Sun. At this stage of its evolution, the outer layers of the star have expanded to an radius which varies from 41 to 45 times that of the Sun.
Delta Cephei is emitting around 1,900 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 M☉ 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. The result of this expelled gas is the formation of a nebula about 1 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 relative to its neighbors. It is a suspected member of the Cep OB6 star cluster, and hence may be around the same age as the cluster: around 79 million years. At an angular separation of 40 arc seconds from Delta Cephei is a magnitude 6.3 companion star with the identifier HD 213307, called component C in most multiple star catalogues, which is visible in small telescopes. HD 213307 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.