Delta Capricorni is a multiple star system about 39 light-years away in the constellation of Capricornus. The primary star in the system is a white giant and the combined light of its members makes it the brightest star in the constellation. The system consists of an eclipsing binary, Delta Capricorni A, and two companion stars, B and C. A's two components are themselves designated Delta Capricorni Aa and Ab. Delta Capricorni is 2.6 degrees south of the ecliptic and can be occulted by the Moon, and by planets.
In 1906 astronomer Vesto Slipher of Lowell Observatory discovered that Delta Capricorni A was a spectroscopic binary. The orbit was determined in 1921 by Clifford Crump using 69 radial velocity measurements obtained at Yerkes Observatory. However the eclipsing binary nature of the system was not discovered until 1956 by Olin J. Eggen at Lick Observatory. Lunar occultations have been observed in 1951, 1962, and 1988. Delta Capricorni was recognised as a metallic-line star in 1957 and included in a 1958 catalog of magnetic stars. It has also been associated with extreme ultraviolet and radio sources, believed to be from coronal activity in the secondary star.
Stellar system
Delta Capricorni A is an Algol-type eclipsing binary star, with an orbital period of 1.022768 days and an inclination close to the line of sight from the Earth. The peakapparent visual magnitude of the pair is 2.81. During an eclipse of the primary, this magnitude drops by 0.24. When the primary is eclipsing the secondary, the magnitude decreases by 0.09. Delta Capricorni A has an overall stellar classification of A7m III, indicating that it is a giant star that has exhausted the supply of hydrogen at its core. More specifically, this is a chemically-peculiarAm star with a spectral type of kA5hF0mF2 III under the revised MK system. This notation indicates that the calcium K-line matches the temperature of an A5 star, the hydrogen spectral type matches an F0 star, and the metallic absorption lines match an F2 star. In the past this star was suspected of being a Delta Scuti variable, which is rare for an Am star. This categorization was brought into question during observations in 1994 and it is most likely not inherently variable. The primary has double the Sun's mass and nearly twice its radius. It is rotating rapidly with a projected rotational velocity of 105 km s−1. Note that it is unusual for an Am star to have such a high rotational velocity. The outer envelope of the star is radiating energy at an effective temperature of 7,301 K, giving it the white-hued glow of an A-type star. The secondary component is a G-type or K-type star with around 90% of the mass of the Sun. There are two optical companions. A fifteenth magnitude star is one arcminute away, and a thirteenth magnitude star is over two arcminutes away from the primary star and that distance is increasing.
