DELTA CEP (Delta Cephei). Surely among the most famed of all stars, fourth magnitude (4.1 or thereabouts) Delta Cephei, set at the southeastern corner of dim Cepheus (the King), does not even have a proper name. It is, however, the only star that has given its constellation name over to represent a whole class of stars, the "Cepheids." While most stars look down steadily upon us, Delta Cep is one of the few easily- watchable variables, its magnitude changing from 3.5 to 4.3 and back over a regular period of 5 days 8 hours 47 minutes and 32 seconds, the star acting like a natural clock. We can't even pin down the class. Listed as a yellow-white class F (F5) supergiant, the star actually changes from F5 to cooler G2 in synchrony with its light variations, the temperature going from about 6800 Kelvin at the warmest to 5500 at the coolest. The term "supergiant" is apt, as at a distance close to 900 light years (see below), the star pours an average of 2000 solar luminosities into space from a surface swollen to some 40 solar diameters. At the pinnacle of a vast class of stars, Delta Cep has a few naked-eye cousins that include Mekbuda (Zeta Geminorum) and Eta Aquilae (actually the first of the breed to be discovered), both of which are visually brighter. With a mass of around six times that of the Sun, Delta was born less than 75 million years ago as a hot class B2 or B3 dwarf. Like all Cepheids, as a supergiant it long ago exhausted its hydrogen core. Now dying, it has lost a sense of equilibrium, and regularly expands and contracts, pulsing like a celestial heart. Maximum brightness occurs not at largest or smallest radius, but at highest expansion velocity (and vice versa).

The Cepheids' deep astronomical importance lies in Henrietta Leavitt's 1912 discovery that their luminosities are directly related to their periods of pulsation (which run from about a day to over 50 days). Since the period gives the luminosity, we need only measure the apparent luminosity (the visual magnitude) to find the distance (after allowing for dimming by interstellar dust). Cepheids are so luminous that they are easily seen in nearby galaxies, their presence then giving the galaxies' distances. Edwin Hubble's discovery of a Cepheid in the Andromeda Galaxy allowed the first decent distance to be determined and showed it and others like it to be external systems outside our own Galaxy. The observation of Cepheids in more distant galaxies was a "key project" of the Hubble Space Telescope, one that has allowed the establishment of an accurate cosmological distance scale. The various distances found for Delta itself are in good agreement with each other. Parallax (second Hipparcos reduction) gives 865 light years, while the period-luminosity relation gives 898, within the 37 light year uncertainty of the parallax and that of the relation. Moreover, Delta Cephei does not pulse in lonely splendor, but appears to be a member of the extended "Cepheus OB6 association" that also includes Zeta Cephei ("associations" being loose, expanding systems of massive birth-related stars). Cep OB6 is centered a satisfying 880 light years away, right in the middle of the other two determinations. As Delta Cep plows along through space, it and its wind create a shock wave in the interstellar gases in front of them much like that made by a speedboat going through water. Closer in, Delta has a 6th magnitude (6.3) class B (probably B7) 500 or so solar-luminosity companion 41 seconds of arc away. Separated by at least 11,000 Astronomical Units, the two take at least a third of a million years to orbit each other. Even at that distance, the companion would shine in Delta's sky about as bright as our Venus. Watching Delta vary by over a full magnitude from the companion would be a fascinating sight, not there would be anybody to watch, the stars being much too youthful to have any life on any planets that may not have been there in the first place.

Written by Jim Kaler 12/14/01; revised 5/21/13. Return to STARS.