Delta Cephei. Image: © T. Credner & S. Kohle, AlltheSky.com.
Delta Cephei is the second Cepheid variable to have been identified and the prototype for the category known as Type I Cepheids, or Delta Cephei stars. With a change in apparent magnitude of 3.5 to 4.4 over a period of 5.366 days, Delta Cep's entire range of variability can be followed with the naked eye.
At a declination of +58°, Delta makes a perfect target for northern observers, especially those who see Cepheus as a circumpolar constellation. Also, it lies conveniently between two bright stars that shine close to each end of its range of variability: Zeta Cep (magnitude 3.6) and Epsilon Cep (magnitude 4.2). For those observing with binoculars or a modest telescope, Delta Cep's companion stars can be discerned. A seventh magnitude mate is located 41" from the variable itself and is thought to be physically associated with it. A second nearby star of 13th magnitude, located 20.9" from Delta, is most likely a line-of-sight object.
|visual magnitude||4.07 (mean)|
|distance||890 light-years (273 pc)|
|position||R.A. 22h 29m 12s,
Dec. +58° 25' 08"
|other designations||27 Cep, HR 8571, BD +57°2548,
HD 213306, SAO 34508, FK5 847,
AAVSO 2225+57, IRAS 22273+5809,