Cepheus lies between Cassiopeia and Draco. In the Northern Hemisphere, it is a circumpolar constellation, meaning it can be seen throughout the year. It is best viewed from August through January. It is not visible for observers in the Southern Hemisphere.
History and MythologyIn Greek mythology, Cepheus is Cassiopeia's husband and Andromeda's father. Cepheus was king of Joppa, which is now modern Israel. Cepheus was one of the Argonauts who traveled with Jason in search of the golden fleece. His daughter, Andromeda, was saved by Perseus from the sea monster Cetus, to whom she was to be sacrificed.
Notable Stars
The constellation looks somewhat like a lopsided house with a pointed roof. The five main stars in Cepheus are in the southwestern part of the constellation. The northern parts of Cepheus are devoid of any conspicuous stars.
The brightest star in Cepheus is Alderamin, which marks the king's right shoulder. It is a giant star of spectral type A at a distance of 49 light-years. The name Alderamin means "right arm". It is the westernmost of the two stars at the base of the "house" of Cepheus. The second brightest star is Alfirk, or Beta Cephei, a giant star which gets its name from the Arabic meaning "flock of sheep". It is a double star with components of magnitude 3.3 and 8.0, and is also slightly variable.
Gamma Cephei, also Errai, is among the visually brightest stars known to have a planet. This star is only 45 light years away, and also has a red dwarf companion.
Delta Cephei is a famous variable star discovered in 1784. It changes in brightness over a period of about 5 days in a very predictable manner. Because it was the first variable star of its kind discovered, other similar stars are now called Cepheid variables. The period of a Cepheid variable is directly related to its brightness. This relationship permits Cepheids to be used to measure the distances to external galaxies.
Just below the bottom part of the "house" is the red giant Mu Cephei. It is sometimes called the Garnet star because of its strong red color. It is 1,800 light-years away and is 14,000 times more luminous than our Sun. It is one of the reddest stars visible to the naked eye. Mu Cephei serves as a north star for the planet Mars, since it is the star closest to the north celestial pole of Mars.
Clusters, Nebulae, and Galaxies
Because part of the Milky Way passes through Cepheus, the constellation is rich in clusters and nebulae. There are several open star clusters in Cepheus brighter than 10th magnitude. Ninth magnitude NGC 188 is one of the oldest open clusters known, with an estimated age of 5 billion years. It is within 5 degrees of the north celestial pole. NGC 7160 is an open cluster with about 25 stars, the brightest of about 7th magnitude.
IC 1396 is an extremely large region of nebulosity of very low surface brightness. It is difficult to see visually, but makes spectacular photographs. The Elephant Trunk nebula within IC 1396 is a dark, dense globule with a bright, sinuous rim - a concentration of interstellar gas and dust.
NGC 7822 and Cederblad 214 are a very large area of emission nebulosity located in the constellation Cepheus, right at the Cepheus/Cassiopeia border. The Cave Nebula is another faint region of hydrogen emission and surrounding dust. These nebulae are also difficult targets to detect visually.
NGC 7023 is a wonderful example of an open star cluster associated with a reflection nebula. It is located about 3.5 degrees southwest of Beta Cephei (Alfirk). Sometimes called the Iris Nebula, NGC 7023 is about 6 light-years across, and 1,300 light-years away.
The planetary nebula NGC 40, sometimes referred to as the "Bow tie" nebula, is also located in Cepheus. It is 11th magnitude, and about 48 arcsconds across.
NGC 6946 is a rather nearby but dim spiral galaxy, on the border between the constellations Cepheus and Cygnus. It is called the "Fireworks Galaxy" because it has produced more supernovae (nine since the start of the 20th century) than any other galaxy.