in astronomy, a large north circumpolar constellation. In the Northern Hemisphere, Cepheus is well above the horizon from June through February, and north of 40° N. latitude it never sets. It reaches its highest point in the sky at 10:00 pm in late September, when a few stars in this long constellation are visible south of the equator. Its northern terminus is only 1.5 degrees from the north celestial pole. Cepheus is one of the 48 constellations cataloged by the Greek astronomer Ptolemy in the 2nd century ad, but it was delineated as a constellation many centuries earlier and is probably of Euphratean origin. In Greek mythology, Cepheus was the king of Ethiopia. Cepheus, which comes from the Greek word meaning “gardener,” is home to an important variable star, Delta Cephei, after which the Cepheid variables—stars used to estimate distances in the universe—are named.
Cepheus is one of four mythologically related constellations, called the Royal Family, that are found in close proximity in the sky. The constellation is bordered on the east by Cassiopeia, the king’s wife, and Cassiopeia in turn borders Andromeda and Perseus, representing the royal couple’s daughter and son-in-law. Andromeda was the unfortunate maiden chained to the rocks as a sacrifice to Cetus, the sea monster, in retribution for Cassiopeia’s vanity. Perseus, the hero, rescued Andromeda and later won the benefaction of Cassiopeia by turning his rival into stone at a wedding banquet Cepheus gave the young couple. Some legends place Cepheus with Jason on the Argos in search of the Golden Fleece. Cepheus and 17 of his 20 sons died in battle with Hercules over control of Sparta. Because Cepheus was distantly related to Zeus, it was easy for him to gain a place in the heavens. Cassiopeia joined him in the sky, where she sits in a chair.
The main group of stars in Cepheus resembles a house or a tower with a steeple. Cepheus the king is usually portrayed in royal robes and stretching out both hands; Ptolemy described him as wearing a Persian-type tiara. Although the constellation contains no especially bright stars, it has many named stars, double stars, and prototype stars, as well as clusters, nebulae, and some galaxies. A tongue of the Milky Way crosses the southern part of the constellation.
Three named stars form the western outline of the figure. Alderamin (Alpha Cephei), whose name comes from the Arabic meaning “right arm,” which it identifies in the constellation, is a second-magnitude star with unusually rapid rotation. It is prominent in the northern sky on June 21, the summer solstice. Alderamin and nearby stars were called the “stars of the flock” by the Tartar astronomer Ulug-Beg, grandson of Timur Lenk (Tamerlane). Alderamin has three companion stars and is 48 light-years from Earth. It will replace Polaris as the north polar star in about 5,000 years.
Alfirk (Beta Cephei), from the Arabic word for “flock,” lies northeast of Alderamin and is sometimes used to indicate the king’s right hip or knee. Alfirk is a hot, blue-white giant star about 1,000 light-years from Earth. Its eighth-magnitude visual companion, a white star, is only half that far away. Alfirk is also a variable star, its magnitude fluctuating between 3.27 and 3.16 within a period of 4.6 hours. It is the prototype star of a class of short-period variable stars. Errai (Gamma Cephei), from the Arabic for “shepherd,” sometimes represents Cepheus’ dog and a flock of sheep. This third-magnitude orange subgiant emits 11 times more light as energy than the sun. As a result of precession—the slow change of direction of the Earth’s axis with respect to the stars—it will become the north polar star in 2,000 years. It is about 48 light-years from Earth. Back in the southern portion of the constellation the orange supergiant Zeta Cephei and the faint Lambda Cephei mark the king’s head, while the stars Delta and Epsilon Cephei mark his crown. The galactic equator passes just south of Delta and Epsilon Cephei and almost parallel to a line drawn between them.
Cepheus contains three famous prototype stars—Beta, Delta, and Mu Cephei. The brightness changes of the short-period Beta Cephei variable stars are not noticeable to the eye but can be picked up with instruments. Study of Delta Cephei variable stars, usually known simply as Cepheid variables, has helped astronomers determine the size of the universe and the distances between various elements within the universe. Cepheid variables have become “standard candles” for measuring distances because their luminosity is directly related to their period of variability, and because they are common stars. More than 700 Cepheid variables have been discovered in the Milky Way galaxy, and some in other galaxies as well. After determining luminosity-period relationships for Cepheid variables whose distance from Earth was known, scientists were able to calculate the distances to other stars and galaxies. Delta Cephei ranges in apparent visual magnitude between 4.4 and 3.5 over a period of 5.4 days. The third prototype star in Cepheus is Mu Cephei, a prototype of the long-period semiregular variable stars. Mu Cephei’s magnitude varies between 5.1 and 3.4 over a period of about 2 years. It is a red supergiant whose color has earned it the name the Garnet Star. As the constellation is conventionally drawn, it marks Cepheus’ left elbow.
Most of the deep-sky objects in Cepheus are faint. About 5 degrees south of Polaris is the open cluster NGC 188, which consists of very old—12 to 14 billion years old—late-type yellow giants. In a small spur of Cepheus that extends into the neighboring constellation Draco is a spiral galaxy with unwound arms; five supernovas have been discovered in this galaxy. Less than 1 degree north of it is the eighth-magnitude rich star cluster, NGC 6939, , , ,
Critically reviewed by James Seevers