Cepheus is a northern constellation named after a king in Greek mythology, whose wife Cassiopeia and daughter Andromeda are also represented among the stars. It is the 27th-largest of the 88 recognized constellations, covering 588 square degrees, and is visible from latitudes between approximately 90°N and 10°S. In much of the Northern Hemisphere, Cepheus is circumpolar, meaning it never sets below the horizon, although November is an especially good month to observe it.
Despite having few particularly bright stars, Cepheus contains several fascinating objects, including Delta Cephei, the prototype of an important class of variable stars, the enormous red supergiant known as the Garnet Star, and the Fireworks Galaxy. It contains no Messier objects, but offers several worthwhile targets for binoculars and telescopes.
Perseus Family of Constellations
Cepheus belongs to the Perseus family of constellations, alongside Cassiopeia, Andromeda, Perseus, Pegasus, Cetus, Auriga, Lacerta, and Triangulum.
Mythology
In Greek mythology, Cepheus was the king of Aethiopia, husband of Cassiopeia, and father of Andromeda. Their troubles began when Cassiopeia boasted that she or her daughter was more beautiful than the Nereids, the sea nymphs. Angered by the insult, Poseidon sent a sea monster, represented by the constellation Cetus, to devastate the kingdom. An oracle declared that the only way to save the land was to sacrifice Andromeda, who was subsequently chained to a rock beside the sea.
Before the monster could claim her, the hero Perseus arrived and rescued Andromeda. In some versions of the story, he used the severed head of Medusa to turn the creature to stone. Perseus later married Andromeda, although their union provoked a confrontation with Phineus, to whom she had previously been promised. Cepheus, Cassiopeia, Andromeda, Perseus, and Cetus were all immortalized as constellations, preserving the story in the night sky.
Principal Stars
- Alderamin (Alpha Cephei) is the brightest star in Cepheus, shining at magnitude +2.5 from approximately 49 light-years away. It is a white, A-type main-sequence star notable for its rapid rotation, completing a turn in roughly half a day. Its name derives from an Arabic expression meaning “the right arm.” Due to Earth’s axial precession, Alderamin will become a northern pole star in roughly 5,500 years.
- Alfirk (Beta Cephei) is a multiple-star system approximately 700 light-years away, with a combined apparent magnitude of around +3.2 to +3.3. Its principal component is a hot blue-white star whose brightness varies slightly over several hours. Alfirk is the prototype of the Beta Cephei class of variable stars, whose pulsations provide valuable information about the interiors of massive stars.
- Errai (Gamma Cephei), also known as Alrai, is a binary system about 45 light-years away, shining at magnitude +3.2. Its primary is an orange evolved star accompanied by a faint red dwarf, and it also hosts a confirmed exoplanet called Tadmor. As Earth’s rotational axis slowly changes direction, Errai will become a useful northern pole star in the fourth millennium.
- Delta Cephei is one of astronomy’s most important variable stars. Its brightness changes from approximately magnitude +3.5 to +4.4 over a remarkably regular period of about 5.37 days, as the star’s outer layers expand and contract. It is the prototype of the classical Cepheid variables, whose pulsation periods are closely related to their intrinsic luminosities. By comparing a Cepheid’s true brightness with how bright it appears from Earth, astronomers can estimate its distance. This relationship became a cornerstone of the cosmic distance ladder and helped establish the immense scale of the universe beyond the Milky Way.
- Mu Cephei, commonly called the Garnet Star, is a red supergiant famous for its striking reddish-orange color. William Herschel described its appearance as resembling a deep-colored garnet, inspiring the nickname. Mu Cephei is hundreds of times wider than the Sun, although estimates of its precise size and distance remain uncertain. It is also a semiregular variable star, changing in brightness over months and years. Like other massive red supergiants, Mu Cephei is in an advanced stage of stellar evolution and is expected eventually to end its life in a supernova explosion.
Notable Deep-Sky Objects
- NGC 7142 is an old open star cluster approximately 6,000 light-years away. It lies close to the reflection nebula NGC 7129, although the two objects are physically distinct. NGC 7142 is especially interesting because it contains blue stragglers — stars that appear younger and hotter than expected for a cluster of its age.
- NGC 7538 is a large star-forming nebula roughly 9,000 light-years away. Hidden within its clouds of gas and dust are young massive stars and protostars still gathering material. Observations at infrared and radio wavelengths reveal activity that is largely obscured in visible light.
- The Fireworks Galaxy (NGC 6946) is a spiral galaxy located approximately 22 million light-years away, near the border between Cepheus and Cygnus. It earned its nickname because at least ten supernovae have been observed there since 1917, an unusually high number for a single galaxy. Discovered by William Herschel in 1798, it is a rewarding target for larger amateur telescopes under dark skies.
- The Iris Nebula (NGC 7023) is a beautiful reflection nebula approximately 1,300 light-years away. Its blue glow comes from starlight scattered by surrounding interstellar dust, rather than gas emitting its own visible light. The nebula is embedded within a much larger dark molecular cloud.
Other objects of interest include the open clusters NGC 188 and NGC 7380, the latter surrounded by the emission nebula commonly known as the Wizard Nebula.