Star Constellation Facts: Cepheus

Star Constellation Facts: Cepheus

The constellation Cepheus is named after a mythical king from Greek mythology, who was the husband of Cassiopeia, and the father of Andromeda, both of whom are represented by neighbouring constellations. It is the 27th largest of the 88 recognised constellations, taking up an area of 588 sq/degrees of the celestial heavens, and can be observed between latitudes of +90° and -10°, although this northern circumpolar constellation is best seen during November. Despite mostly containing third and fourth magnitude stars, Cepheus does have several notable stellar objects, among which is the enormous red giant Garnet Star, as well as several well-known deep sky objects, such as the Fireworks Galaxy, and several beautiful star clusters and nebulae. However, the constellation contains no Messier objects, and has no meteor showers associated with it.

Perseus Family of Constellations

Cepheus is a member of the Perseus Family of Constellations, together with Cassiopeia, Andromeda, Perseus, Pegasus, Cetus, Auriga, Lacerta, and Triangulum.


According to legend, Cassiopeia, the wife of King Cepheus of Aethiopia, brought the wrath of Poseidon upon them after she claimed that their daughter Andromeda was more beautiful than the Nereids, one of whom was the wife of the Sea God. An angered Poseidon then sent a sea creature, nowadays represented by the constellation Cetus, to ravage their kingdom, leading Cepheus to seek out the wisdom of an oracle, who advised him to sacrifice his daughter to the sea monster. Cetus found Andromeda chained to a rock, but was subsequently slain by the hero Perseus on his flying horse Pegasus, before then claiming Andromeda for his wife. This caused problems with Cepheus’ brother, Phineus, who had previously been promised her hand in marriage,  and after a major fight broke out, Perseus killed many of his enemies by using the head of Medusa to turn them into stone. Determined that Cassiopeia should be punished for her insult, Poseidon later placed her in the sky as a constellation where she circles the celestial pole, sometimes upside-down, while her husband Cepheus also suffered the same fate.

Principal Stars

– Alderamin (Alpha Cephei), the constellation’s brightest star, is a blue-white subgiant located about 50 light years away with an apparent visual magnitude of 2.514. This makes it an easy naked-eye target for stargazers, especially since it never sets below the horizon for observers in Europe, Canada, North American, and Northern Asia. A remarkable feature of Alderamin is its very high rate of rotation, which is estimated to be at least 246 km/second at its equator, making for one complete rotation every 12 hours. The stars name derives from the Arabic for “The Right Arm”.

Cepheus– Alfirk (Beta Cephei), the second brightest star in Cepheus, is a triple star system about 690 light years from our solar system with a combined apparent magnitude that varies between 3.15 to 3.21. Its main component star is a blue giant (B1 IV) which has 6 times our sun’s radius and 12 times its mass. Alfirk is a slow rotator, at around 28 km per second, meaning it takes 51 days to complete a single rotation.

– Alrai (Gamma Cephei), the constellation’s third brightest star, is a binary star system found 45 light years distant of magnitude 3.22. Its main component is an orange subgiant (K1III-IV) that is 6.6 billion years old, while its companion is believed to be a red dwarf. The precession of the equinox will cause Gamma Cephei to replace Polaris as the north celestial pole star about one thousand years in the future.

Other stars of interest in Cepheus includes the blue supergiant Nu Cephei; the yellow-white dwarf Epsilon Cephei; the orange subgiant Zeta Cephei; and the orange giants Eta Cephei and Iota Cephe. Also worth mentioning are two famous variable stars, namely Delta Cephei and Mu Cephei:

Star Constellation Facts: Cepheus

– Delta Cephei is a binary star system 890 light years from the Sun which shines with an average apparent magnitude of 4.07. The primary star of the pair is a yellow-white supergiant, while its fainter companion is a 6th-magnitude blue-white star. Delta Cephei serves as the prototype for a class of variable stars called Cepheids, which spend their lives on the main sequence as B-class stars. Their variability stems from the instabilities in their helium cores that cause them to expand and contract with almost metronome-like regularity, which in this case results in the primary star of the pair varying enough in brightness to change its classification from an F5 to a G3 star every 5 days, 8 hours, 47 minutes and 32 seconds. Although Cepheids are massive stars that are in the process of dying, they are bright enough to be easily spotted and used as distance markers, since their luminosity is directly related to their pulsation periods. Thus, by measuring their apparent magnitudes, astronomers use Cepheids as a means to calculate distances to their home galaxies.

– Garnet Star (Mu Cephei), one of the biggest stars ever discovered, is a red supergiant (M2e Ia) estimated to be about 2,400 light years away, making it too far away for its distance to be determined with any degree of certainty. Its name derives from an observation made by Sir William Herschel in 1783 who upon seeing it described it as a “very fine deep garnet colour. Mu Cephei has an apparent magnitude of 4.08, which at its estimated distance, makes it very luminous indeed. Moreover, it has a diameter around 2,536 that of the Sun, and if not for the interstellar dust obscuring much of its light, it would have an apparent magnitude of 1.97. Mu Cephei serves as the prototype of a class of stars known as Mu Cephei variables, which in the case of the Garnet Star varies from magnitude 3.62 to 5.0 over a period of between 2 and 2.5 years. To date, no definitive pattern in the stars variability has been found, but what is known is that the star has begun to convert helium into carbon, with it’s observed instability evidence that it is nearing the end of its life, which could occur as a supernova in the next several million years.

Notable Deep Sky Objects

NGC 7142– NGC 7142 is an open cluster located about 6,200 light years away that is one of the most perplexing examples of its kind yet found. The cluster towards the right of the image is obscured by the nebulous cloud (NGC 7129) to the left of the cluster, which makes estimating its age very difficult. Some scientists have suggested that NGC 7142 is over 3 billion years old, although the presence of many young, hot, blue stars in what is believed to be a very old cluster is perplexing as current models state that they should all have died off a long time ago. Nevertheless, blue straggler stars are present in significant numbers, and research into their presence is continuing.

– NGC 7538, located about 9,100 light years away, is a huge emission nebula that is experiencing a burst of star formation. While there may be nothing remarkable about this, what is special, however, is that the nebula contains the biggest proto-star ever discovered. This contracting clump of gas and dust is about 300 times as big as our solar system, and although it will shrink as the star forms, it is likely to give birth to one of the biggest, and most massive stars in the entire Milky Way.

Fireworks Galaxy

The Fireworks Galaxy (NGC 6946, Arp 29, Caldwell 12) is a spiral galaxy about 22 million light years from Earth, that was discovered by William Herschel in 1798 on the border between Cepheus and Cygnus. Although the Fireworks Galaxy is smaller than most other galaxies, its major claim to fame is the fact that nine supernova explosions have been observed in it during the last 100 years or so.

Other deep-sky objects of interest in Cepheus includes the open clusters NGC 7380, NGC 188, and NGC 7142; and the reflection nebulae NGC 7538, and NGC 7023.