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 the the 27th largest of the 88 recognised constellations,  taking up an area of 588 sq/degrees of the nothern sky, and can be seen by observers located between latitudes +90° and -10°. Despite mostly containing third and fourth magnitude stars, Cepheus does have several notable stars, among which is the enormous red giant Garnet Star, as well as several well-known deep sky objects, such as the Fireworks Galaxy, the Iris Nebula, and the Wizard Nebula. However, the constellation contains no Messier objects, and has no meteor showers associated with it.

Perseus Family of Constellations

The nearby constellations which belong to the Perseus family of constellations are Andromeda, Auriga, Cassiopeia, Cepheus, Cetus, Lacerta, Pegasus, Perseus, 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, today 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 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, Perseus killed many of his enemies using the head of Medusa to turn them into stone. Unfortunately, Cepheus and Cassiopeia failed to look away in time, and sadly, both were instantly turned into stone as well.

Notable Stars

– Alderamin (Alpha Cephei), located about 50 light years away, is a white, late-stage class A main sequence star that is rapidly evolving into a sub giant. What makes it remarkable is its very high rate of rotation, which is estimated to be at least 246 km/second at its equator, for a complete rotation once every 12 hours. With an apparent visual magnitude of 2.514, the star is an easy naked-eye target, and it never sets below the horizon for observers in Europe, Canada, northern Asia, and even most North American urban centres. The stars original name, Alderamin, derives from the Arabic words ad-dira‘ al-yamin, which roughly translates into “The Right Arm”.

Star Constellation Facts: Cepheus

– Alfirk (Beta Cephei) is actually a triple star system about 690 light years distant, with a combined apparent magnitude that shows variations between 3.15 to 3.21. The most luminous star in the Beta Cepheid system, Alfirk, is a class B2IIIev blue giant, as well as a slow rotator at around 28 km per second, meaning it takes 51 days to complete a single rotation.

– Delta Cephei is a binary star system 890 light years away, with an average apparent magnitude of 4.07. It also serves as the prototype of another 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, with the primary star of the pair, a yellow-white F-class supergiant, 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. The companion star has an apparent magnitude of 7.5, and is thought to be a B-class star. The system has

Although Cepheids are massive stars that are in the process of dying, they are nevertheless bright enough to be spotted easily in neighbouring galaxies, where they serve as distance markers, since their luminosities are directly related to their pulsation periods. Thus, by measuring their apparent magnitudes, astronomers use Cepheids as a means to calculate the distances to their home galaxies.

– Garnet Star (Mu Cephei) is not only one of the biggest stars ever discovered, it is also among the most luminous stars known. Sir William Herschel once referred to the star as “…a very fine deep garnet colour, such as the periodical star o Ceti.” Hence the name, Garnet Star. The star is estimated to be about 2,400 light years away, too far for its distance to be certain with any degree of certainty.

It is classified as a M2Ia star, and has an apparent magnitude of 4.08, which at its estimated distance, makes it very luminous indeed. Moreover, it is at least 1,650 bigger than the Sun, has a diameter that is equal to 7.7 AU, and if not for the fact that interstellar dust obscures 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, and in the case of the Garnet Star, its variability varies from magnitude 3.62 to 5.0. over a period that varies between 2 and 2.5 years. To date though, no definitive pattern in the stars variability has been found. What is known, however, is that the star has begun to convert helium into carbon, and that it’s observed instability is 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 7142NGC 7142; located about 6,200 light years away, the open cluster (NGC 7142), toward the right in this image, is one of the most perplexing clusters yet found. It is thought that much of the cluster is obscured by the nebulous cloud ( NGC 7129) to the left of the cluster, which makes estimating the cluster’s age very difficult. It is not certain how much of the cluster is actually obscured, but more perplexing is the presence of many young, hot, blue stars in what is believed to be a very old cluster. Current models state that old clusters should not contain young, hot stars, since 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, this huge emission nebula is experiencing a burst of star formation. There may be nothing remarkable about this, but what is special 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 the 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): The aptly named Fireworks Galaxy was discovered by William Herschel in 1798 on the border between Cepheus and Cygnus, about 22 million light years away. 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.