Cepheus (constellation)

Cepheus is a constellation in the deep northern sky. It is named after Cepheus, a king of Aethiopia in Greek mythology. Cepheus is one of the 48 constellations listed by the second century astronomer Ptolemy, and it is still one of the 88 constellations recognized today.

The brightest star in Cepheus is Alderamin (Alpha Cephei), which has an apparent magnitude of 2.5. Another important star in this constellation is Delta Cephei, which is the prototype of a special kind of star called a Cepheid variable. Cepheus also includes some of the largest stars known, such as RW Cephei, an orange hypergiant, and several red supergiants like Mu Cephei and VV Cephei.

Additionally, Cepheus contains a very bright object called the quasar S5 0014+81. This quasar has an extremely massive black hole at its center, estimated to be about 40 billion times the mass of our Sun. This makes it one of the most massive black holes known, much larger than the central black hole of the Milky Way.

History and mythology

Cepheus was the King of Aethiopia. He was married to Cassiopeia and was the father of Andromeda. All three of them are shown as constellations in the night sky today.

Features

See also: List of stars in Cepheus

Alderamin, also known as Alpha Cephei, is the brightest star in the constellation, with an apparent magnitude of 2.51. Gamma Cephei, also known as Errai, is the second-brightest star in the constellation, with an apparent magnitude of 3.21. It is a binary star, made up by an orange giant or subgiant and a red dwarf. The primary component hosts one exoplanet, Gamma Cephei Ab (Tadmor). Delta Cephei is a yellow-hued supergiant star 980 light-years from Earth and the prototype of the class of the Cepheid variables. It was discovered to be variable by John Goodricke in 1784. It varies between 3.5^m and 4.4^m over a period of 5 days and 9 hours. The Cepheids are a class of pulsating variable stars; Delta Cephei has a minimum size of 40 solar diameters and a maximum size of 46 solar diameters. It is also a double star; the primary star also has a wide-set blue-hued companion of magnitude 6.3.

There are four red supergiants in the constellation that are visible to the naked eye. Mu Cephei is also known as the Garnet Star due to its deep red colour. It is a semiregular variable star with a minimum magnitude of 5.1 and a maximum magnitude of 3.4. Its period is approximately 2 years. The star's radius has been estimated to be from 972 solar radii (4.52 AU) to 1,420 solar radii (6.6 AU). If it were placed at the center of the Solar System, it would likely extend past the orbit of Jupiter. The second, VV Cephei A, is a semiregular variable star, located approximately 5,000 light-years from Earth. It has a minimum magnitude of 5.4 and a maximum magnitude of 4.8, and is paired with a blue main sequence star called VV Cephei B. The red supergiant primary is around 1,050 times larger than the Sun. VV Cephei is also an unusually long-period eclipsing binary, but the eclipses, which occur every 20.3 years, are too faint to be observed with the unaided eye. The third, Zeta Cephei, is not as large as Mu Cephei and VV Cephei A with a diameter less than 200 times that of the Sun; however, its surface would lie between the orbits of Venus and Earth if placed at the center of the Solar System. Zeta Cephei has an apparent magnitude of 3.35, being the fourth-brightest star in the constellation. The last and faintest is V381 Cephei Aa with a maximum magnitude of 5.5. It is part of a triple star system similar to VV Cephei, and has a diameter 980 times that of the Sun. All four stars have initial masses more than eight times that of the Sun.

Nu Cephei is a blue supergiant similar to Deneb with an initial mass of over 20 solar masses. It belongs to the Cepheus OB2 stellar association along with Mu Cephei and VV Cephei, which have similar initial masses.

There are several prominent double stars and binary stars in Cepheus. Omicron Cephei is a binary star with a period of 800 years. The system, 211 light-years from Earth, consists of an orange-hued giant primary of magnitude 4.9 and a secondary of magnitude 7.1. Xi Cephei is another binary star, 102 light-years from Earth, with a period of 4,000 years. It has a blue-white primary of magnitude 4.4 and a yellow secondary of magnitude 6.5.

Krüger 60 is an 11th-magnitude binary star consisting of two red dwarfs. The star system is one of the nearest, being only 13 light-years away from Earth.

Deep-sky objects

The constellation Cepheus is rich in star-forming regions and emission nebulae of various kinds. Most of the structures visible in the wide-field view lie in the Orion Arm of the Milky Way, roughly 2,800 to 3,600 light-years away. Some notable deep-sky objects include:

• NGC 188 is an open cluster that is the closest open cluster to the north celestial pole and one of the oldest-known.
• NGC 6946 is a spiral galaxy sometimes called the Fireworks Galaxy.
• The nebula NGC 7538 is home to one of the largest-known protostars.
• NGC 7023 is a reflection nebula with an associated star cluster, located near Beta Cephei and T Cephei.
• Sh 2-155, also known as the Cave Nebula, is a dim and diffuse bright nebula.
• The quasar 6C B0014+8120 is one of the most powerful objects in the universe.
• Sh 2-131 is ionized by the stars of the cluster IC 1396. Embedded within it is a dark nebula known as the Elephant's Trunk.
• NGC 7822 (also known as Sh 2-171) is an H II region with an apparent diameter of about 3°

Visualizations

In pictures, Cepheus is often shown with his arms raised, asking the gods to protect Andromeda's life. He is also sometimes shown as a king sitting on his throne.

Equivalents

In Chinese astronomy, the stars of the constellation Cepheus are part of two special areas. These areas are called the Purple Forbidden enclosure and the Black Tortoise of the North.

Namesakes

There have been ships named after the Cepheus constellation. Two of them were part of the United States Navy: the USS Cepheus (AKA-18) and the USS Cepheus (AK-265). In the video game Stellaris, there is also an update called "Cepheus".