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.

History and mythology

Cepheus was the King of Aethiopia. He was married to Cassiopeia and was the father of Andromeda. You can see all three of them as constellations in the night sky today.

Features

See also: List of stars in Cepheus

Alderamin, also called Alpha Cephei, is the brightest star in the constellation. It has an apparent magnitude of 2.51. Gamma Cephei, or Errai, is the second-brightest star. It has a magnitude of 3.21. It is a binary star with an orange giant or subgiant and a red dwarf. Gamma Cephei has an exoplanet named Gamma Cephei Ab (Tadmor). Delta Cephei is a yellow star 980 light-years from Earth. It is the main star for the Cepheid variables. It was found to change in brightness by John Goodricke in 1784. Its brightness changes every 5 days and 9 hours. The Cepheids are a group of pulsating variable stars. Delta Cephei can be as small as 40 solar diameters and as large as 46 solar diameters. It is also a double star with a blue companion.

There are four red supergiants in Cepheus that we can see without a telescope. Mu Cephei, called the Garnet Star, is deep red. Its brightness changes over about 2 years. It is very large—if it were placed where the Sun is, it would extend past the orbit of Jupiter. VV Cephei A is another red supergiant about 5,000 light-years away. It pairs with a blue star and is much larger than the Sun. Zeta Cephei is smaller than Mu Cephei and VV Cephei A. If placed at the Sun's place, its size would be between the orbits of Venus and Earth. The last is V381 Cephei Aa, which is part of a triple star system and very large.

Nu Cephei is a bright blue star with a mass over 20 times that of the Sun. It is part of the Cepheus OB2 group with Mu Cephei and VV Cephei.

Cepheus has many double stars and binary stars. Omicron Cephei is a binary star 211 light-years away. Xi Cephei is another binary star 102 light-years away.

Krüger 60 is a faint binary star just 13 light-years from Earth.

Deep-sky objects

Cepheus has many star-forming areas and gas clouds. Most of these are in the Orion Arm of the Milky Way, about 2,800 to 3,600 light-years away. Some well-known objects are:

• NGC 188 is an open cluster near the north celestial pole.
• NGC 6946 is a spiral galaxy called the Fireworks Galaxy.
• The nebula NGC 7538 contains one of the largest-known young stars.
• NGC 7023 is a reflection nebula near Beta Cephei and T Cephei.
• Sh 2-155, the Cave Nebula, is a faint cloud of gas.
• The quasar 6C B0014+8120 is one of the most powerful objects in space.
• Sh 2-131 is lit by stars in the cluster IC 1396. Inside it is a dark cloud called the Elephant's Trunk.
• NGC 7822 (Sh 2-171) is a region of glowing gas about 3° wide.

Visualizations

In pictures, Cepheus is often shown with his arms raised, asking the gods to help Andromeda. 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

Some ships were named after the Cepheus constellation. Two 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 an update called "Cepheus".