Cepheus Constellation

Located at high latitudes in the northern sky, the constellation Cepheus, with its many bright stars, looks like a large house drawn by a child, but is often overlooked because its neighbouring constellations are more dazzling. On the northern shore of the Milky Way, it holds many forgotten ‘treasures’.

Cepheus in Greek mythology represents the king of Ethiopia, husband of Cassiopeia and father of the princess Andromeda. Cepheus constellation is famous for the variable stars in it, as the prototypes of at least three different types of pulsating stars have been found in it. The brightest of these, β Cepheus (Uranus I), is a blue-white giant star, 595 light-years from Earth, which varies in brightness by 0.1 mag every 4.6 hours and requires special equipment to observe its rapid fluctuations in brightness. In contrast, the brightness of δ Cepheus varies between 3.5 and 4.4 magnitudes every 5.7 days. This light variation is easily recognisable compared to the surrounding stars, and is a typical ‘parental variation’, often used by astronomers as a benchmark for measuring the distances of other galaxies.

Mu Cephei and the Nebula

Nebula 1C 1396, located near the famous star Mu Cephei (μ Cepheus). At 5,000 light-years from Earth and 4.0 mag, μ Cepheus is a red giant star that emits as much energy as 350,000 suns and bursts into unexpectedly bright bursts of light from time to time. Nebula 1C 1396 is 2000 light-years away and contains richly detailed features of the dark dust belt.

The Definition of Cepheus Constellation

Right Ascension22h
Rank (area)27
Number of Bright Stars1
Adjoining ConstellationUMi, Dra, Cyg, Lac, Cas, Cam
Apparent Magnitude of the Brightest Star2.45
Meteor Shower/
Brightest Starα Cep
Optimal observational durationSeptember~October

Cepheus is one of the circumpolar constellations. The whole constellation is visible all year round, especially at night in autumn. It is close to the North Star and far opposite the Big Dipper. Cepheus is largely immersed in the Milky Way, forming a slender, skewed pentagon.

The Location and Appearance of Cepheus Constellation

Cepheus is a dim northern constellation, located at north of Cygnus and west of Cassiopeia. It is visible all year round because it belongs to the circumpolar constellation. However, it takes some work to find Cepheus that it is about as bright as the stars around it. 

The whole Cepheus looks like the King. The three stars δ, ε, and ζ make up the crown and the king’s head; ι and α are his left and right shoulders, and γ near the North Pole is his left foot.

Then, how to find Cepheus in the night sky?

First, face north and look up to find the Big Dipper in Ursa Major. Then, find the Ursa Minor where Polaris lies in, Cepheus is next to the UMi.

The Stars in Cepheus Constellation

 α Cep

Alderamin/α Cep is a white main sequence star in Cepheus. With an apparent magnitude of 2.5141, it is the brightest star in the constellation. It is 49.05 light years from Earth.

Alderamin is a main sequence star of spectral type A8Vn. It has a mass 1.74 times and a radius 2.3 times that of the sun. With an effective temperature of 7,740 K, it is 17 times brighter than the sun. The star is thought to be moving out of the main sequence belt and becoming a subgiant.

The Alderamin is an exceptionally fast spinner with an estimated rotation speed of 246 km/s at the equator. It completed a circle in less than 12 hours, much faster than the sun, which takes nearly a month.

Alderamin has a slightly varying brightness of 0.06 magnitude and is a suspicious Delta Scuti variable star, also known as dwarf Cepheid variables. Delta Scuti stars exhibit luminosity fluctuations due to radial and non-radial pulsations on their surfaces.

δ Cep 

Cepheus has a very famous variable star called δ Cep/Alrediph. It contracts and expands every 5.37 days, and its brightness changes accordingly, which makes it a typical pulsating variable star of high brightness. More than 600 such variable stars have been discovered, known collectively as cepheid variable stars.

δ Cep is the fourth star in Cepheus and located on the tip of the nose of Cepheus. It is white at its brightest and yellow at its darkest. It was 30 times larger in diameter than the Sun, but only six thousandths as dense. For each expansion and contraction process, diameters differ by as much as five million kilometers. δ Cep also has an accurate variability period: 5 days, 8 hours, 46 minutes and 39 seconds.

This star is visible to the naked eye, which means you don’t need a telescope or binoculars to see it. Alrediph is a binary or multi-star system. Alrediph is 865.15 light-years away from Earth, using the latest available data from Ibaku in 2007. 

μ Cep

MuCephei/μ Cep is a red supergiant or supergiant in Cepheus and one of the largest and brightest stars known in the Milky Way galaxy. It is also called Garnet star for its garnet red color. Its star is classified as M2e Ia. Astronomers began using garnet’s optical spectrum as a benchmark for classifying other stars in 1943.  Its diameter is estimated to be 1,420 times that of the Sun. 

β Cep

There are many different names of β Cep/Alfirk, like HD 205021, TYC 4465-2643-1, HIP 106032 and so on. It is a triple-star system with the apparent magnitude ranged from 3.15 to 3.21 in 0.1904844 days. β Cep is about 690 light years from Earth.  

The name Alfirk comes from the Arabic al-Firqah, which means “flock of birds”. Alfirk is the prototype for a class of stars called cepheid beta variable stars. These stars are main-sequence stars with masses between 7 and 20 solar masses. Pulsations on their surfaces cause changes in brightness. These stars typically vary in brightness by 0.01 to 0.3 magnitudes with periods of 0.1 to 0.6 days. The brightest part of the Cepheid beta system, Afirk A, is a blue giant, classified in the B2IIIev stellar spectrum. The suffix ev stands for “spectral radiation with variability”. The star spins slowly, taking about 51 days to complete a revolution at 28 kilometers per second.

γ Cep

Errai/γ Cep is a spectroscopic binary system. It is a major star in Cepheus and forms the outline of the constellation. Errai is orange to red in color, depending on the spectral type of the star (K1IV).

This star is visible to the naked eye, which means you don’t need a telescope/binoculars to see it. Its calculated age is 6.600 billion years. This information comes from ExoPlanet.

Errai has at least one extrasolar planet thought to be in orbit around its star. Using the latest data from Ipaku in 2007, Ere is 46.00 light years away from Earth.

Deep Sky Objects

Iris Nebula

Iris Nebula is also called Rainbow Nebula, Blue Iris Nebula, numbered NGC 7023, C4. It  is a bright reflection nebula that located in Cepheus. Its brightness of about 7 magnitude, and about 1300 light-years away from the Earth, 6 light-years in diameter.

Elephant’s Trunk Nebula

Elephant’s Trunk Nebula, numbered IC 1396, is a star-forming region of gas and dust clouds about 2,400 light-years from Earth at magnitude 3.5.

Ghost Nebula

Sh2-136(also known as the Ghost Nebula) is a reflection nebula in Cepheus, located at the edge of the Cepheus Flame molecular cloud complex on the galactic plane. It is about 1200 light-years away from Earth. The dense cloud core within this reflection nebula shows telltale signs of collapsing in the early stages of star formation.

The Mythology of Cepheus Constellation

Cepheus Constellation represents the Ethiopian king Cepheus, who is also the ancestor of beauty Lo.

In Greek mythology, Andromeda was the daughter of Ethiopia King Cepheus and Queen Cassiopeia. Her mother’s constant flaunts of her beauty offended Amphetrite, the wife of Poseidon, the god of the sea. Amphetrite wanted Poseidon to revenge for her, so Poseidon sent Cetus to rape Ethiopian. Cepheus was frightened and asked oracle for help. Oracle revealed the only way is to offer Andromeda.

Andromeda was locked by her parents with iron rope on a huge boulder where the sea monster (represented by Cetus) would pass. Then the hero Perseus happened to catch a glimpse of the tragedy, so he immediately took out the head of Medusa, and petrified Cetus. Finally, Perseus killed the sea monster and rescued Andromeda.

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