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Cepheus


Transit Date of principal star:
11 August


Cepheus is the name of two mythological kings. One was the son of Aleus, from Arcadia. He would become the king of Tegea (a community on the Peloponesian peninsula), would father twenty children, and would sail with Jason as an Argonaut.

The other Cepheus was the son of Belus, king of Egypt (who was himself the son of Poseidon). This Cepheus grew to become the King of Ethiopia (or Joppa). He married Cassiopeia and they had a daughter Andromeda. (Yes, the whole family eventually winds up in the heavens.)

Cassiopeia was incredibly beautiful but immensely vain. She was also proud of her daughter's beauty. In fact she continually boasted that the two of them were more beautiful than any of the fifty sea nymphs who attended Poseidon's court.

These nymphs (the Nereids) complained to Poseidon, who felt he had to defend his own reputation. So he sent a flood to devastate Cepheus' kingdom. The oracles told Cepheus that in order to save his people he must sacrifice his daughter to a great sea monster: Andromeda was tied to a rock along the coastline, dressed only in her jewellery. The monster would be along in due time to take his prize.

At that moment Perseus came flying by. He had just killed the Gorgon Medusa and was carrying the severed head back to Athene.

Medusa was one of the three Gorgon sisters; they were once very beautiful but Medusa slept one night with Poseidon, in Athene's temple. This infuriated Athene so much she turned Medusa's hair to snakes and turned her into a terrifying monster with huge teeth and claws. One look from Medusa would turn the viewer to stone.

With the assistance of Athene, Medusa's sworn enemy, Perseus tricked Medusa by looking at her reflection. He then sliced off her head (Pegasus and a warrior named Chrysaor sprang fully-formed out of Medusa's dead body).

So Perseus arrives at the scene and has a quick chat with Cepheus and Cassiopeia; it is agreed that should he rescue their daughter, he can marry her. So he skims across the water and his shadow on the waters confuses the monster, which Perseus then beheads.

Far from delighted, Cepheus and Cassiopeia balk at their daughter marrying Perseus. However Andromeda insists, so the marriage ceremony is performed on the spot.

Halfway through the ceremony Agenor, a family relative, shows up and claims Andromeda as his bride. (It's pretty certain now that Cassiopeia put him up to it.) Understandably, this angers Perseus and a great battle breaks out. However, outnumbered, Perseus has to resort to desperate measures. So he shows Medusa's head, instantly turning everyone to stone, including Andromeda's parents.

Poseidon then put Cepheus and Cassiopeia into the heavens, but with a twist: he made the vain Cassiopeia spin around on her chair, spending half the year upside down. As for Cepheus, Poseidon gave him a number of medium sized stars that go to make his square face with a pointed crown.


If not very bright, the constellation is still quite noticeable, just to the west of Cassiopeia's chair. The stars are mostly third and fourth magnitude.

The constellation has numerous binaries, several significant variables, and a few interesting deep sky objects.


Alpha Cephei is known as Alderamin ("The Right Shoulder"). In another 5500 years this will become the Pole Star.

Beta Cephei is called Alfirk ("The Herd"). This is a visual binary as well as a variable (see below).

Gamma Cephei is Er Rai (Shepherd). Long before Aldemarin becomes the Pole Star, this one will assume that title (around 4000 AD).

Delta Cephei is a prototype for one of the more significant types of variables (see below). The star is also a very fine binary with a colour contrast.

Mu Cephei is a brilliantly coloured star, a deep red which moved William Herschel to call it "The Garnet Star". The colour depends on the size of one's telescope; the larger scopes bring out an orange element. It is also a semiregular variable (see below).


Double stars in Cepheus:

The constellation has a number of very fine binaries, some quite easily resolved by small scopes, others rather more difficult.

Beta Cephei is a blue giant with a faint companion easily resolved: 3.2, 8; PA 250, separation 13.6"

Delta Cephei is a fixed double, a yellow giant with blue companion: 3.8, 7.5; PA 191, separation 41".

Xi Cephei is considered to be the most attractive binary in Cepheus, a blue-white primary and yellow (or reddish) companion that orbits every 3800 years: 4.4, 6.5; PA 275, separation 8.2". 6.5;

Kruger 60 is a famous binary only 12.9 light years away, comprised of two red dwarfs. Observers had reported seeing flare-ups on the surface of the companion, which is orbiting the primary every 44.6 years.

AB: 9.8, 11.4; presently the companion is at PA 109 and separation 3.2".

The binary is less than one degree SSW of delta Cephei. Burnham has a finder's chart (p. 600).

Struve 2816 is a multiple binary, a very attractive triple:

AC: 6.3, 8.1; 121, separation 11.7"
D: 8.0; PA 339, 19.9"

In the same field are Struve 2813 and Struve 2819, all centred in the middle of the large but faint diffuse nebula IC 1396, just south of mu Cephei.

More binaries can be found in the Binocular Section; use the link below.


Variable stars in Cepheus:

Beta Cephei is the prototype of a class of pulsating variables. These are hot, luminous, and very massive stars with a specral type of O9 to B3. The group is also called 'beta Canis Majoris' stars, for this star too is a member of the group. Indeed, it is the brightest member of the group.

The variations in visual magnitude are of very small amounts, barely as much as a quarter of a magnitude. Beta Cephei varies only 0.04 magnitude from its typical brightness of 3.2 and a period of 4 hours 34 minutes.

Delta Cephei is also the prototype of a class of variables.

Delta Cephei variables are immensely useful stars. Studies of the period of pulsation and the apparent magnitude led investigators to devise a method of gauging the distance to outlying galaxies (the so-called period-luminosity relation).

Delta Cephei varies from 3.48 to 4.37 every 5 days, 8 hours, 47 minutes, 31.9 seconds. This means its maximum brightness can be calculated with a reasonable degree of accuracy.

After the maximum is reached, a gradual diminuition of magnitude occurs over the next three days, only for the star to again increase over the following two days until it again reaches its maximum.

Comparing its magnitude with zeta, just to the west, will tell you if it has reached its brightest. Zeta has a visual magnitude of 3.4, while delta Cephei varies from 4.4 to 3.5. That is, for most of its cycle, it will be rather more dim than zeta, but as it reaches its maximum, it should appear to have a magnitude quite similar to its neighbour.

Mu Cephei is a semiregular supergiant that also varies roughly from 4.5 to 3.5, with a very long period: 730 days.

S Cephei is a long-period Mira-type variable, 7.4-12.9, with a period of 486 days.

T Cephei is also a long-period Mira-type variable, ranging from 5.2 to 11.3 every 388 days.


Deep Sky Objects in Cepheus:

NGC 188 is a faint globular cluster of 150 stars. Its significance lies in the fact that it is extremely old: it is estimated to have formed five to seven billion years ago.

The easiest way to find it is drop down four degrees from the Pole Star, toward the star called "2 UMi", which is nevertheless in Cepheus. The cluster is just to the southwest of this star.

NGC 6939 is a fine open cluster of about eighty stars in a very rich field that includes NGC 6946, a face-on spiral galaxy.

The cluster is found about 2.5 degrees south of theta Cephei, or about two degrees SW of eta Cephei (thus it forms a rough triangle with these two stars).

For a closer appreciation of Cepheus, visit the Binocular Section.


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All files associated with The Constellations Web Page are
Richard Dibon-Smith.