Cepheus (constellation)

Cepheus is a northern constellation representing the legendary King of Ethiopia Cepheus, husband of Cassiopeia and father of Andromeda. It is one of the 88 modern constellations and one of the 48 constellations named by Ptolemy.

Notable features

Constellation of Cepheus

The brightest star in the constellation is Alderamin (α Cephei), a white subgiant whose luminosity is 17 times that of the Sun and located 48.8 light-years away. It is followed in brightness by γ Cephei, officially called Errai. It is a binary star composed of an orange subgiant of spectral type K1IV with a radius of 4.8 solar radii, and a red dwarf of class M1V that orbits it at a distance between 12 and 26 AU. A planet has been discovered around the subgiant, called Tadmor, 60% more massive than Jupiter with a separation of 2 AU from its star; It takes 903 days to complete one orbit.

Cepheus contains two variable stars of special interest in astronomy. β Cephei—called Alfirk—is a main-sequence star of type B1V with an effective temperature of 26,900 K. It is the main representative of a class of variables (Beta Cepehi variables) that show fluctuations in their luminosity due to pulsations. on its surface. They present small oscillations of brightness; thus, Alfirk's brightness varies between apparent magnitude +3.16 and +3.27 over a period of 0.1905 days (4.57 hours). On the other hand, δ Cephei is the prototype of the well-known Cepheids, stars whose luminosity varies with a very regular period and which play an important role as distance indicators. John Goodricke already observed the variability of this star in 1784, which occurs with a periodicity of 5 days, 8 hours, 47 minutes and 32 seconds.

ζ Cephei is an orange supergiant of spectral type K1.5Ib. A small variation of 0.04 magnitudes in its brightness with a periodicity of 533 days suggests the presence of an as yet undetected companion star. Also, Kurhah, name of ξ Cephei, is a triple star system whose main component is an A3m star whose atmosphere is enriched in metals. Around it, an F7V type star completes an orbit every 810.9 days. In turn, another yellow dwarf of type F8V orbits around the inner pair whose orbital period is approximately 3800 years.

Another star of interest is λ Cephei, a very hot blue supergiant of spectral type O6I—38,900 K surface temperature— and with a luminosity 377,000 times that of the Sun. It has a high relative speed with respect to the Sun, which is why it is thought to be a runaway star; its movement through space indicates that 2.5 million years ago λ Cephei was ejected from the Cepheus OB3 stellar association.

In this constellation are also located μ Cephei and VV Cephei, two of the largest stars in the Milky Way. The first, which receives the title of "garnet star" for its intense red color, is a red hypergiant of spectral type M2Ia or M1I; it has a diameter — calculated from the direct measurement of its apparent diameter — of 0.021 seconds of arc considering that it is 2,400 light-years away—some 1,200 times greater than solar, equivalent to 13.5 AU. VV Cephei is also a variable red hypergiant, whose radius is approximately 1400 times the solar radius, which implies that if it were in the place of the Sun —considering the maximum value— its surface would extend beyond the orbit of Jupiter. Both μ Cephei and VV Cephei are semiregular variables, and the latter is also an eclipsing binary. Of different characteristics is XX Cephei, an eclipsing binary whose orbital period is approximately 2.3 days, the main component being a star of type A4V. There are also eclipsing binaries WX Cephei —of joint spectral type A5V and with a period of 3.3785 days—and EK Cephei, whose orbital period is 4.4278 days. On the other hand, GP Cephei, a distant quadruple star system made up of two binary stars: the main pair has an orbital period of 6.6884 days and consists of a hot WN6-type Wolf-Rayet star accompanied by a blue supergiant type O6I.

Spiral Galaxy NGC 6946, where ten supernovaes have been observed since 1917.

HD 26367 is a "barium dwarf" of type F7V, so classified for its high levels of barium and strontium, much higher than usual; As in other barium stars —which are giants—, it is thought that the chemical peculiarities have their origin in superficial contamination —mass transfer— from a stellar companion when it was in the giant asymptotic branch, today converted into a dwarf white.

On the other hand, the Kruger 60 system is made up of two small red dwarfs 13.15 light years away from the solar system, being the closest star of the constellation. The spectral type of the two components is M3V and M4V respectively, and the separation between them varies between 5.5 ua in the periastrum and 13.5 ua in the apoastrum.

As for deep sky objects, NGC 188 is an open cluster of about 120 stars located about 1800 light-years above the galactic plane. It is one of the oldest open clusters with an age of 6.8 billion years. By contrast, NGC 7510 is a young cluster 10 million years old, 11,400 light-years distant.

Another object of interest in Cepheus is the Iris Nebula (NGC 7023), a reflection nebula 1,300 light-years away: it is actually a star cluster embedded within the NBL 487 nebula. Instead, NGC 40 and NGC 7354 are two planetary nebulae within the constellation's boundaries. In the first one, the central star is a Wolf-Rayet star with a temperature of 50,000 K; in the second one can distinguish a circular outer shell and an elliptical inner one.

In this constellation is also found CTA 1, a composite supernova remnant, in which a shell-like structure is observed in radio band and a full morphology from the center in X-rays. Inside is a pulsar —RX J0007.0+7302— which does not emit radio frequencies but does emit X-rays and gamma rays.

Similarly, various galaxies are located in Cepheus, including NGC 6946, known as the “Fireworks Galaxy” due to the large number of supernovae observed in it, ten in the last hundred years. Distant about ten million light-years, it lies quite close to the plane of the Milky Way.

In this constellation is located the blazar S5 0014+81, one of the most energetic objects in the universe. It contains a supermassive black hole with a mass 10,000 greater than the black hole at the center of our galaxy, equivalent to 40 billion suns.

Main Stars

• α Cephei (Alderamin), the brightest star of constellation with magnitude 2,44, white.
• β Cephei (Alfirk), prototype of the variables Beta Cephei, white-azulada variable brightness between magnitude 3,15 and 3.21. It is also a Be star that expels matter from its surface.
• γ Cephei (Errai), with magnitude 3,22 is a binary star with an extrasolar planet around the main star, an orange subgigante.
• δ Cephei, one of the most famous stars in the sky for being the prototype of the Cepheid variables. It's also a double star.
• ε Cephei (Phicares), whose name is of Phoenician origin, is a Delta Scuti variable of magnitude 4,18.
• ج Cephei, (Tsao Fu), supergigante orange of magnitude 3,39 to 725 light years away.

Image of μ Cephei, one of the largest stars in the galaxy.

• MIL Cephei, orange subgigante of magnitude 3,41 located at 47 light years.
• ι Cephei, an orange giant of magnitude 3.51.
• λ Cephei, bright blue supergigante and fugitive star of magnitude 5.05.
• μ Cephei, the "Granate star", a red supergiant and one of the visible stars whose size is greater.
• . Cephei, white supergiant of magnitude 4,29.
• ・ Cephei (Kurhah or Alkurhah), a very visible double star whose components are separated by 8 seconds of arc.
• ρ Cephei (Al Kalb al Rai), white star of magnitude 5.50.
• 9 Cephei (V337 Cephei), light blue supergigante of magnitude 4,79.
• 14 Cephei (LZ Cephei), variable blue star of magnitude 5.55.
• 19 Cephei, luminous O-type star of magnitude 5,11.
• 31 Cephei, white-yellow star with a metallic content 3.2 times higher than the Sun.

Comparison of size between the Sun (the yellow point on the left) and VV Cephei A.

• S Cephei, variable Mira whose brightness varies between magnitude +7.4 and +12.9 over a period of 486,84 days.
• RW Cephei, semi-regular variable of average magnitude 6.51.
• ST Cephei, red supergigante and irregular variable.
• VV Cephei, one of the largest known stars; it is also an eclipsing binary, formed by a red hypergigante (VV Cephei A) and a blue star of the main sequence (VV Cephei B).
• WX Cephei and EK Cephei, eclipsing binaries of magnitude 8.7 and 7.88 respectively.
• XX Cephei, triple system of magnitude 9,21 that contains a Algol-type eclipsing binary.
• AH Cephei, also an eclipsing binary of average magnitude 6,88, formed by two white-blue stars.
• CQ Cephei, one of whose members is a star of Wolf-Rayet.
• DH Cephei, binary composed of two hot blue stars that forms part of NGC 7380.
• GP Cephei, quadruple system, one of whose members is a star of Wolf-Rayet.
• OV Cephei, a red giant only 3.o of the north celestial pole that is 500 light years away.
• V419 Cephei, red supergiant of magnitude 6.62.
• Kruger 60, a star system composed of two red dwarfs distant only 13 light years from the Solar System.
• HR 285 (HD 5848), an orange giant of magnitude 4,24 also known as 2 Ursae Minoris, being previously located in the Osa Menor.
• HR 8938 (HD 221525), white 5,56-spectacularly 2.5.o of the north pole.
• HR 9038 (HD 223778), triple to 35.2 light years whose main component is a spectroscopic binary orange dwarf.
• HD 26367, "babyana" of magnitude 6.56.
• HIP 106924, very low metallic halo star.
• Gliese 793 and Gliese 133, red dwarfs 26 and 46 light years respectively.
• Gliese 842.2 (GJ 9764), red dwarf surrounded by a circum-stellar dust disk.
• G 261-43, binary system whose components are two very different white dwarfs.

Deep Sky Objects

Image of NGC 7354 obtained with Hubble space telescope.

• NGC 188, open cluster, observable at less than 5 degrees from the north celestial pole. It is estimated that its distance is about 5400 light years.
• NGC 6939, AR: 20h 31m 24.0s Dec: +60°38'00" (Epoch 2000). Open volume 2.5 degrees south of θ Cephei.
• NGC 7142, distant open cluster 6200 light years.
• NGC 7510, also an open cluster located about 11 400 light years. He's young, 10 million years old.
• NGC 6946, a spiral galaxy in which ten supernovaes have been observed, more than in any other galaxy. This is why it is also known as "Galaxia of fireworks". Its diameter is approximately 40 000 light years, one third of the size of the Milky Way.
• NGC 7023 or Nebulosa Iris, small diffuse nebula that is located 3.5.o to the southwest of Alfirk (β Cephei).
• NGC 40 and NGC 7354, planetary nebulae.
• CTA 1, rest of distant supernova 4500 light years.
• S5 0014+81, quasar with a large black hole supermassive center, the greatest known.

Mythology

Cepheus, king of Ethiopia.

Cassiopeia is the mother of Andromeda and wife of Cepheus, king of Ethiopia. Cassiopeia was so proud of her beauty that, to compete with the Nereids, they asked Poseidon, god of the seas, for revenge and he in response sent the sea monster (Cetus) to the coasts of the country causing great evils.

To deal with this situation, Cepheus consulted the oracle of Amun, who advised sacrificing his daughter Andromeda, exposing her tied to a rock on the cliff so that she would fall victim to the monster. Thus, Andromeda was offered to Cetus. Perseus, who was coming back from his journey after defeating Medusa, saw the victim and immediately fell in love with her. He proposed to Cepheus to free her from her, in exchange for her hand being granted to him. Perseus kills the monster and later marries Andromeda.

Cepheus, the father of Andromeda, was catasterized by the will of Athena.