Cetus

Cetus or Keto(also known as the Whale or the Sea Monster) is a constellation in the southern hemisphere, in a region known as Water, close to other constellations like Aquarius, Pisces and Eridanus.

Notable features

Constellation of Cetus.

Cetus's brightest star is β Ceti —officially called Diphda and also known as Deneb Kaitos—, a yellow-orange giant of spectral type G9.5III that lies 96 light-years away. It is one of the brightest stars in X-rays in the vicinity of the solar system; such radiation originates from the corona heated to several million K due to the stellar magnetic field.

Next in brightness is Menkar (α Ceti), a red giant of type M1.5IIIa whose diameter is about 80 times larger than the Sun and which is three times further from us than Diphda. Its luminosity in the visible spectrum is 380 times greater than that of the sun, but because it is a cold star, a large amount of its light is emitted as non-visible infrared radiation, in such a way that its radiation in the entire electromagnetic spectrum is 1800 times greater than the one coming from the Sun.

γ Ceti, called Kaffaljidhma, is a binary composed of a white main-sequence star of type A3V and a somewhat cooler companion of type F3V; the separation between both stars is at least 70 au.

The constellation has several orange giants, a fairly common class of stars in the night sky: Deneb Algenubi (η Ceti), 118 light-years away and of spectral type K2IIIb; Baten Kaitos (ζ Ceti), 260 light-years away light years and 25 times larger than the Sun; e υ Ceti, 293 light-years away and whose diameter is twice that of Baten Kaitos.

Image of Mira obtained with Hubble Space Telescope

But the constellation's most notable star is Mira (ο Ceti), the first variable star discovered. Throughout its period of 331.96 days it reaches a maximum magnitude of 2.0 —being at that time the brightest in the constellation— to later fall to magnitude 10.1. Mira is in the last phases of its stellar evolution and its variability comes from pulsations on its surface, changes in the size of the star —which can amount to 15% in each pulsation— that also affect its temperature and luminosity. R Ceti is another such variable in the constellation, varying in brightness between magnitude +7.2 and +14.0 over a period of 166.24 days. In contrast, T Ceti is a semiregular SRC variable whose brightness fluctuates between magnitude +5.0 and +6.9. Of spectral type M5-6Se, its effective temperature is only 2400 K.

A white dwarf in this constellation, ZZ Ceti, is the prototype of a class of variables named after it, ZZ Ceti variables. They are characterized because the variability is due to non-radial pulsations, with typical pulsation periods between 100 and 1200 s. The variability of ZZ Ceti was discovered in 1970 by B.M. Lasker and J.E. Hesser.

There are several stars in Cetus that have exoplanets. Two planets with orbital periods of 403.5 and 751.9 days have been detected in the aforementioned Deneb Algenubi. τ Ceti, the second closest yellow dwarf to the Sun, has a planetary system with up to nine possible planets. At around 75 Ceti, a yellow giant of type G3III, it orbits a planet at a distance of 2.1 AU, and there is some evidence that there might be a second planet in the system. Likewise, 94 Ceti is a binary whose components are a yellow dwarf and a red dwarf; around the first moves a planet more massive than Jupiter in a markedly eccentric orbit. Finally, Axolotl, the official name of HD 224693, is a subgiant with a giant planet orbiting at 0.19 AU.

In addition to τ Ceti, other solar analogues are located in this constellation such as 9 Ceti, a star of type G2.5V at 66 light years, and κ1 Ceti, a young yellow dwarf of type G5V distant 30 light years; both are chromospherically active BY Draconis variables.

In this constellation there are also interesting stars close to the solar system. The nearby Luyten 726-8 star system, 8.73 light-years away, is made up of two red dwarfs of spectral type M5.5V and M6V. Both are flare stars: one of them, called UV Ceti, is one of the most famous flare stars, this class of eruptive variables being known as UV Ceti variables. In 1952 UV Ceti's brightness increased 75 times in a matter of 20 seconds. Somewhat further away —at 12.1 light years— is also the eruptive variable YZ Ceti; in 2017 the discovery of three extrasolar planets in orbit around this red dwarf was announced. Another nearby system, Gliese 105, consists of a K3V-type orange dwarf, an M3-type red dwarf, and a second M7V-type red dwarf whose diameter should not be much larger than that of Jupiter. This system is 23 light years distant from Earth.

Image of NGC 246 obtained with the Spitzer space telescope.

Image of the Wolf-Lundmark-Melotte galaxy (WLM) obtained with the VLT.

Another object of interest is the pulsar PSR J0108-1431, the second closest to Earth. With an estimated age of 166 million years and a rotation period of 0.8 seconds, it is one of the oldest known pulsars.

NGC 246 is a planetary nebula whose central star, probably a remnant of a Wolf-Rayet star, has a temperature of 200,000 K, making it one of the hottest known stars.

Numerous galaxies can be observed in Cetus, among which M77 stands out, a barred spiral galaxy about 47 million light-years from Earth. Classified as a type II Seyfert galaxy, it is the closest and brightest representative of this class of active galaxies. NGC 1055 is another barred spiral galaxy that, like M77, is part of the M77 Group. On the other hand, NGC 1087 is an intermediate spiral galaxy located 80 million light years away. Much more distant, at 250 million light-years, NGC 17 is a galaxy that appears to have formed as a result of the merger of two other galaxies.

Of a different nature is Wolf-Lundmark-Melotte (WLM), an irregular galaxy located in the confines of the Local Group and which is isolated from other galaxies, since its nearest neighbor, IC 1613, is one million of light years. Quite elongated in shape, its extension is of the order of about 8,000 light-years, including a halo of very old stars discovered in 1996.

The ecliptic passes very close to the boundary of Cetus, so planets and asteroids can be found in this constellation for brief periods. The asteroid 4 Vesta was discovered in Cetus in 1807.

Main Stars

X-ray image of Diphda obtained with the Chandra observatory

Image of Mira in ultraviolet light, where you can see the trace left by the star.

• α Ceti (Menkar), red giant of magnitude 2.54 that forms a double optic with star 93 Ceti.
• β Ceti (Deneb Kaitos or Diphda), the brightest star of constellation with magnitude 2,04, is a yellow-orange giant.
• γ Ceti (Kaffaljidhma), yellow and blue binary star through the telescope; both components are separated about 3 seconds of arc.
• δ Ceti, white-azulada subgigante of magnitude 4,08 and Beta Cephei variable that is located in the celestial equator.
• ج Ceti (Baten Kaitos), an orange giant of magnitude 3.9.
• Cristian Ceti (Deneb Algenubi), an orange giant of magnitude 3.45, located 118 light years from the Solar System.
• κ Ceti, the name of Bayer shared by two different stars: κ1 Ceti, yellow dwarf at 30 light years that emits large flames, and κ2 Ceti, a yellow giant.
• λ Ceti, white-azulada star of magnitude 4,71.
• μ Ceti, white-yellow subgigant of magnitude 4,27, Delta Scuti type variable.
• roga Ceti, gives name to two different stars: φ1 Ceti, a yellow supergigante and spectroscopic binary, and ¢2 Ceti, a white-azulada star.
• ο Ceti (Look, in Latin "marvelous"), was the first variable star discovered in 1596. Prototype of long-term variables (332-day cycle varying in magnitude 2,2 to 10) that carry their name (variables Mira).
• σ Ceti, white-yellow star of magnitude 4.75.
• Δ Ceti, yellow dwarf of magnitude 3, less than 12 light years from Earth and where he began to seek extraterrestrial life because it is a very similar star to the Sun.
• ♫ Ceti, orange giant of magnitude 4,01.
• φ Ceti is a Bayer denomination shared by four different stars; φ1 Ceti is an orange giant and φ2 Ceti is a white-yellow star distant 50 light years.
• 3 Ceti, supergigante distant orange more than 2000 light years.
• 6 Ceti, star of magnitude 4,89 that can be surrounded by a belt similar to the Kuiper Belt.
• 9 Ceti (BE Ceti), yellow dwarf very young and variable BY Draconis.
• 13 Ceti (BU Ceti), multiple star and variable RS Canum Venaticorum.
• 18 Ceti, yellow dwarf of magnitude 6,15 with a possible star companion.
• 60 Ceti, white giant of magnitude 5.43.
• 75 Ceti, yellow giant with a planet.
• 79 Ceti and 81 Ceti, respectively a subgiant and a G-type giant; each of them has discovered an extrasolar planet.
• 94 Ceti, binary system composed of a yellow dwarf and a red dwarf; around the first a planet has been discovered.
• Luyten 726-8, the sixth star closest to Earth forms a binary system of two red dwarfs. Luyten 726-8 B, also called UV Ceti, is a typical example of sparkling star.
• R Ceti, variable Mira whose brightness varies between magnitude 7.2 and 14.0 in a cycle of 166.24 days.
• T Ceti, variable semi-regular variable between magnitude 5,0 and 6,9.
• YZ Ceti, also a 12-year-old light star of the Solar System.
• ZZ Ceti, pulsating white dwarf, prototype of a group of variables bearing its name.
• AB Ceti (HR 710), binary system composed of an Ap star and a white dwarf.
• BG Ceti (HD 3326), Delta Scuti variable of magnitude 6.06.
• CD Ceti, dark red dwarf distant 28 light years.
• DK Ceti (HD 12039), variable BY Draconis with a circumestelar disk.
• EX Ceti (HD 10008), also variable BY Draconis; is part of the Hércules-Lyra Star Association.
• HD 1461, yellow dwarf with a confirmed extrasolar planet.
• HD 15115, star where a markedly asymmetrical ring disk has been discovered.
• HD 15477, is a star of apparent magnitude +8,75.
• Gliese 84 and Gliese 87, dim red dwarfs 29.8 and 34.0 light years respectively.
• Gliese 105 (HR 753), triple star system to 23.5 light years.
• GJ 2012, white dwarf close to 29.5 light years.
• BD-17 63, distant orange dwarf housing a planet.
• HIP 5158, orange dwarf also with a planet.
• WD 0053-090, hot white dwarf whose surface temperature exceeds 80,000 K.
• PSR J0108-1431, one of the oldest pulsars as well as one of the closest to the Solar System.

Deep Sky Objects

M77, lead member of the M77 Group. It is 47 million light years away.

Sculptor Group spiral galaxy NGC 247.

• M77 (NGC 1068), one of the largest galaxies in the Messier catalogue, a dominant member of a group of galaxies bearing its name (M77 Group). Its bright central area spans about 120 000 light years, but its faintest extensions reach about 170 000 light years. AR: 02h 42m 42.0s Dec: -00°01'00" (Epoca 2000).
• NGC 17, galaxy resulting from the merger of two other galaxies, recent star formation has been observed in their central regions.
• NGC 45, spiral galaxy of very low surface brightness.
• NGC 217, spiral galaxy, NGC 1042, coiled spiral galaxy, and NGC 1052, elliptical galaxy.
• NGC 247, a spiral galaxy member of the Sculptor Group, one of the galactic groups closest to the Milky Way. It's 9 million light years away. AR: 00h 47m 06.0s Dec: -20°46'00" (Epoca 2000).
• NGC 615, also a spiral galaxy. AR: 01h 35m 06.0s Dec: -07°20'00" (Epoca 2000).
• NGC 908, spiral galaxy barred to 60 million light years. It is the main galaxy of a group of galaxies bearing its name.
• NGC 1055, SBb-type spiral galaxy that is about 52 million light years away. It forms a binary system with M77.
• A degree to the east-south of M77 can be seen the spiral galaxies NGC 1087 and NGC 1090. These two galaxies are not part of the same galactic grouping.
• IC 1613 Galaxy, a dwarf galaxy belonging to the Local Group. Along with the Magellan Clouds, it is the only irregular dwarf galaxy in the Local Group where RR Lyrae variable stars have been found.
• Wolf-Lundmark-Melotte Galaxy, of magnitude 11,0 is an irregular galaxy within the limits of the Local Group.
• NGC 246, small diameter planetary nebula.
• Abell 370, cluster of galaxies at 5 billion light years.

Mythology

Cetus.

Cassiopeia was 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.

However, there is ambiguity as to whether the name of the sea monster was really cetus as a generic name for a sea monster or whether it refers to Ceto in particular as a sea deity; the last option seems to be the most accepted.

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

Cerberus, in ancient mythology, was a dog with three heads and a snake for a tail whose job was to guard the gates of Hades, so that no human could enter without permission and at the same time, so that no ghost could get out of the hells. On the banks of the River Styx, the border between the living and the dead, and with the only company of the boatman Charon, the ever-alert Dog Cerberus, was the nightmare for all those brave heroes who dared to cross those gates without the permission of Hades.. Its origin is related to the constellation of Cetus, in which you can sense the shapes of the gates of the underworld closed and a three-headed beast in the center guarding them.

Text references

General references

• _ Catasterism (Καταστερισμο).
  • 36: Ballena (ωος; Cetus): the sea monster Poseidon sent to devour Andromeda.
    • Latin text in Google Books; electronic facsimile.
      • Greek text on the Internet Archive; electronic facsimile.

• HIGINO: Poetry astronomy (Astronomica).
  • 31: Ballena.
    • English text on Mary Grant's Theoi site; 1960 trad.