In this article, we’ll be sharing with you everything you need to know about the constellation of Cetus, including how to find it, deep space objects contained within it and how to use it to find other night sky objects more easily.

The Constellation of Cetus 

Cetus represents a whale and is one of the constellations Greek astronomer Ptolemy cataloged over 2,000 years ago.

In Greek mythology, Cetus was the monster Poseidon sent to destroy Egypt after Queen Cassiopeia boasted that she and her daughter Andromeda were more beautiful than any of Poseidon’s nymphs.

Andromeda was chained to a rock near the shore in hopes of appeasing the gods and saving the kingdom. Perseus used Medusa’s head to kill Cetus and save Andromeda. Ancient Greeks depicted Cetus as a hybrid creature with forelegs, huge jaws and a scaly body.

To help you spot Cetus, here’s what SkySafari 6 shows.

Cetus as shown by SkySafari
Cetus as shown by SkySafari. Click for full-screen.

Cetus has an area of 1,231 square degrees making it the 4th largest of the 88 recognized constellations. It resembles a rough circle of stars (its head) connected to an irregular hexagon (its body) by a long string of stars (its neck). 

The boundaries and neighboring constellations for Cetus
The boundaries and neighboring constellations for Cetus. Click for full-screen.

In the next section discover how to find Cetus.

How To Find Cetus In The Night Sky

Cetus is part of the Perseus family of constellations and is visible to observers at latitudes between +70° and -90°. Northern Hemisphere and Southern Hemisphere observers can see it in evening skies from October through February.

The constellation of Cetus is bordered by the constellations Aquarius, Aries, Eridanus, Fornax, Pisces, Sculptor and Taurus. Cetus is located in the ‘water oriented’ part of the night sky and is one of the constellations of ‘The Celestial Sea’.

Cetus at 9:00 p.m. on the 3rd of December
Cetus at 9:00 p.m. on the 3rd of December. Click for full-screen.

To find Cetus, do a naked-eye search for The Great Square of Pegasus. Draw an imaginary line from Markab to Algenib (~16°) then extend this line ~36° to the rough circle of stars that represents Cetus’s head.

Alternatively, connect the left ‘foot’ of Orion (Saiph) to the bright star Rigel (the right ‘foot’) and extend this onwards for about 36° towards the head of Cetus.

You can measure these degree distances using your hand, see here (opens a new tab).

To find Cetus’s exact position for your location on any night, use software such as Stellarium (free) or SkySafari.

Cetus’s Brightest Stars 

Since it is such a large constellation, it shouldn’t come as a surprise that Cetus contains several stars brighter than magnitude five that can comfortably be seen under a moderately dark sky.

Each of them is shown in the star chart below and examined in more detail underneath that.

The brightest stars of Cetus
The brightest stars of Cetus. Click for full-screen.

Alpha Ceti (Menkar) – Despite being the ‘alpha’ star, Menkar is actually the second brightest in the constellation, shining at magnitude 2.54. It is an orange-red giant located about 220 light-years away from us. ‘Menkar’ is derived from the Arabic word for ‘nostril’. 

Beta Ceti (Diphda, Deneb Kaitos) – It is this ‘beta’ star that shines at magnitude 2.04 and is the brightest in Cetus. Also an orange giant, Diphda is half the distance to Menkar at 96 light-years away. ‘Deneb Kaitos’ is Arabic for ‘the whale’s tail’ and ‘Diphda’ is derived from the Arabic for ‘first frog’. 

Gamma Ceti (Kaffaljidhma) – This triple star system is the fourth brightest star in Cetus and marks the point where the whale’s head joins its neck.

The magnitude 3.47 white main-sequence primary is accompanied by a magnitude 6.18 main-sequence secondary that is 2.0 arcseconds away. The third component is a magnitude 10.1 orange dwarf.

Kaffaljidhma is 82 light-years away and its name is derived from the Arabic phrase for ‘part of a hand’.

Delta Ceti – This magnitude 4.07 blue subgiant is 650 light-years away. It’s a pulsating variable ranging in magnitude from 4.05 to 4.10 over 0.16 days. 

Epsilon Ceti – This double star is 70 light-years away. The magnitude 4.82 yellow-white main-sequence primary is just 0.1 arcseconds from the magnitude 6.50 secondary component. 

Zeta Ceti – This variable double star is 235 light-years away. The magnitude 3.73 orange giant primary and magnitude 10.14 secondary components are 189 arcseconds apart. This is an eruptive variable ranging in magnitude from 3.68 to 3.74.

Eta Ceti (Deneb Algenubi) This double star is the third brightest star in Cetus and is 124 light-years away from Earth. The magnitude 3.46 orange giant primary and magnitude 10.52 secondary components are 260 arcseconds apart. Astronomers think that this may be a multiple system. 

Theta Ceti – This double star is 114 light-years away. The two components are a magnitude 3.60 orange giant primary and a magnitude 14.80 secondary that are 78.9 arcseconds apart. This may also be a multiple system. 

Iota Ceti – This variable double star is 275 light-years away. The magnitude 3.54 orange giant primary and its magnitude 12.87 secondary component are 68.4 arcseconds apart.

Kappa1 Ceti – This variable double star is one of the closest stars to us at less than 30 light-years away. It’s composed of a yellow main-sequence primary shining at magnitude 4.84 and a secondary component that are 263.6 arcseconds apart.

Lambda Ceti – This magnitude 4.71 blue-white giant is 576 light-years away. Its mass is 6.4 Solar masses, its diameter is 5.1 Solar diameters and it’s 2.4 times hotter than the Sun. 

Mu Ceti – This variable double star is 84 light-years away. The magnitude 4.26 yellow-white subgiant primary and magnitude 13.60 secondary components are 158.4 arcseconds apart.

Nu Ceti – This double star is 340 light-years away. The magnitude 4.88 yellow-orange giant primary and magnitude 9.08 secondary components are 8.4 arcseconds apart.

Xi1 Ceti – This variable double star is 383 light-years away. The magnitude 4.36 yellow-orange giant primary and secondary components are too close to each other to be separated in a backyard telescope.

Xi2 Ceti – This magnitude 4.28 blue-white giant is 193 light-years away. Its mass is 4.6 Solar masses, its diameter is 2.6 Solar diameters and it’s 1.9 times hotter than the Sun. 

Omicron Ceti (Mira) – This variable double star is the fifth brightest star in Cetus and is found in Cetus’s neck.

A magnitude 4.94 orange-red giant is the primary star and it’s joined by a magnitude 14.07 secondary component lying 71.5 arcseconds away. This pulsating variable ranges in magnitude from 2.00 to 10.10 with a period of 332 days.

Mira is the most famous and the brightest long-period pulsating variable and the ‘Mira’ class of stars is named for it.

Mira is 420 light-years away and its name is derived from the Latin word for ‘wonderful’ or ‘amazing’. 

Pi Ceti – This magnitude 4.25 blue-white main-sequence star is 393 light-years away from our solar system. Its mass is 4.5 Solar masses, its diameter is 5.0 Solar diameters and it’s 2.2 times hotter than the Sun. 

Rho Ceti – This magnitude 4.88 blue-white main-sequence star is 456 light-years away. Its mass is 3.3 Solar masses, its diameter is 5.0 Solar diameters and it’s 1.8 times hotter than the Sun. 

Sigma Ceti – This double star is 87 light-years away. The magnitude 4.73 yellow-white subgiant primary and magnitude 8.77 secondary components are 346 arcseconds apart. This may be a multiple system.

Tau Ceti – This double star is even closer than Kappa1 Ceti at just 11.9 light-years away. The magnitude 3.49 yellow main-sequence primary and magnitude 13.09 secondary components are 90 arcseconds apart. 

Upsilon Ceti – This magnitude 4.00 orange-red giant is 293 light-years away. Its mass is 1.2 Solar masses, its diameter is 55.1 Solar diameters and it’s 36% cooler than the Sun. 

Phi1 Ceti – This magnitude 4.76 orange giant is 234 light-years away.

Chi Ceti – This double star is 77 light-years away. The magnitude 4.65 yellow-white giant primary and its magnitude 6.81 yellow giant secondary component are 193 arcseconds apart. 

Star Hopping From Cetus 

Cetus is the starting point for one well-known star hop.

M77 – Identify Cetus’s head then Kaffaljidhma. Hop ~3° south to Delta Ceti, then 0.75° east to M77, which is a magnitude nine spiral galaxy near the region where Cetus’s ‘neck’ meets its ‘head’.

Objects To See Within Cetus 

Cetus contains one Messier object and one other deep sky object suitable for small telescope users. 

M77 (NGC 1068, Cetus A) – This spiral galaxy, known as the Squid Galaxy, has a magnitude of 8.96 and an apparent size of 6.1 x 5.6 arcminutes. It’s 60 million light-years away and is at right ascension 02h 43m 47s and declination 00° 04’ 42”.

Read our guide on observing M77 the Squid Galaxy.

NGC 247 – This spiral galaxy has a magnitude of 9.10 and an apparent size of 19.7 x 5.5 arcminutes. It’s 12.2 million light-years away and is at right ascension 00h 48m 13s and declination -20° 38’ 35”.

Summary

While Cetus is the 4th largest constellation, it’s fairly dim. There are two magnitude two stars, and all of the rest are fainter than that.

It is notable for its size and shape and makes a home for a couple of deep sky objects that work well for small telescope users.

Look for it east of The Great Square of Pegasus this fall.

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