Cetus, the “Sea Monster”, is the fourth biggest constellation taking up 3% of the night sky, and is best observed in winter by observers located between latitudes of +70° and -90°. The constellation contains only one Messier object, a large galaxy called M77, but it is home to several notable stellar objects, amongst which is the red giant, Omicron Ceti, which defines an entire class of variable stars, as well as 14 stars with confirmed planets.
Menkar, the star depicting the head of Cetus is found near the constellation of Taurus, with its long body then stretching all the way across a part of the sky referred to as “The Sea”. There are found a group of other constellations evoking watery images, including Eridanus (a river), Aquarius (the water bearer), and Pisces (fish), Piscis Austrinus (southern fish), Capricornus (sea-Goat), and Aquarius (water-carrier).
Cetus (“the Whale”) most commonly represents a sea-monster that was sent to devour Andromeda, after her mother Queen Cassiopeia foolishly claimed that her daughter was more beautiful than the Nereids. The story goes that Poseidon, the sea-god, was so incensed by Cassiopeia that he sent a sea monster to destroy the lands belonging to Andromeda’s father, King Cepheus. To ward off disaster, the Oracle of Apollo proposed that the King and Queen sacrifice their daughter to the sea monster, but before the monster could devour the princess, the young hero Perseus happened by and rescued Andromeda after killing the monster.
– Deneb Kaitos (Beta Ceti, Diphda, ), located about 94 light years away, is the most luminous star in Cetus with an apparent visual magnitude of 2.04. It is an orange giant star (K0 III), meaning that it has evolved off the main sequence, and is rapidly turning into a red giant. One of its traditional names, Deneb Kaitos, derives from the Arabic for “the southern tail of Cetus”. Its other traditional name, Diphda, comes from the Arabic for “the second frog”, to distinguish it from the star Fomalhaut in the constellation of Piscis Austrinus, which is sometimes referred to as “the first frog.”
– Menkar (Alpha Ceti, Menkab), the constellation’s second brightest star, is a red giant (M1.5IIIa) located 220 lights year from Earth of magnitude 2.54. Menkar represents the ‘Nose of the Whale’, and has twice the Sun’s mass, but 89 times its radius. Viewed through a telescope, it appears to be a double star with a faint blue-white companion, 93 Ceti, although the two are not physically related.
– Mira (Omicron Ceti) is a 6 billion year old red giant (M7 IIIe) located 420 light years from our solar system whose visual magnitude varies anywhere from 2.0 to 10.1. Its name in Latin means ‘the amazing one’, in reference to the stars extreme variability in brightness. Mira is actually a binary system whose main component, Mira A, is an oscillating variable star that defines an entire class of stars, known as Mira variables. There are between 6,000, and 7,000 stars discovered in this class, all of which are red giants with surfaces that oscillate in a way that produces variations in their luminosity that can stretch from about 80 days to periods of more than 1,000 days. Mira B, on the other hand, is a hot white dwarf that is busy pulling material off Mira A, thus forming the closest symbiotic binary system to the Sun. It is interesting to note that Mira A is shedding material as it moves through space, and the Galaxy Evolution Explorer space telescope has revealed a 13-light year-long trail of stellar material behind the star.
– Tau Ceti is a rather unremarkable yellow dwarf star (G8.5 V) located only 11.9 light years from the Sun with an apparent magnitude of 3.5. It has about 80% the Sun’s mass, and only 55% of the its brightness, but what distinguishes Tau Ceti from other perhaps more deserving star candidates to mention is that along with Epsilon Eridani in the constellation Eridanus, it was chosen for Project Ozma, the ground-breaking SETI experiment that took place in 1960. Frank Drake of Cornell University ran Project Ozma in an attempt to discover artificial, (i.e. intelligent) radio signals, but despite his best attempts, he did not find anything that even remotely sounded like an alien communication.
Other stars of interest in Cetus includes the orange giants Dheneb Algenubi, Deneb Kaitos Shemali, Theya Ceti, Baten Kaitos, Upsilon Ceti; and the blue-white subgiant Delta Ceti; and the blue-white giant stars Xi-2 Ceti and Lambda Ceti.
Notable Deep Sky Objects
Despite being the 4th largest constellation, Cetus is situated far from the galactic plane and so contains few deep sky objects. Those that is does, however, make good targets for astronomers as their view is unobscured by dust from our Milky Way.
– Messier 77 (M77, NGC 1068) is a large barred spiral galaxy that is around 170,000 light years wide and about 47 million light years distant. Although it has an apparent visual magnitude of just 9.6, its bright center makes it an easy target for binoculars of 15x70s and above, as well as telescopes with an aperture of at least 3.1-inch. M77 was discovered in 1780 by Charles Messier’s associate, Pierre Méchain, who described it as a nebula, while Messier and William Herschel both thought the object to be a large cluster of stars. M77 is a known radio source due to its active nucleus, which is obscured by dense intergalactic dust. Look for M77 at 0.7 degrees east-south-eastward of Delta Ceti.
– NGC 1055, another radio source, is only 0.5 degrees removed from M77. It is oriented nearly edge-on to our line of sight, and with a diameter of about 115,800 light years, it is the largest member of a cluster of galaxies that includes N1073 and some smaller, irregular galaxies.
– NGC 1087 is an intermediate spiral galaxy that is 86,800 light-years across and found about 80 million light years from Earth. However, it is difficult to discern any spiral structure in visible light, and the only detail that is clearly defined is a small central bar, and the many irregular features that make up the body of the galaxy, such as the extremely small nucleus.
Other objects of interest in Cetus includes the spiral galaxies NGC 17 and NGC 1042; the intermediate spiral galaxy NGC 247; and the barred spiral galaxies NGC 45 and NGC 47.
Three meteor showers are associated with Cetus, namely the Omicron Cetids, the Eta Cetids, and the October Cetids, none of which is particularly productive.
– The Omicron Cetids, one of only a few daylight showers, runs from May 7th to June 9th, with a peak that can occur at any time from May 14th to 25th. Daylight makes the meteors in this stream all but invisible to the naked eye, but suitable radio-echo equipment could potentially reveal up to 18 meteors per hour during the peak period.
– The Eta Cetids has a long duration period from September 20th to November 2nd, which usually peaks during the first week of October. However, its activity for the most part is almost non-existent, although large fireballs have been known to occur unexpectedly at any time between the stated dates.
– The October Cetids is a weak shower whose highest count happened in 1916 when just 4 meteors per hour were observed during its most active period. The peak date is not certain, and it is recommended that an optical aid be used to spot meteors between October 20th and 26th.