Amazingly, Cetus the sea monster and Cetus the whale are the same constellation. Yet in the ancient Greek myths, Cetus is not a plankton-eating humpback singing to its pod in the depths. It's the terrifying monster that Poseidon sent to devastate the kingdom of ancient Ethiopia.
The classical gods were inclined to be capricious, arrogant and prickly - rather like many humans, but much more powerful. If you know the myths you'll know what lay behind Poseidon's action. Cassiopeia, the queen of Ethiopia, had been bragging that she and her daughter Andromeda were so gorgeous that they were even more beautiful than the nereids, the sea nymphs. The nereids were annoyed by a mere human making such a boast. Poseidon's wife Amphitrite was herself a nereid, so when they complained to the sea god, he took it seriously. Probably you or I would have dealt with it without the overkill, but of course, then there wouldn't have been much of a story without the voracious Cetus.
Cetus is depicted as an amalgam of horrors: it had a monstrous head with great gaping jaws, and a body whose front had two legs like a land animal, but was attached to a scaly hind end like that of a giant sea serpent. I'm pretty sure we wouldn't want to meet such a creature on the beach, but in some of the old star atlases, it really looks rather comical. In this depiction from the 1776 French edition of John Flamsteed'sAtlas Coelestis, Cetus seems to be looking more mildly irritated than really ferocious.
Cepheus, the king of Ethiopia, was in despair at the damage that Cetus was doing, so he consulted an oracle. The oracle said that he could only save his kingdom by sacrificing his daughter Andromeda to the monster. Oracles could be misleading, but this one was spot on. Andromeda was chained to a cliff, awaiting her awful fate, when Perseus happened on the scene. He killed Cetus and married Andromeda. Whew!
The gods put all of the main characters of the story in the sky. That even included Cetus, but the creature did end up in a different part of the sky to the royal family.
Cetus is one of the largest constellations in the sky. In the northern hemisphere it's very low on the southern horizon and you can only see the whole thing from October to January. The stars are also not particularly bright, almost as though Cetus were hiding away in murky depths. Here is the shape of the constellation. Its brightest star is the second-magnitude Beta Ceti, an orange giant only 96 light years from us. Its traditional name is Deneb Kaitos, "deneb" meaning tail.
The best known star is Mira. It was the first star to be documented as a variable, i.e., a star whose brightness changes. David Fabricius first described the star in the late sixteenth century, but it took another forty years to work out its period of variability. In the mid seventeenth century Polish astronomer Johannes Hevelius named it Mira (Latin for 'amazing'). Then it was still the only known variable star. We now know not only that Mira pulsates (expands and contracts), but that it also has a white dwarf companion. A white dwarf is the remnant of a medium-sized star that has collapsed after burning all of its hydrogen fuel.
Tau Ceti is the nearest sunlike star to the Solar System - just under 12 light years away. It's about twenty percent less massive than the Sun and so far no planets have been discovered. But it does have a debris disk, so it's quite possible that there are planets, or at least planets in the making. A debris disk is a disk of material orbiting a star - it's the material available for making planets.
Deep sky objects
About 1600 light years from us lies the planetary nebula NGC 246, discovered by William Herschel in 1785. A planetary nebula is created when a medium-sized star has run out of hydrogen fuel and sloughed off some of its outer layers. In this photo taken by Greg Crinklaw, you can see why its nickname is the Skull Nebula.
There is an unusual face-on spiral galaxy located nearly sixty million light years away. It's a Seyfert Galaxy, a sort of mini-quasar with an active black hole at its center. Using modern telescopes that let us study it in different wavelengths, we can see many of the details, but the galaxy is bright enough that French astronomer Pierre Mechain discovered it in 1780. Charles Messier listed it as M77 in his catalog of nebulae. At that time, you couldn't distinguish between true nebulae (great clouds of gas and dust), clusters of stars, and galaxies, so they were all called nebulae.
Dust obscures the view when we look towards the Milky Way. It's much clearer looking outwards from the center, as we do when we look towards Cetus. There are a large number of galaxies to see here, but the most amazing must be the cluster of galaxies JKCS041, the most distant yet discovered. Astronomers calculate that it's around ten billion light years away. We're detecting it as it was when the universe was a quarter of its current age.
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