Astro Bob: Sea beast of the southern sky
Cetus emerges from the sea of night to prepare the way for Orion. How to find this enormous yet unfamiliar constellation.
If you face south around 8 p.m. local time in late November, Jupiter seems to be the only thing happening. There are no other bright stars or constellations until you tilt your head far enough back to see the Great Square of Pegasus. It's almost as if the gods let the autumn sky lie fallow in preparation for the arrival of the spectacular winter star groups.
But don't let the gods deceive you. A giant sea monster follows close on Jupiter's tail. Name Cetus (SEE-tus) and befitting a beast, it's the fourth largest constellation, spanning more than 40 degrees — equal to four fists measured off at arm's length against the sky. There's only one problem with Cetus, and it's the reason so few of us are acquainted with it. It has only two moderately bright stars: Menkar at magnitude 2.5 and Deneb Kaitos at 2.0.
They're as easy to see as the Big Dipper stars. All the others take more effort, but I promise it'll be worth your while. For your trouble a large section of the southern fall sky will no longer seem so empty.
It's actually helpful that Cetus' two luminaries lie at opposite ends of the constellation. Deneb Kaitos, an Arabic name meaning "the southern tail of Cetus," naturally marks the tail's terminus, located about two fists below and left of Jupiter. Finding it is pretty easy. Just extend a line from Markab, the star in the lower right corner of the Great Square, through Jupiter. The next bright star you'll encounter is Deneb Kaitos.
Menkar, Arabic for "nostril," shines from the bulbous head of the beast. There are several ways to locate it. You can connect the star into a giant triangle with Jupiter and Deneb Kaitos. Or you can step down from the dipper-shaped Pleiades star cluster, past the "eyes" (a close pair of equally bright stars in Taurus), and bend right to Menkar.
Once you've found Menkar, work your way around the five short sides that outline the monster's head to add several fainter stars then amble southwest toward Deneb Kaitos, connecting and collecting the remaining stars in the figure along with way. There are about a dozen in all — not many considering the creature's size! Most are around magnitude 3.5, with a couple as faint as 4.5. To spot those dimmer ones you may want to keep a pair of binoculars close.
The constellation's most famous star is currently too faint to see. Mira , whose name means "wonder star," glows in the monster's throat. It can shine as bright as magnitude 2 or better and temporarily outshine every other star in the group. A few months later, it fades to magnitude 9 or 10 and only shows itself in a telescope.
Mira is a giant, red pulsating variable star that physically expands and contracts, changing in brightness over a period of 11 months. You could easily see it with the naked eye last summer; right now it's magnitude 8.0 and requires binoculars. Once you get to know Cetus, watch for Mira's return. When bright, it acts as a "connector star" joining head to tail.
Sea monsters of antiquity get a bad rap. They're usually called in to ravage things as is the case with Cetus, which is also depicted as a whale. In fact, Cetus is Latin for "whale," while Ketos, the original Greek name for the animal, means "sea creature."
Anyway, it all started when Cassiopeia bragged that she was more beautiful than the sea nymphs called the Nereids. This angered the sea god Poseidon, who sent a terrifying aquatic dragon (Cetus) to ravage the king and queen's lands along the coast of the eastern Mediterranean. Only by sacrificing their daughter Andromeda to the beast could they placate Poseidon and ward off certain disaster. So they chained her to the rocky cliffs overlooking the water and left her to await her fate.
Meanwhile, Perseus happened to be passing by and got wind of the nasty business. He swooped down upon Cetus and repeatedly thrust his sword into the creature until it collapsed dead into the sea. He then rescued Andromeda, fell in love with her, and they married.
I still feel bad for the whale, but at least he holds a prominent place in the sky to this day along with the likes of Perseus, Andromeda, King Cepheus and the self-absorbed Queen Cassiopeia who started it all.