For northern observers, November and December are the best months to explore the lovely constellation Cassiopeia. High overhead this time of year, this W-shaped constellation is situated in the plane of the Milky Way, so it’s full of bright stars and enough open star clusters to fill many nights of pleasant observation, either with binoculars or a small telescope.
In Greek mythology, Cassiopeia was the wife of King Cepheus of Ethiopia. She was beautiful but vain, and boasted she and her daughter Andromeda were more beautiful than all the Nereids, the nymph-daughters of the sea god Nereus. Poseidon, the main sea god, did not take kindly to this boast and threatened to flood the kingdom of Ethiopia.
An oracle advised Cepheus and Cassiopeia to appease Poseidon by sacrificing their daughter Andromeda. The beautiful princess was chained to a rock at the edge of the sea, and left to be mangled and eaten by the dreaded sea monster Cetus. But the hero Perseus, flying back on the winged horse Pegasus, and after slaying the Gorgon Medusa, arrived in time to save Andromeda and turn Cetus to stone with Medusa’s severed head. Soon after, Perseus and Andromeda married. But Poseidon still punished Cassiopeia by casting her and Cepheus into the heavens, where they circle the celestial pole, never rising or setting (at least not from the latitude of Mt. Olympus).
Let’s have a look at three of the finest open clusters in Cassiopeia, all of which are worthy of a little examination through the chilly fall air.
M52. Messier 52 is easy to find… just continue a line from beta Cass (Shedir) to alpha Cass (Caph) a little more than the same distance. It’s visible in binoculars as a hazy patch of magnitude 7. A 3-4 inch telescope will reveal a few dozen mostly blue-white stars, and a couple of yellow giants which evolved off the main sequence. The cluster is fairly tightly packed and hard to resolve. It looks small because it’s far away… about 5,000 light years. In the 1800’s John Herschel saw the cluster as round, while the redoubtable amateur Admiral Smyth saw it as triangular or fan-shaped. What do you see?
M103. This is the last object in Messier’s original catalog (it was later padded to include 6 more objects). It’s also easy to find, about 1 degree northeast of Ruchbah (or delta Cass). At 8,500 light years away, it’s one of the most distant open clusters in Messier’s catalog. In a small scope, the cluster is unmistakably triangular and displays perhaps two dozen stars. The star triple star Struve 131 is the bright star at the north vertex of the cluster; you can easily resolve all three stars in Struve 131 in a scope, even at low power.
NGC 7789. A personal favorite. This open cluster is old (> 1 billion years), far (>8,000 light years), and faint (most stars are magnitude 11 or 12). Because it’s so old, many blue-white stars have turned into red giants, so the cluster is quite colorful in photographs. It was discovered by Caroline Herschel in 1783. You can’t resolve this cluster in binoculars, but a 3-inch scope resolves a spray of tiny pinpoints of light on a hazy background. This open cluster, unlike many, looks better with higher magnification. Find NGC 7789 at one vertex of the right-angled triangle it forms with Shedir and Caph.
Star clusters M52, M103, NGC 7789, and NGC 457 in Cassiopeia (click to enlarge)
A bonus object: Another favorite cluster in Cassiopeia is the lovely NGC 457, named after a famous character in a Steven Spielberg classic, and described on p. 171 of our own guide Secrets of the Deep Sky. You can read more about this guide here.
That’s it for today. But we’ll look at a few more sights in Cassiopeia in an upcoming issue.