This map shows the northern sky around 8:30 p.m. the first week of fall. Cassiopeia is the bright W of stars well-placed halfway up in the northeastern sky. — created with Stellarium
The moon’s at last or third quarter phase, having completed 3/4 of its monthly orbit around the Earth. It has the same "half a pie" shape like the first quarter moon but reversed, with the left half lit. The moon still provides enough illumination for distinguishing forms in the night landscape but clearly its light is slipping away. Dark skies are again at hand.
With the stars back in full force and Cassiopeia the Queen in her glory, this coming week will be a good one to find the Perseus Double Cluster. Even though it’s in the constellation of Perseus the Hero, the Double Cluster is found more easily using Cassiopeia’s stars. Just drop one outstretched fist below the left side of the W and look for a small, puffy cloud. If you can see the Milky Way in Cassiopeia, the Double Cluster will look like a brighter condensation within it. It’s not hard to spot from suburban neighborhoods. The ancient Babylonians and Greeks knew this little cloud too. The Greek astronomer Hipparchus included it in his catalog of the sky in 130 B.C.
The Double Cluster is directly below Cassiopeia and one of the sky’s best binocular objects. You can also use the W to point you to the Andromeda Galaxy, described in an earlier blog. Photo: Bob King / Duluth News Tribune
To the naked eye, there’s not much more to look at than an unresolved glow about the size of two full moons, but binoculars reveal the cloud as a splendid pairing of two individual star clusters. With my favorite pair of 10x50s, I can resolve lots of stars in each cluster and enjoy their different characters. The cluster on the left is called NGC 884, its companion to the right is NGC 869. 869 is richer in stars and more compact than its neighbor. Both clusters are some 300 light years apart and approximately 7000 light years from Earth.
This photograph of the Double Cluster gives you a good idea how it appears in an amateur telescope. NGC 884 is at left , and NGC 869, right. Notice the red stars, especially around 884. Photo: N.A. Sharp, NOAO, AURA, NSF
We’ve seen that some pairings in the sky are chance alignments but that’s not so with Double Cluster. Both of them are anchored together in an enormous cloud of gas and young stars called the Perseus OB1 association, located in an outer spiral arm of our galaxy. If you sweep the sky around the Double Cluster with binoculars, I guarantee you’ll bump into even more clusters.
I’ll never forget my first look at the Double Cluster as a boy from my suburban Chicago neighborhood. Even there, I was knocked over by the hundreds of stars I could see in my low power telescope. Several ruby-red stars sprinkled about added an additional dimension of beauty to the scene. A little horseshoe of stars at the center of 869 is one of my favorite asterisms in the sky.
All star clusters are gravitationally-bound collections of stars born from clouds of gas of dust called nebulas. Astronomically speaking, both of our featured clusters were born recently: six million years ago for NGC 869 and 14 million for NGC 884. For comparison, consider that the sun is five billion years old. The Double Cluster is filled with freshly-minted supergiant and giant stars that shine with a fierce brilliance. They’re a highlight of the new season now at hand.