Open Clusters are beautiful, varied and relatively simple to observe and image. As the Summer constellations move westwards, Cassiopeia and Perseus start to take over in the evenings. There are a number of good Open Clusters well-placed at this time of year so have a look at some of them.
At first sight it might seem that Open Clusters are just a bunch of stars, loosely associated with each other and that’s it. Well that’s sort of true, but they are much more beautiful than that description might suggest.
You can see many with binoculars and many more with small telescopes. They differ in colour, structure, size and setting. Because most Open Clusters, also know as Galactic Clusters, exist in the plane of the Milky Way, these clusters can have a backdrop of many thousands of fainter stars, making viewing them a very rich experience. (Globular Clusters differ in that they inhabit a halo surrounding our galaxy.)
Here are a few to get you started, but there are many more that you can find using your favourite planetarium program.
What are Open Clusters?
An Open Cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre.
Constellation: Cassiopeia Mag: 7.4, Size: 6′ One of the most distant Open Clusters (8,000 – 9,500 light years). About 40 member stars
Constellation: Cassiopeia Mag: 5, Size: 13′ Almost 200 members, distance uncertain. M52 has the added bonus of the Bubble Nebula (NGC 7635) nearby (upper right corner of image).
Constellation: Perseus Mag: 5.5, Size: 35′ (just larger than the size of the Moon) Over 100 members, distance 1,500 light years.
NGC 457, Owl Cluster
Constellation: Cassiopeia Mag: 6.4, Size: 13′ Distance 7,900 light years. Discovered by William Herschel. Called the Owl because of the two bright stars that appear like eyes.
NGC 7789, White Rose Cluster
Constellation: Cassiopeia Mag: 6.7, Size: 16′ Distance 7,900 light years. Discovered by Caroline Herschel, younger sister of William, who was a talented astronomer in her own right. A beautiful, rich cluster, called the White Rose (or sometimes “Caroline’s Rose”) because the loops of stars and dark lanes look like the pattern of rose petals from above.
NGC 869 & 884, Double Cluster
Constellation: Perseus Mag: 3.7 & 3.8, Size: 60′ Distance 7,500 light years. Relatively young at 12.8 million years old. Both clusters are visible to the naked eye under dark skies and the radiant of the annual Perseid meteor shower is approximately located at these clusters. A beautiful sight with a wide field telescope or binoculars.
Of course there are all the Open Clusters of Auriga too – but that’s maybe a topic for another post as Auriga rises higher in the evenings.