Star*Points for June, 2014 Open or Galactic Star Clusters Stars are regularly found in star clusters. This may not seem to be the case to the casual stargazer because so few clusters are visible without some kind of optical aid. There are two broad categories of star clusters ? open and globular. This month we?re discussing the open clusters, sometimes referred to as galactic clusters. This is the most common type of star cluster. To say that stars are found in open clusters doesn?t mean that stars form clusters. That is, mature stars don?t meet-up, get together and form star gangs. Rather stars originate in clusters where they are formed. In other words, an open cluster will typically consist of younger stars. With time gravitational tugs from other clusters and molecular clouds of gas and dust passing by causes cluster members to eventually disperse. Open clusters have been included in many catalogs prepared by observers. The names of these catalogs include, among others, Berkeley, Collinder, Dolidze, Harvard, King, Melotte, Messier, Ruprecht, Stock, and Trumpler. The most famous open clusters visible to the unaided eye are the Pleiades (Messier 45) and Hyades, two star clusters in the winter sky located in the constellation Taurus the bull. The best known clusters that are visible to the unaided eye are the Pleiades (Messier 45) and Hyades, two star clusters in the winter sky belonging to the constellation Taurus the bull. Closely related to the open clusters is the asterism. Asterisms are recognizable stellar groupings whose members may or may not be physically associated with each another. In other words, its stars may be present in their unique arrangement simply due to their chance alignment, with some stars being closer to us, while others are farther away. Perhaps the best known asterism are seven stars in the constellation Ursa Major (greater bear) commonly referred to as the Big Dipper. The Big Dipper?s stars are not a chance alignment. They are believed to have a shared origin. That means that the Big Dipper is also what astronomers refer to as a ?stellar association.? Furthermore, most of the Big Dipper?s stars, as well as stars in several nearby constellations, happen to be traveling in roughly the same direction making them part of a stellar ?moving group.? The Big Dipper is also known to astronomers by its catalog designation Collinder 285. So, if you?ve ever seen it, you should log it in your astronomical journal as you track and keep count of objects you?ve viewed. Open star clusters are asymmetrical in shape and each, like a snow flake, is different. Astronomers classify open clusters in several ways. First is by their size. Not their true size, but by their angular size as they appear in the sky. They are also classified by brightness ? the combined brightness of the stars in the cluster or the brightness range of its member stars. Open clusters are also classified by their concentration, from loose and irregular to rich and compressed. There are many examples of open clusters. In the remaining space we?ll discuss two clusters visible this time of year. Both are relatively unknown, yet fairly easy to locate and view in binoculars. The first cluster is near Leo the lion in a moderately unfamiliar constellation whose official name is Coma Berenices, or Berenice?s hair. Leo?s most prominent feature is an asterism of stars in the shape of a backwards question mark known as the ?sickle? formed by stars including 1st magnitude Regulus which marks the ?dot? in the punctuation mark . The sickle represents the head and mane of the lion and Regulus its heart. To the left of the sickle is a triangle of stars representing Leo?s hind quarters. Around 10 p.m. ET in early June, Leo is low in the southwest and tilted so that its hind quarters are above and to the left of the sickle. From a dark sky site a rich smattering of faint stars about 30 degrees above Leo?s hind quarters should be visible. This is the star cluster known as the Coma Star Cluster, or Melotte 111. If you don?t see it, sweep straight upward from the hind quarters using binoculars. The cluster?s big, about 7«ø wide, so it may not fit all at once in your binocular?s field of view. If you need more help finding it there are many references, charts, and images on the web. The other open cluster is known as Brocchi?s Cluster or the Coathanger, and is cataloged as Collinder 399. It is an example of a random grouping of unrelated stars that happen to stand out. Its prominence is especially due to its coathanger shape ? a bar and a hook ? formed by 10 stars. The Coathanger is located in the constellation Vulpecula, the little fox. To find it in early June try looking around 11 p.m. by which time it should be clearing the trees. You can hop to it using known stars. If you?re unfamiliar with the following directions don?t worry. There are charts and images waiting to help you on the web. Find the star Albireo in Cygnus the swan, also known as ? Cygni. It represents the swan?s head. To its lower right, locate à Vulpeculae, the brightest star in Vulpecula. Using your binoculars sweep from the former to the latter, and then continue about 1« times the distance between them. You should now be looking at the coathanger. If not, try again, and don?t give up too soon. Double check that you?ve identified the correct stars and that they are the ones you?re seeing in your binoculars. It?s easy to get mixed up when you?re using a hand held optical device. Next month we?ll discuss globular clusters.