What is a star cluster? A star cluster is a grouping of stars that are relatively close together. A star cluster is easy to spot in the sky as a grouping of stars against a background where the stars are more random and spread out. Viewing them can range from knock your socks off beauty to barely detectable fuzz. Star clusters are categorized into two main types, “globular” and “open.” We will first discuss the globular.

A globular star cluster is a tight and dense gathering of stars that takes its name from the fact that it looks like a globe or ball. Globulars range from the spectacular M13 great cluster in Hercules and M-22 in Sagittarius, to faint little NGC fuzz balls. The one thing they all have in common is that they are round and hundreds if not thousands of stars are gathered in a concentrated area. Comet hunter Charles Messier identified many globulars for his catalogue because in a lousy telescope, these fuzzy balls may look just like a comet without a tail. Globulars take magnification well but filters do not usually help. I cannot think of any globulars that contain significant nebulosity so nebula filters will only dim the view and take away from the spectacular gleam of the individual stars. For a thrill, get to know someone with a large telescope and have them aim it at M-33, the Triangulum galaxy. This large face-on galaxy contains several visible globular clusters in its arms. Those are the only star clusters I am aware of outside of our own galaxy that are visible in amateur telescopes in the northern hemisphere. After that, move the scope over to Saggittarius. Just north of the tip of the spout in the Teapot reside a pair of globulars that are easy to fit in a medium wide eyepiece field.

The second type of cluster, the open, is a different animal altogether. Open clusters range from as little as two stars, as in M-40 (a Messier mistake?), up to a hundred or so. Many of the smaller clusters of less than ten stars are called asterisms though they may be classified as an open cluster in star catalogues and on star charts. Some are round, some are almost square, but whatever the shape, they are always interesting. If you look at many of these objects, you will find a few surprises. Some of them are surrounded by nebulosity. A good example is the Pleiades. Several of these bright blue stars are surrounded with a halo of nebulosity that is clearly visible with an O-3 nebula filter. Another nice surprise is M-46. Within this cluster is a planetary nebula that is visible in 12” or larger scopes. The key to viewing that nebula is to know it is there or you may miss it because it is small. The Lagoon Nebula, M-20 contains an open cluster. The Rosetta Nebula in smaller telescopes is simply an open cluster.

A quick rundown of the classic open clusters must include, of course, the Pleiades, the double cluster in Cassiopeia, the beehive cluster, and the Christmas Tree cluster. Just the names alone give you an idea of the variety of shapes out there. The stars in an open cluster may be all the same magnitude, or their magnitudes may vary greatly giving the impression they are not related to each other which in some cases, may be true. This varying magnitude adds to the variety of shapes and gives some of them a 3-D appearance. Many open clusters take magnification well while others are completely lost when the power is cranked up. In my 16” I usually use the same 32mm Erfle eyepiece for everything (82X) and have yet to feel the urge to try a higher power eyepiece even on the smaller clusters.

Recently in my quest for the Herschel objects, I ran across an open cluster that consisted of a bright yellow central star surrounded by about twenty or thirty bluish stars. What helped this cluster stand out was that it was round and concentrated in a small area, yet not dense enough to be considered a globular. That same night I found an open cluster that looked like a flattened “H” and later one that looked like a mess with two uneven lumps but no definable shape. The variety is endless. In a small scope, the most numerous objects visible besides individual stars are the open clusters. My guess is that there are more open clusters within range of a 60mm refractor than all the nebulas and galaxies combined for that same aperture. Regardless of the size telescope, when these clusters all start to look the same, you are either too tired or are not looking hard enough! Take some time, look closely, you may surprise yourself.