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Star Clusters Overview The dictionary definition of a cluster is simply a group of objects. A cluster of apples, for example, is just a group of apples. In astronomy, on the other hand, a cluster is used to refer to a grouping of stars. There are two types of clusters: Open and Globular. Open Clusters
Sometimes, there are more prominent stars, while others tend to be less conspicuous, such as the Pleiades at the left. This cluster, known since prehistoric times, with the earliest written mentions dating back to between 700-1000 B.C. Known popularly as the "Seven Sisters," this is also known as M45, and lies approximately 440 light-years from Earth. Popularly, there are seven stars that are visible to the naked eye, but before the invention of the telescope, as many as 14 were identified on star charts. However, these are just the brightest in the cluster -- there are actually about 500 stars that belong to the cluster. In a young open cluster, the stars will still have material from their parent nebula in between them. This isn't the case in the Pleiades; they are currently moving through a nebula. Usually, the stars will stay together, for since they were born from the same nebula, they all retain its general velocity and direction through space. Sometimes, however, the cluster will disperse, and become unrecognizable, due to the gravitational effects from unrelated stars. Globular Clusters A globular cluster is very different from an open cluster. First of all, globular clusters do not lie within our galaxy, but rather on the outskirts of it, forming a "galactic halo." Our Milky Way galaxy has 147 known globular clusters. Second, they are in a practically symmetrical spherical shape, whereas open clusters tend to take on the shape of their nebula. Also, they contain literally thousands or even millions of stars. Globular clusters are thought to be the first structures of our galaxy to form, and they last (have lasted) for billions of years. Their longevity makes them very useful for setting a lower limit of the universe's age, for the universe cannot be younger than its oldest stars. All globular clusters look relatively the same, for they are simply a big ball of stars. If our sun were a member of a globular cluster, everything on Earth would cast dozens of shadows, and there wouldn't be any real night, for the density of stars is much greater than the density of stars in our stellar neighborhood *. The above picture was taken by the Hubble Space Telescope, and shows what is known as M80 and also NGC 6093. It was originally discovered in 1781 by Charles Messier. It is approximately 87 light-years in diameter, and lies 27,400 light-years from Earth. *The average distance between stars in the sun's neighborhood is about 4 light-years. In a globular cluster, the density of stars can grow to be several stars per cubic light-year. 
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Open clusters are also sometimes called galactic clusters because
they lie relatively close to us, and on the disk of the galaxy. Open clusters
can contain anywhere from a few dozen to a few hundred stars. It is assumed that
they are all from the same parent 