Clusters

Star Clusters

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.

Pleiades Open Cluster - M45 Open clusters are also sometimes called galactic clusters because they lie relatively close to us, and they are within 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 nebula, and so have the same relative velocity. This is what keeps them from heading off in different directions and dissipating the cluster. Clusters can be destroyed, however, such as by a passing massive star whose gravitational effects change the motions of some of the cluster's stars.

Sometimes, there are more prominent stars, while others tend to be less conspicuous, such as the Pleiades at the left. This cluster has been 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 called 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 because the stars have not existed long enough for their radiation to effectively destroy it. This isn't the case in the Pleiades; they are currently moving through a nebula that is unrelated to their birth.

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 150 known globular clusters. Other galaxies can also have globular clusters, though it is not a rule. For example, M31, the Andromeda galaxy, has thousands of globular clusters, while the Sagittarius Dwarf Galaxy has none.

Second, globular clusters are in a practically symmetrical spherical shape, whereas open clusters tend to take on the shape of their nebula. Finally, globular clusters contain literally thousands or even millions of stars.

Globular clusters are thought to be one of the the first structures of a galaxy to form, and they last (have lasted) for billions of years. A recent research paper by Dr. Lawrence Krauss et al, featured in a winter, 2002 issue of Science, finds that globular clusters in the Milky Way have a minimum age of 11.3 billion 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.

Most globular clusters look relatively the same, for they are simply a big ball of stars, though they vary in size. 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. For more information about this globular cluster, see the Hubble Heritage Archive - 1999.

*The average distance between stars in the sun's neighborhood is about 4 light-years (0.02 stars per cubic light-year). In a globular cluster, the density of stars can grow to be several stars per cubic light-year.