Astronomical Clustering of Stars
Globular and Galactic (Open) Clusters and O and B Associations
Stars cluster together in groups ranging from binary star systems to galactic and globular clusters of stars and at larger scales galaxies and clusters of galaxies.
Binary Star Systems
They are not really clusters, but the smallest groupings of stars are binary star systems. These systems are simply two stars mutually orbiting each other. There are also many multiple star systems consisting of three or more stars mutually orbiting each other.
O and B Associations
The next largest grouping of stars is O and B associations. Not really clusters, these associations are very loose groupings of a few dozen massive O and B spectral class stars. The stars in these associations are recently formed from a giant molecular cloud. As the stars age, these associations will gradually dissipate just as children born in the same family spread out when they reach adulthood. The gravitational forces in O and B associations is not strong enough to bind the stars together.
Open or Galactic Clusters
Open clusters and galactic clusters are two different names for the same thing. Occasionally the term "galactic cluster" is used to refer to clusters of galaxies rather than clusters of stars, but this usage is incorrect. Galactic or open clusters are groups of stars not groups of galaxies.
Galactic or open clusters typically contain a few hundred stars that are spread out enough that we can usually resolve the individual stars in a small telescope. The stars in galactic clusters are relatively young stars that contain about 2% elements heavier than hydrogen or helium - referred to as metals by astronomers, even though they don't meet the standard chemical definition of metals. Astronomers call these stars Population I stars.
The seven sisters of the Pleiades in the constellation Taurus is perhaps the best known and most easily visible open cluster. Despite its name, only six stars are visible to the naked eye, but a telescope reveals a few hundred.
Globular Clusters
Globular clusters typically contain a few hundred thousand stars that are so densely packed together that a small telescope can resolve only a few near the outskirts. The central portions of globular clusters blend into a fuzzy white haze.
The stars in globular clusters and the globular clusters themselves are about 10 to 12 billion years old. These stars contain insignificant amounts of metals, which to astronomers are elements heavier than hydrogen and helium. They are the first generation of stars and are called Population II stars.
Perhaps the best known globular cluster is M13 in the constellation Hercules. It is not visible to the naked eye, but is spectacular in a telescope.
Galaxies
The globular and galactic clusters that we see with small telescopes are part of a much larger collection of stars - our Milky Way Galaxy. The Milky Way is a spiral galaxy containing a few hundred billion stars. Galaxies also group together in clusters of galaxies, which are a much larger scale than galactic clusters.
In our Milky Way galaxy the older globular clusters are distributed in a spherical halo, while the younger galactic, or open, clusters are distributed in the disk. This observation tells us that the galaxy has flattened over the past 10 to 12 billion years. The galaxy spinning as stars orbit the center of the galaxy causes this flattening, just as pizza dough will flatten when an old fashioned pizza maker spins it overhead.
Further reading
Morrison, D., Wolff, S. and Fraknoi, A., Exploration of the Universe, Saunders, 1995.
