The Basics: Star Clusters
What is a star cluster? A star cluster is a group of stars that are gravitationally related and formed from the same dust cloud. When galaxies form there is a large dust cloud called the parent dust cloud but within that cloud there are several smaller dust clouds that can form star clusters. The stars in the cluster are the same age and of the same type because they all formed from the same dust cloud at the same time. There are two different types of star clusters, open and globular.
Open clusters are a loose collection of stars and can contain from several hundred to tens of thousands of stars. The Pleiades is an example of an open cluster and a Reflection Nebula. The open cluster is the source of illumination for the blue nebulous gas.
Globular clusters are much tighter in appearance and look like small cotton balls. They typically contain hundreds of thousands of stars but can contain millions of stars. Omega Centauri is a fine example of a huge globular cluster. Most globular clusters are estimated to be 10 billion years old or older. Globular clusters typically contain older low mass stars, RR Lyrae variable stars and blue straggler stars. RR Lyrae variables vary in size and brightness with a regular cycle. Blue stragglers were baffling when first discovered, how can you have young hot stars in an old cluster where all of the stars formed at the same time? The answer is that they are thought to have formed from two older cooler stars that have had gravitational interactions causing the stars to merge in to one younger hotter star. Globular clusters are rated according to the distribution of their stars. This rating system was formed by Harlow Shapley and Helen Sawyer Hogg and is known as the Shapley-Sawyer Concentration Class. The ratings use Roman Numerals starting at I and ending at XII. Roman numeral I is used for globular clusters with most of their mass at the core with little surrounding stars away from the core. Roman numeral XII is used for globular clusters with a very loose core with minimal structure.