1. Star clusters are groupings of stars held together by a common gravitational bond. They vary greatly in size and shape as well as the number of stars. They also vary in age from just thousands of years to billions of years old. Many of these clusters can be seen with the naked eye on a dark night. Astronomers have divided star clusters into two main types according to their shape and number of stars. The can all be classified as either open clusters or globular clusters. Open Cluster M45 (The Pleiades) Image courtesy http://antwrp.gsfc.nasa.gov/apod Star Clusters

2. Open Clusters Open clusters usually contain between a dozen and a thousand stars. Open clusters are formed when several stars are formed at the same time from the same cloud of dust and gas. These clusters are composed of hot, relatively young stars. In our galaxy, the Milky Way, they tend to be found in the spiral arms. There are about 20,000 open star clusters in our galaxy. The reason open clusters are so young is because they don't last very long. Gravitational interactions between the stars and other objects eventually cause them to break apart. Open cluster M7, image courtesy http://www.seds.org Open cluster NGC 7789 image courtesy http://skytonight.com Coathanger cluster, image courtesy http://www.seds.org Open cluster M44 (Beehive), image courtesy http://skytonight.com

3. Globular cluster M3, image courtesy http://antwrp.gsfc.nasa.gov Globular cluster M4, image courtesy http://www.seds.org Globular cluster M13, image courtesy http://www.wendelstein-observatorium.de Globular cluster M92, image courtesy http://www.bellatrixobservatory.org Globular Clusters Globular clusters are much older cluster. They usually contain between ten thousand to a million stars and are more tightly bound together. Globular clusters are found just outside our galaxy, as well as other galaxies. There are about 200 known globular clusters surrounding our galaxy. The stars are usually in a spherical arrangement with the greatest number of stars occurring in the center of the cluster.

4. Why are clusters important to Astronomy? Star clusters have proven crucial to uncovering the secrets of stellar evolution and for answering basic questions about how galaxies form and even when the universe began. Star clusters also gave astronomers the first hints that our solar system was not located at the center of the Milky Way, as was commonly thought until the 1930s, but actually farther out, near the edge. Globular clusters are some of the oldest objects in the universe. Observations of these objects helped astronomers discover that the universe was older than they had thought. Image courtesy science.psu.edu