1. Cluster

2. Star Clusters A nebula often results in a cluster of stars. A cluster of stars from the same molecular cloud: –were formed at about the same time –are of similar chemical composition –are about the same distance from us Clusters are classified by appearance. They can measure stellar lifetimes and distances.

3. Open Clusters Clusters of 100 to 1000 stars in a region of 50 ly are called open clusters. –Stars a few ly apart –Some bright, hot stars Individual stars can be seen by eye or with a telescope. Open clusters are usually young stars and have significant amounts of heavy elements like carbon and oxygen.

4. Mass Matters Once on the main sequence, fusion is the source of energy. The core stops compressing. –Radiation pressure out = gravitational pressure in For a main sequence star the luminosity depends on mass. more mass more gravity more fusion higher temperature brighter star

5. Open Cluster Age Open clusters can be dated with their H-R diagram. –About the same age –Low mass stars form slowly The lowest mass star on the H-R diagram sets the age of the whole cluster. Lightest star on the main sequence

6. Globular Star Cluster Clusters of 100,000 stars in a 50-300 ly region are called globular clusters. –Very dense –Few bright stars –Almost all hydrogen and helium These were originally mistaken for nebulae.

7. Globular Cluster Age Globular clusters can also be dated by age. –Older stars moving off the main sequence –Large stars – short life The highest mass star on the H-R diagram sets the age of the whole cluster. Heaviest star on the main sequence