Star Clusters

Star Clusters

·  Gravitationally-bound collections of stars are known as star clusters. The stars within a cluster orbit around the cluster center-of-mass, sometimes in very complex orbits.

·  Astronomers have grouped star clusters into two classes, based on the their shape and the amount of “compactness” in their visible appearance.

·  OPEN clusters:

4  Appear irregularly-shaped

4  Contain 10s to 1000s of stars, loosely packed

4  Are found to be relatively young, ages of 3 to a few 100 million years,

4  Often contain blue, O & B type main sequence stars.

4  Are still forming today in the Galaxy

·  GLOBULAR clusters:

4  Appear spherically-shaped

4  Contain 10,000s to millions of stars, densely packed

4  Globular clusters in the Galaxy are very old, ages of 9-13 billion years

4  Contain only cool, red (GKM dwarfs & giants) stars, plus some white dwarfs.

4  No young globular clusters exist in the Galaxy

·  All stars in a cluster formed at the same time from the same parent gas cloud. Therefore all stars in a cluster have the same age and chemical composition.

·  Star clusters are fundamental natural laboratories for studying stellar evolution. Why ?

4  Same distance – the relative brightness of each star in a cluster reflects their true brightness differences, since all stars in the cluster are located at the same distance from Earth.

4  Same age – the evolutionary state of individual stars within a cluster depends on their relative masses, not or different “birth dates”, since they were all formed at the same time.

4  Same composition – the stars’ evolution doesn’t depend on different fuel mixtures, since they all formed with the same composition.

Because of these three attributes of star clusters, astronomers can assign an age to a star cluster by examining the cluster’s HR diagram. By inference, that age applies to all of the stars in the cluster.

·  Open clusters gradually “dissolve” with time. The cluster loses stars through small gravitational forces acting on outlying cluster members, which may cause those stars to drift away from the cluster into the galaxy.

·  Globular clusters, formed with a more compact distribution of stars, appear to be very long lived. These clusters contain large numbers of stars initially, so that even when they lose stars to random gravitational forces, the cluster’s appearance is not measurably altered.