- Star clusters
- Measuring the age of a star cluster


[SLIDE 1]

The giant clouds of gas where stars and planets form usually contain enough material for many stars. This means that stars usually form in groups known as star clusters.

Star clusters are useful for study because all of the stars in a cluster lie at a similar distance from Earth and were formed at roughly the same time. This allows astronomers to compare the properties of stars that have similar ages.

There are two basic types of clusters. Open clusters tend to have just a few thousand stars and are about thirty light-years across, while globular clusters contain more than a million stars and might be 60 to 150 light years across.

This tells you that the stars in a globular cluster are very densely packed together with 10,000 stars fitting into a space just a few light years across.

Globular clusters contain the oldest stars in the universe and are found in the halo of the galaxy. Open clusters are relatively young and are always found in the disk of the galaxy.


[SLIDE 2]

By plotting the stars from a star cluster on an H-R diagram, we can determine the age of the star cluster.

Since the hottest stars run out of fuel first, the point on the diagram where the cluster's stars begin to fall off of the main sequence is called the main-sequence turnoff point.

In the diagram above, you can see the stars of an open cluster called Pleiades (pronounced PLEE-uh-deez) plotted on an H-R diagram. Only its hottest stars, the type O stars, have begun to fall off the main sequence, indicating that they are running out of hydrogen. Some of the hotter B-type stars have also fallen off. Since we know that those types of stars have a lifespan less than 100 million years, we can estimate that the Pleiades star cluster is about 100 million years old.

To summarize, the age of a star cluster is approximately equal to the hydrogen core-fusion lifetime of the most massive stars that are left on the main sequence.