Our Galaxy – Globular Star Clusters

What are these…

Globular Star Clusters are much different than Open Star Clusters in most regards.

Globular Clusters:• Old stars• Densely packed• Located in the Halo of the

Galaxy

Open Clusters:• Newly born stars• Less dense than globular cluster• Located in the Disk of the Galaxy

Density

• Globular star clusters are much denser than open star clusters.• The span of distance across the cluster is approximately 150 light-years, but this can

vary from 50 light years for small clusters to 300 light years for large clusters.• Within this cluster than can be anywhere from 100 Thousand to 1 million stars.

Towards the edge of the cluster, there is about 0.5 stars per cubic parsec.

At the center of the cluster, that rises to 100 stars per cubic parsec!

• “metal-poor” stars: star made from the 1st atoms:

• H, He, Li created in the BIG BANG

• Unlikely to have planets

Age

• Globular clusters are very old – the youngest are 10 billion years old• Their formation is not understood, but the lack of young globular

clusters suggest that these formed early in the universe.• Some studies have found globulars to be as old or older than the

galaxies they reside in – perhaps they formed before galaxies???

Location

• Another peculiar characteristic about globular star clusters is that they do not orbit the galaxy inside the bright disk, like the sun and other stars do.

• This location is tough to explain with current models of galaxy formation (accretion)

• They ‘plunge’ through the galaxy for their orbital motion, rather

than have a circular orbit around the galaxy.

Harlow Shapely’s Globular Discovery

• Shapely used a relationship of variable stars to determine how far away globular star clusters were.

• What we saw was that an overwhelming majority of known clusters were distributed spherically around galaxy.

• He assumed that a spherical distribution would imply they were located at the center of the galaxy – therefore, however far away globular clusters are from us, is how far away we are from the center of the galaxy.

• This was important for showing the sun is NOT in the center of the galaxy!