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Stars - Stellar Populations - Globular Clusters

Like open clusters, globular clusters give astronomers a unique insight to how stars evolve. Globular star clusters are found orbiting the center of our galaxy in the halo.


(Image credit: Brooks/Cole Thomson Learning)

The yellow dots are the distribution of globular clusters.

They are generally comprised of old, metal poor stars - that is contain little elements heavier than helium. This means globular clusters formed a long time ago, before our galaxy contained more heavy stars. By contrast, open clusters have more heavier metals.

Here are some examples of globular clusters:


( 2005 Russell Croman, www.rc-astro.com)

The image above is of one of the most dramatic clusters around - Omega Centauri.

The image below is of M56, and contains stars that are more red.


( 2005 Russell Croman, www.rc-astro.com)

Until recently, we have been able to spy a look at the core of a globular cluster.

This Hubble Space Telescope image of M15 shows what appears to be a concentration of stars at the very center. This has led some to suggest that a black hole is at the heart of this (and others) cluster.

What provides insight to this is the high concentration of white dwarfs found.

This image of M4 shows a ground view as well as a Hubble Space Telescope image. The HST image shows at least 7 white dwarfs (circled).

Just like open clusters, astronomers can use a CMD to determine cluster age:


(Image credit: Brooks/Cole Thomson Learning)

Because globular clusters are old, it will be rare to see the CMD distribution like the one on the left. The distribution of stars will be more like the diagram on the right. Most of the stars will have already entered the Red Giant stage. This is called the Main-Sequence Turn-Off point (MSTO). The age of the cluster is determined by the stars that have already entered the MSTO stage.

Here is a summary chart of star clusters:
Characteristic: Open Clusters: OB Associations: Globular Clusters:
Diameter (pc): <10 30 - 200 20 - 100
Number of Stars: 50- 1000 10 - 100 104 - 106
Mass (Solar): 100 - 1000 100 - 1000 104 - 106
Density (Solar Mass/pc3): 0.1 - 10 <0.01 0.5 - 1000
Shape: Irregular Irregular Spherical
Color (Common): Red or Blue Blue Red
Metallicity: High High Low
Location: Disk of Galaxy Disk of Galaxy Halo of Galaxy

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