Globular Clusters
Globular Clusters
Goals: View the globular star clusters and investigate how their age relates to their location in the Milky Way.
Before starting, turn on: oc
You will be using: gc, galaxy, see command
The final data group that we will examine within the Galaxy is the globular clusters.
Globular Clusters Are Distributed Spherically
If you orbit the Sun from outside the Galaxy, you'll see the globular clusters are not concentrated in the disk at all but are found above and below the disk. You will also see that they are distributed about the Galactic center.
In addition to their Galactic position, globular clusters differ from open clusters in other ways. For one, they are far more dense, packing 100,000 to 1 million stars into a cluster typically 30 to 100 light-years in diameter. With your new sense of scale, imagine stuffing a million stars inside Earth's radio sphere.
Globular Cluster Stars Are Older and Cooler
The stars that populate these two cluster types are different as well. Open clusters are young objects filled with young stars. Globular clusters contain stars that are older and cooler and that formed as the Galaxy was taking shape.
Turn off the open clusters to see the globulars on their own. Among all the data in the Milky Way Atlas, the globular clusters are the most complete, with only a few missing that we cannot see. These are likely on the opposite side of the Galaxy.
Metallicity Is a Measure of Age
Astronomers use a quantity called metallicity to determine the age of a globular cluster. Metallicity is the fractional abundance of elements heavier than helium. In astronomy, all elements other than hydrogen and helium are “metals.” A chemist would not be happy with this definition, but this is how astronomers use the word “metal.” Clusters with a low metal abundance are older, while higher metal abundances imply younger clusters.
You can see where the older and younger clusters reside using the pre-defined selection expressions in the mw.cf file. Type
see oldgcto see the older globulars. The distribution remains the same, but fewer clusters are present. Now look at the younger clusters with
see younggcThe young clusters are concentrated in the center of the Galaxy and are close to the Galactic plane (turn on the galaxy group to see this plane).
Perhaps this makes sense. Younger objects tend to be found in the spiral arms within the Galactic disk. The disk is also generally more metal-rich. Heavy elements are created inside stars, and supernovae release them into the Galaxy. These elements, then, are going to be found where the stellar life cycle is active.
The older clusters, on the other hand, are found out of the disk. This part of the Galaxy is called the halo. The stars that formed here are from an era when the Galaxy was forming. After this star formation, the hot, blue stars exhausted their fuel quickly and became stellar remnants. What we see now are the cool stars that burn their fuel slowly and have very long lifetimes. These stars are metal-poor and very faint, making them difficult to see.
If you inspect all the clusters (type see all), you may notice some are quite far away. Astronomers are still discovering what lies around our Galaxy. Recently there has been speculation about some of the globular clusters possibly belonging to other galaxies, or the Milky Way capturing them from passing galaxies.
In the next section, we will discuss the larger structural elements of the Galaxy and begin to talk about what is outside our Galaxy as we wrap up our discussion of the Milky Way and head into the Extragalactic Atlas.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott