Digital Universe Guide - Hayden Planetarium

90 3. THE MILKY WAY ATLAS
7,200 light-years away. Some of the farthest clusters lie at the edge of the Galactic halo and perhaps
even beyond it (more on this in the “The Galactic Outskirts” tutorial).
Cluster Ages The age of a globular cluster is directly related to something called the metallicity.
(In astronomy, all atomic elements heavier than helium are called metals, and the metallicity is the
fractional abundance of metals in an object.) Clusters with stellar populations that have higher
metallicities are typically found near the Galactic center, while clusters that are deficient in metals are
found in the Galactic halo. Use the following Partiview commands to invoke preset selection expressions
to see these data subsets. (Remember see all returns all data to view.)
see oldgc
see younggc
Having formed about 10 billion to 12 billion years ago, globular clusters are among the oldest objects
in the Galaxy. They were around when the Galaxy formed, perhaps even before the disk evolved to the
shape it is today. Some of the oldest stars in the entire Galaxy are found in these clusters.
The metal-rich clusters are several billion years younger than the metal-poor clusters. This may be a
reflection of galaxy interaction and tidal captures from other small galaxies. In fact, astronomers are
now studying whether some of the globular clusters previously thought to belong to the Milky Way are
bound to other small satellite galaxies.