Messier: 4

Date: June 2008

Additional Comments:

The name "globular" cluster, coined by William Hershel, is very descriptive of their appearance - a glob of stars. There are fewer than 200 globular clusters known around the Milky Way, and unless you are familiar with specific ones, they all look very similar. These clusters condensed while the Milky Way was still forming, and reside in a spherical halo around the disk. Unlike objects in the disk, which have a mostly orderly rotation about the galactic center, the orbits of globular clusters are random. It was the three dimensional distribution of globular clusters that Harlow Shapely used to find the center of the Milky Way. All large galaxies are observed to have globular clusters in a halo. Globular clusters are ancient. Generally comprising what are called Population II stars (old ones with fewer heavy elements), there are no longer any luminous, blue, main sequence stars, and the remaining distribution of stars is noticeably redder than that found in open clusters in the spiral arms. Lots more information can be found here: http://en.wikipedia.org/wiki/Globular_cluster

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