Globular Clusters

So far, all of the objects we have discussed are relatively nearby (within, say, 10,000 light years). They are, in fact, quite close to our location in the Milky Way galaxy. So let's step back and take a more global view of our home galaxy.

The Milky Way galaxy is a large disk about 100,000 light years in diameter and about 1,000 light years thick. It is visible as a fuzzy band which raps round the entire sky. This time of year, the band of the Milky Way is relatively faint. However, on summer evenings you may see a brighter band of light. The reason for this difference in brightness is due to the fact that our galaxy has a disk shape. We are located about halfway between the center and the edge of the disk (therefore the center, or nucleus, of our galaxy is about 25,000 light years away). When we look at the region of the sky displayed here, we are looking out towards the edge of the galaxy. Six months from now, the evening sky points in the opposite direction and we see a brighter band because we are looking toward the center of the galaxy. Surrounding the disk of our galaxy is a giant halo of very old stars. In this halo are very crowded clumps of stars called globular clusters. Whereas open clusters lie in the galactic plane and contain dozens to hundreds of stars, the globular clusters mainly lie outside of the plane of the galaxy and contain tens of thousands to millions of stars. The most dramatic example of a globular cluster which is visible from our latitude is the Great Globular Cluster in Hercules (M13/NGC 6205). Here's a picture.
The total number of stars in this cluster is estimated to be over a million. The diameter of this cluster is about 160 light years. Although the stars appear very crowded, the density of stars is about one star per cubic light year. The cluster is about 25,000 light years away. Therefore the light we see when we look at this cluster left its source when the stoneage was in full swing. Hercules is not up this time of year, but will be visible in the late evening sky starting in April. This object is just barely visible to the naked eye, and is quite impressive through a telescope. The small fuzzy spot in the photo here is a distant galaxy NGC 6207. So let's move farther out and explore these large galactic inhabitants of our universe.

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