When I show star clusters to people at public star parties, one question I get asked a lot is, "What's the difference between a 'globular' and an 'open" cluster?" I asked a couple astronomer friends of mine and got different answers, (no big suprise there). But the two most frequent answers were "Age of the stars" and "Mass of the cluster as a whole". I'll take some time here to fill in the blanks a bit.
There are a lot of differences between open clusters and globular clusters. The age difference comes in when the clusters formed, and how. An open cluster is relatively made of new stars, usually less that a billion years old, and more often just a few tens of millions of years old. They orbit the center of our galaxy along the plane of the disk, just as our Sun does. Open cluster form from molecular clouds of gas and dust that collected after the galaxy itself formed.
Globular clusters formed along with the galaxy, and so contain stars that are mostly as old as the galaxy itself. They also orbit across the plane of the galaxy
Some time after the Big Bang, or actually a very long time after the Big Bang, the matter in the universe began to clump together by virtue of its gravitational attractions. As the larger structures formed, namely the galaxies we see all around us, there was a certain amount of matter that clumped together around their edges. These coalesced to form the very dense patches of stars we call "Globular Clusters". My Websters dictionary defines "glob" as being a "large, rounded clump". This defines, as well as anything, the appearence of a globular cluster. They are large, roundish clumps of stars. Because of their distances, on the order of tens of thousands of light years, they most times appear very faint. For example, M22 pictured here 110 light years across, and contains approximately 70,000 stars. The same area around our sun contains less than 500. There are exceptions, however. They orbit our galaxy perhaps once every 250,000,000 years. These orbits are highly elliptical and pretty much stay centered around the core of our galaxy as one axis and are generally in a polar orbit, as opposed to open clusters that orbit the core along with the spiral arms....
ebmaster : Steve
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