Star clusters are groups of stars that occur close to each other in space, appear to have roughly similar ages, and therefore, seem to have had a common origin. Star clusters are typically classified into one of two large subgroups, galactic clusters and globular clusters. Galactic clusters are sometimes also known as open clusters. Astronomers have identified thousands of galactic star clusters in the Milky Way, but no more than about 200 globular star clusters.
The two types of star clusters found in the Milky Way differ from each other in a number of ways. First, galactic clusters occur in the plane of the galaxy, while globular clusters are found outside the galactic plane in the region known as the galactic halo. Second, globular clusters tend to be much larger than galactic clusters with an average of a few thousand to a million stars in the former and a few hundred stars in the latter. In fact, some galactic clusters contain no more than a half dozen stars. Probably the most famous of all galactic clusters is the Pleiades, or the Seven Sisters. This grouping consists of six or seven stars as seen by the naked eye (depending on the accuracy of one's eyesight), but of many more when viewed by telescope. Third, globular clusters, as their name suggests, tend to have a rather clearly defined spherical shape with a higher concentration of stars at the center of the sphere. In contrast, galactic clusters, as their alternative name also suggests, tend to be more open and lacking in any regular shape.
Fourth, the compositions of stars found in each kind of cluster are quite different. The stars that make up galactic clusters tend to consist primarily of hydrogen (more
than 90%) and helium (almost 10%), with small amounts of heavier elements (less than 1%). Stars in globular clusters, on the other hand, contain even smaller amounts of heavier elements. This difference suggests that the stars in galactic clusters are much younger than those in globular clusters. When the latter were formed, the universe still consisted almost entirely of hydrogen and helium, so those were the only elements used in the formation of globular cluster stars. Much later in the history of the universe, some heavier elements had been formed and were present at the formation of galactic cluster stars.
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