Star Clusters and Nebulae Help
Star Clusters—Open Clusters
Some star systems contain hundreds, thousands, or even millions of individual stars, all held in each other’s vicinity by their mutual gravitation. Some huge gas-and-dust clouds condense into star clusters . There are three main types of star clusters found within our galaxy and presumably within other galaxies.
The Pleiades , also known as the “seven sisters,” are a familiar star cluster that can be seen easily with the unaided eye. On a clear night, most people can see six individual stars in this group. People with sharp vision can see seven stars; with a good pair of binoculars, it becomes apparent that there are a lot more than seven stars in this cluster. The Pleiades are an example of an open star cluster . This type of star group is also known as a galactic cluster . The stars are held together by gravity, but they are not tightly packed. They do not show a striking central concentration nor an orderly pattern or structure.
How do we know that the stars in an open cluster are associated and are not just accidentally close to each other because they all happen to fall along the same line of sight relative to Earth? One way to find out is to measure the radial speeds of the individual stars relative to us and then compare these speeds. If the stars are in a common group, held in each other’s vicinity by gravity, then we should not observe much difference in the radial speeds of the stars. If, however, we’re just seeing a coincidental lineup of stars, the individuals in the swarm should have much different radial speeds, just like stars chosen at random in the sky. It turns out that the stars in an open cluster all have radial speeds that are nearly identical.
The most spectacular star groups, besides galaxies themselves, are known as globular clusters . They get this name because of their symmetrical appearance. Some of these clusters contain more than 1 million stars, 50 percent of which lie inside a spherical shell whose radius is called the median radius . The concentration of stars is greatest near the center of the cluster and decreases uniformly in all directions away from the center, as shown in Fig. 13-6.
An excellent example of a globular cluster is M13 in the constellation Hercules. This can be seen in the summer sky from the northern hemisphere. When viewed at low magnification through a large amateur telescope, M13 is spectacular. More than 100 globular clusters have been observed in and around the Milky Way. Globular clusters, unlike open clusters, are arranged in a large spherical halo around the galaxy. There are many globular clusters that lie outside the disk-shaped main part of the Milky Way whose stars are arranged in spirals like the pattern of rainbands in a hurricane.
Globular star clusters are believed to be very old, among the oldest objects in the observable Universe. Such clusters contain large numbers of variable stars, much greater in proportion than the stars in the rest of the galaxy. The stars in these clusters are mostly metal-deficient . The term metal in this context refers to any element other than hydrogen and helium, the two most abundant and oldest elements in the Cosmos. This fact suggests that globular clusters formed from the original stuff of the Universe, the original gases hydrogen and helium, and that few or none of the stars in the clusters are the result of congealed gas and dust from previously blown-up stars.
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