Monday, June 11, 2012

Star clusters

Star cluster is a group of stars detained jointly since regards the result of gravitation. Star clusters are classified into two groups: open clusters, which have no definite shape, and globular clusters, which are spherical or nearly spherical. The open consist of a few hundred young stars, while globular clusters contain more than a thousand times that amount, and are generally very old stars. Globular clusters form a halo around our galaxy, the Milky Way, while the open is located in the spiral arms. Open clusters are much more numerous than globular: about 1,000 are known in our galaxy and there are only 140 globular.

Open clusters
The two best-known open clusters are the Pleiades and the Hyades, together observable to the naked eye in the group Taurus. The cluster of the Hyades is about 150 light years from Earth and has a diameter of about 15 light years. The Pleiades cluster has a similar diameter, but is about 400 light years, so it looks smaller. Open clusters are formed from clouds of gas and dust in the arms of a spiral galaxy. The denser regions contract under it is own gravity, leading to individual stars. The Orion Nebula is an example of a region where stars are still forming. In the center of the nebula is a group of old stars, the "Trapezium of Orion."The nebula contains enough gas to form hundreds of stars of the same type. It is known as "stellar association" a group of stars resembling a cluster, but spread over a larger area. They are often open clusters within an association, in areas where the density of the gas from which the partnership was formed is greater. Members of a cluster are born together and continue moving together through space. This is to find their distances. Measuring the motion of the star along the line of sight and across the line of sight, one can calculate the distances that separate the solar system. This technique is known as the moving cluster method.
Globular Clusters
 The two brightest globular clusters Omega Centauri and 47 are Tulane, both visible to the naked eye from the southern hemisphere. The most remarkable globular cluster in the northern hemisphere is M13 in the constellation Hercules, also visible to the naked eye. In globular clusters, the concentration of stars in the central portion may be 100,000 times higher than in the region of space occupied by us, and terrestrial perspective may seem that the stars are merged together. Globular clusters contain some of the oldest stars in the Milky Way, with ages of 10,000 million years, double the Sun’s age of a cluster is estimates putting their stars in a Hertz sprung-Russell diagram. As the speed of evolution of a star depends on its mass, the point at which the star begins to leave the main sequence to become a giant, shows the age of the cluster. Globular clusters formed, when the vast cloud of dust and gas resulted in our Galaxy was collapsing. About the sun is on the outside of the galaxy, most of the clusters found in one half of the sky toward the center of the galaxy.

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