What are star clusters | Earth, Solar System and Universe.

The stars do not appear in isolation, fate forming groups that we call "clusters". A cluster of stars, is a group of related stars held together by gravitation effect.
Star clusters are classified into two groups: open clusters, which do not have definite shape, and globular clusters, which are spherical or nearly spherical. The open are formed by 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 are located in the spiral arms.
Open clusters are much more numerous than the Globular: about 1,000 in our Galaxy are known while there are only 140 globular.

Open clusters

The two best known open clusters are the Pleiades and the Hiadas, both observable to the naked eye, in the constellation Taurus. The Hiadas cluster is located about 150 light years from Earth and has a diameter of about 15 light years. The cluster of the Pleiades has a similar diameter, but is approximately 400 light years, so it is smaller.
Open clusters are formed from clouds of gas and dust in the arms of a spiral galaxy. The most dense regions contract under their own gravity, giving rise to individual stars.
The Orion Nebula is an example of a region that still stars are forming. A group of old stars, "Orion trapezium" is located in the Centre of the nebula. The Nebula contains enough gas to form hundreds of stars of the same type.
Referred to as "Association" to a grouping of stars like a heap, but spread over one larger area. Clusters open inside of an association, in areas where the density of the gas from which the Association was formed is greater are often.
Members of a cluster are born together and continue moving together through space. This serves to find their distances. By measuring the motion of stars along the line of vision and through the line of sight, can calculate the gap that separates them from the Solar System. This technique is known as the moving cluster method.
Globular clusters
The two brightest globular clusters are Omega Centauri and 47 Tucanae, both observable to the naked eye from the southern hemisphere. The most remarkable globular cluster in the northern hemisphere is M13, in the constellation Hercules, also observable to the naked eye. The photo shows the globular cluster NGC 6388.
In globular clusters, the concentration of stars in the Middle can be 100,000 times higher than in the region of space occupied by us, and the terrestrial perspective may seem the stars merge among themselves.
Globular clusters contain some of the oldest stars of the milky way, with ages of 10,000 million years, twice as much as the Sun.
The age of a cluster is calculated by putting its stars in a Hertzsprung-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.
The globular clusters were formed when the immense cloud of dust and gas that resulted in our Galaxy was collapsing. As the Sun is in the outer area of the Galaxy, most of the clusters is one half of the sky toward the center of the Galaxy.
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