The stars do not appear in isolation, but forming groups that we call "clusters." A star cluster, is a group of related stars that are held together by the effect of gravitation.
Star clusters are classified into two groups: open clusters, which have no definite shape, and globular clusters, which are spherical or almost spherical. The open ones are formed by a few hundred young stars, while the globular clusters contain more than a thousand times that amount, and are generally very old stars.
The globular clusters form a halo around our galaxy, the Milky Way, while the open ones are placed in the arms of the spiral.
Open clusters are much more numerous than globular ones: about 1,000 are known in our galaxy while there are only 140 globular.
Clusters of open stars
The two best-known open clusters are the Pleiades and the Hiada, both observable to the naked eye, in the constellation Taurus. The cluster of the Hiadas is about 150 light years from Earth and has a diameter of about 15 light years. The Pleiades cluster has a similar diameter, but it 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 densest regions contract under their own gravity, giving rise 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 "Orion Trapeze." The nebula contains enough gas to form hundreds of other stars of the same type.
It is known as "star association" to a cluster of stars similar to a cluster, but distributed over a larger area. Open clusters are often found inside an association, in areas where the density of the gas from which the association was formed is higher.
The members of a cluster are born together and continue to move together through space. This serves to find their distances. By measuring the movement of the stars along the line of sight and through the line of sight, the distances that separate them from the Solar System can be calculated. This technique is known as the mobile cluster method.
Clusters of globular stars
The two brightest globular clusters are Omega Centauri and 47 Tucanae, both observable to the naked eye from the southern hemisphere. The most prominent globular cluster in the northern hemisphere is M13, in the constellation Hercules, also observable with the naked eye. The photo shows the globular cluster NGC 6388.
In globular clusters, the concentration of stars in the central part may be 100,000 times greater than in the region of space occupied by us, and from the terrestrial perspective it may seem that the stars merge with each other.
The globular clusters contain some of the oldest stars in the Milky Way, with ages of 10,000 million years, double that of the Sun.
The age of a cluster is calculated by putting its stars on 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 formed when the vast cloud of dust and gas that gave rise to our galaxy was collapsing. Since the Sun is in the outer zone of the galaxy, most of the clusters are located in one half of the sky towards the center of the galaxy.
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