To understand what an open star cluster is, it is essential for a person to understand the distinction between constellations of stars, clusters of galaxies, globular clusters of stars and open clusters of stars. A brief description of constellations, galaxy clusters and globular clusters will help to distinguish the characteristics of an open star cluster.
A constellation of stars is not really a cluster of stars at all. From the perspective of a person on earth they appear to be a group of stars in close proximity to each other, however, they usually are separated by many hundreds or thousands of light years. From ancient times people have imagined shapes in the way stars appear to be grouped together when viewed from the earth. Ancient civilizations named these imagined shapes after their gods and their heroes. We know these today as constellations.
A galaxy, such as our own Milky Way, is a system of stars that are held together by gravity and numbering in the millions or billions, they rotate around a central core area. Our solar system, which orbits the star we know as the sun, is a tiny part of Milky Way galaxy. Galaxies have many different forms, such as spiral, disk, whirlpool or elliptical. Galaxies can be in relative close proximity to each other, as are the Milky Way galaxy, Andromeda Galaxy, Triangulum Galaxy and approximately 30 smaller galaxies. This closely bound local group of galaxies is held together in relative close proximity by gravitational forces and is a cluster of galaxies.
A globular cluster of stars is a large group of stars, from 10,000 to 1,000,000, that are held together by a relatively strong gravitational pull in a tight globe shape around a central core. This is why the term globular is used to describe these clusters of stars that, in galactic terms, are very close together. The stars of a globular cluster are very old. Generally globular clusters are found at the outer regions of galaxies where there are fewer disturbances from the much stronger gravitational pull of the center of the galaxy in which they can be found. The stars within globular cluster are old. It could be that because of the age of the stars in a globular cluster, the stars have had time to settle into a tight sphere shape.
As one would then imagine, an open cluster of stars is a much smaller group, only a few thousand, compared to the much larger globular clusters. Generally open clusters are found much closer to the center of the galaxy that they occupy and may be more affected by the gravitational pull from the central core of the galaxy. This is possibly why the form of an open cluster is not as uniform as the globular cluster. Sometimes it is difficult to distinguish a sparsely populated globular cluster of stars from a densely populated open cluster of stars, but an open cluster has younger stars that appear to be bluish in color while globular clusters will have older stars that appear to be yellow within it.
The young stars of an open cluster are held together in a group by a weak gravitational pull and may seem to have an open center. In reality these stars do have a distinct core which holds the open cluster group loosely together in relative close proximity to each other.
In some open clusters the stars are still forming from the irregularly shaped cloud of gas that surrounds them. The stars of an open cluster are formed at approximately the same time from the same molecular cloud and are therefore thought to be of the same age. One theory is that an open cluster of stars is “born” when a giant molecular cloud collapses and there is a burst of star formation. Then the hottest and most massive stars in the cluster will produce ultraviolet radiation which ionizes the molecular cloud. Coupled with stellar winds, pressure from the ultraviolet radiation pushes the gases of the molecular cloud away from the star cluster. In a few million years the cluster will go supernovae and eventually there will be no more gas within the cluster and no new stars will form.
Another theory of open star cluster formation is that star clusters are formed by a violent gas expulsion from a molecular cloud that is contracting to form massive stars. When these massive stars become hot and appear to shine, residual gases are expelled and smaller stars may be formed over a period of thousands of our earth years. Then as the force of this gas expulsion gradually subsides, star formation stops. This gas expulsion can be so violent that the star cluster may lose many or all of its newly formed stars.
Sometimes more than one open cluster may be formed by one molecular cloud and these clusters are called a binary open star cluster. In our own Milky Way galaxy, there are more than 10 known binary open star clusters. One example of a binary open star cluster is the Double Cluster of h Persei and Persei.
Although there are more than 1,000 known open star clusters in the Milky Way galaxy, it is possible that there could be as many as ten times that number. Some of the better known open star clusters are Pleiades, Hyades or the Alpha Persei Cluster and these can be seen with the naked eye. Other open star clusters cannot be seen unless some form of magnification, such as a telescope, is used.
The best time to observe open star clusters would be on a dark night when there is a new moon. It is best to move away from city lights so that you can pick out the open star clusters of Pleiades and Hyades in the constellation of Taurus, or the Alpha Persei cluster in the constellation of Perseus. A telescope or binoculars can be helpful, but these open star clusters can be observed with the naked eye just as ancient people were able to do. To view these constellations, contact a local observatory or log on to a website dedicated to astronomical maps to find out which direction you should look from the location where you will be viewing the night sky.
