Open cluster

As an open star cluster (or galactic clusters ) are collections of about twenty referred to a few thousand stars that were formed from the same giant molecular cloud ( GMC engl. ). Their concentration in the cluster center is relatively small. Nevertheless, they clearly stand out against the star background. From the densely packed globular clusters, they differ in size, location, age and origin, but above all due to the lower density of stars.

Open clusters are found only in spiral or irregular galaxies, in which star formation still takes place ( eg what elliptical galaxies too old ). The clusters are rarely older than a few hundred million years because they lose by the proper motion of the stars, their internal processes or by mutual perturbations members. Sometimes they can also be destroyed by collisions with other clusters and gas clouds.

Young open clusters can still be in that the molecular cloud from which they arose. This is brightened by it, and it creates an ionized H II region. However, the radiation pressure of the young stars causes the molecular cloud is gradually dispersed. Usually 10 % of the mass of the gas cloud for star formation to be used before the radiation pressure blows away the rest.

For the study of star formation, open clusters are very important objects. The reason for this is that all cluster stars have roughly the same age and the same chemical composition. So drop small differences in the properties much faster than if we observe only isolated stars. Nor can their common direction of motion (star Tromp Aral Laxe ) use for distance determination.

Observation history

The best-known open clusters like the Pleiades are considered a group of stars since ancient times. Others were observed as light spots, but could be identified only with the invention of the telescope as star clusters. After further observations, the star clusters were divided into two different types. The one consisted of thousands of stars in a regular, spherical shape and were mainly found in the center of the Milky Way. The other group had fewer stars, more irregular in shape and can be found everywhere in the sky. The first group were given the name of globular clusters and the second one, known as open clusters. Open clusters are sometimes called galactic clusters, since they are located almost exclusively in the galactic plane of the Milky Way.

It was found that the stars in an open cluster have similar properties. The clergyman John Michel calculated in 1767 the probability that a group of stars such as the Pleiades was merely a random arrangement in the night sky, to 1 in 496,000. When the astrometry was accurate, it was found that moving the stars in the cluster with the same proper motion through the night sky. Spectroscopic observations determined you also have the same radial velocity. It was concluded that the stars were created at the same time and are connected to each other as a group.

Although globular clusters and open clusters clearly form separate groups, the differences between sparse globular clusters and very rich open cluster can be low. Some astronomers believe that both types of star clusters, the same mechanisms underlie the difference being that the causes which lead to the formation of large globular clusters, no longer exist in our galaxy.


All stars are formed from multiple star systems, because only a gas cloud with a multiple solar mass is heavy enough to collapse under their own gravity, but it can not collapse to a single star, a heavy cloud.

The emergence of an open cluster begins with the collapse of part of a giant molecular cloud, a cloud of gas to the weight of several thousand solar masses. Many factors may be the trigger for it. Once the giant molecular cloud begins to collapse, star formation begins by the formation of smaller and smaller fragments, from which in the end might be several thousand stars. In our galaxy itself open clusters form every few thousand years.

As soon as the first star formation, the biggest and hottest stars emit a tremendous amount of ultraviolet radiation. This radiation ionizes the surrounding gas of the giant molecular cloud, thereby forming an H II region. Stellar winds of the heavy stars and the radiation pressure displace the surrounding gas. After a few million years, it comes to the first supernova of a star, which further gas is ejected from the system. After a few tens of million years is only remained as much gas left, that it can not come to a star formation. Usually only 10 % is used for star formation from gas initially present. The rest is blown away.

Usually formed from a molecular cloud two or more open star clusters. In the Large Magellanic Cloud, both Hodge 301 and R136 have emerged from gases of the Tarantula Nebula. An example from our galaxy would Hyades and Praesepe. By tracing its movement, it is believed that they have formed from the same cloud of 600 million years ago.

Sometimes form two clusters that are created at the same time and form so-called double cluster. The best known example in the Milky Way is the double cluster h Persei and Chi Persei, but you still know ten more. It has been found in many of the small and large Magellanic Cloud. They are easier to detect in other galaxies, since projection effects in the Milky Way could cause that do not belong together stars look like they would be located close to each other.

Shape and classification

The number of stars in open clusters varies between a few tens of stars to large accumulations of several thousand stars. They usually contain a dense core which is surrounded by a large corona of other stars. The core usually has a diameter of 3 to 4 light-years, while the corona extends at a distance of about 20 light years from the center. In essence, there are about 1.5 stars per cubic light-year ( the stellar density in the area around our sun is approximately 0.0035 stars per cubic light-year )

Open clusters are classified mostly according to a model developed by Robert Trumpler scheme of 1930. These three pieces of information are needed. The Roman numerals I-IV indicate the concentration and detachment from Surrounding Sternfeld (from strong to weak Concentrated ). The Arabic numerals 1-3 indicate how strongly the individual stars differ in brightness (from low to high). The letters p, m, or r indicate whether the Kluster little ( poor ), average ( medium) or many ( rich) star has. A 'n' means that the clusters is in a fog. According to this scheme, the Pleiades are classified for example as I3rn ( highly concentrated with rich population in a fog), the Hyades are classified as II3m ( more dispersed and less stars)

Number and distribution

There are over 1,000 known open clusters in our galaxy, but the real number is likely to be up to ten times higher. In spiral galaxies, they are found almost exclusively in the spiral arms. The reason is that this is where most stars are formed because of the higher gas density and pass the cluster again before they can reach beyond the spiral arms. You are in our galaxy in the galactic plane concentrated with an expansion of the amount of approximately 180 light years ( compared with the radius of the Milky Way is about 100,000 light-years )

In irregular galaxies can be found in the Galaxy open clusters everywhere. Your concentration is greatest, where the gas concentration is highest. They are, however, not found in elliptical galaxies, since the star formation process has stopped many years ago, so that all open clusters have been dissolved.

In our galaxy, the distribution of the age depends. Older clusters are usually found at larger distances from the galactic center. The tidal forces are stronger near the center of our galaxy, so that the clusters are much more easily destroyed. Furthermore, the giant molecular clouds, which can also destroy the open clusters, rather in the inner regions of the galaxy are concentrated. So most star clusters in the inner regions of the galaxy pass much earlier than those in the outer regions.

Composition of stars

Because open clusters disperse before most of their stars die, most of the light comes from young, hot blue stars. These stars are the heaviest and have the shortest life expectancy of a few ten million years. Contrast, older open clusters have more yellow stars.

Some open clusters contain hot blue stars that appear to be younger than their remaining stars. These blue stragglers are also observed in globular clusters. It is believed that they are created when stars collide and merge, and form a much hotter and heavier star. In any case, the stellar density is much lower than in globular clusters, so that star collisions can not explain the number of stragglers. It is rather assumed that most originate in a binary star system. Interactions of the binary system with other stars then lead to the merger of the two stars to one star.

Once a star exhausts its hydrogen supply and thus the nuclear fusion can no longer take place, he pushes off its outer layers, forming a planetary nebula with a white dwarf inside. Most open clusters are, however, scattered, before many of their stars reach the stage of a white dwarf. However, the number of white dwarfs in open clusters again much lower than expected. One possible explanation is the following: If a red giant repels its outer layers, forming a planetary nebula, a small enough asymmetry of the rejected material from in order to give the leftover star a push of a few kilometers per second. This is strong enough to let him get away from the pile.

Fate of the open cluster

The length of time a star cluster has on hand, depends mainly on its initial mass. Many open clusters are unstable since its creation. Their total mass is so small that the escape velocity from this system is less than the average velocity of stars. These star clusters dissolve within a few million years. As the surrounding gas is blown away by the radiation pressure of the young hot stars, reduces the mass, so that rapid dispersal is possible.

Star clusters with a large enough mass to bind the stars permanently by gravity, may exist tens of millions of years but also internal and external processes mean that they are gradually dispersed. Coming too close inside stars, which often means that the velocity of a star is greatly increased, exceeding the escape velocity of the star cluster and he can escape Him thereby. This leads to the slow dissolution of the cluster. The time until loss of half of the stars ranges from 150 to 800 million years, depending on the initial density.

On average, the collision is all half million years an open cluster by an external factor, such as a molecular cloud, destroyed. The effects caused by the gravitational tidal forces then lead to the destruction of the structure of the heap. Finally, from the star cluster, a band of stars which, while not tight enough lying together in order to be called a heap, but all are connected to each other and move in the same direction.

After gravity has become so weak that it is no longer sufficient to bind the stars, most of the stars are still moving in the same direction. So a star association is then known as a mobile pile or moving clusters. Many of the brightest stars in the ' plow ' of Ursa Major were formerly an open cluster, which are now a loose connection, the Ursa Major group.

Studies of star formation

When one enters the stars of an open cluster in the Hertzsprung -Russell diagram, they are usually on the main sequence. The heaviest stars are set away from the main sequence and become red giants. The position of these stars can be used to determine the age of the cluster.

Since all stars in an open cluster have approximately the same distance from the Earth and are created around the same time from the same raw material, the brightness differences depend only on the different masses of the stars. This open clusters are very useful when one wants to test the stellar evolution. Because if you want to compare two stars of a star cluster, drop out most of the parameters.

The study of lithium and Berylliumvorkommen in open clusters are important clues for the evolution of stars and their internal structures. While hydrogen nuclei at a temperature of 10 million K can not fuse to form helium, lithium and beryllium are destroyed at a temperature of 2.5 million and 3.5 million K K. This means that their occurrence is very dependent on what happens inside the star. From the data it can be concluded on the age and the chemical composition.

Distance measurement

To understand an astronomical object, it is imperative to know its distance. The nearer star clusters can be measured with two different direct methods. First, one can determine the parallax, so the apparent displacement of the object with respect to very distant objects, which are actually the result of the movement of the earth around the sun. The second method is the so-called movement cluster method (star Tromp Aral Laxe, see parallax). It is based on the fact to reason that move the stars in a star cluster together on a common vanishing point (vertex ). It then determines from the stellar spectra using Doppler measurements, the radial velocity. Once one knows the radial velocities of the proper motion and the observed angle of star clusters to the vanishing point can be calculated with simple trigonometry the distance. The Hyades are the best known example in which this method was applied. Their removal is 46.3 parsecs.

Once the distance of nearby star clusters is known techniques can be used, which are based on the data obtained at the nearby star clusters for greater distances. The nearby star clusters are known to be categorized their stars at a known distance in the main sequence of the Hertzsprung -Russell diagram, and so one can easily determine the distance of star clusters, the much more are located away from the Earth.

The Earth closest open clusters are the Hyades. However, they are more of a moving star clusters as an open cluster. The most distant open clusters in the Milky Way is the Berkeley 29 with a distance of about 15,000 parsecs. Open clusters are found in many galaxies of the Local Group.

The exact distance of open clusters is important to calibrate the period-luminosity relationship of certain sizes of variable stars ( Cepheids and RR Lyrae stars). These stars are very bright and can be identified in a very large distance. They are therefore used as a "standard candle " to calculate the distance to nearby galaxies in the Local Group.