Open star cluster

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The star cluster Hodge 301 , which is a few million years old, can be seen at the bottom right of the picture . It illuminates parts of the tarantula nebula of the Large Magellanic Cloud .

As open cluster collections are from about a hundred to a few thousand stars denotes that from the same to giant molecular cloud (GMC Engl.) Have formed. Their concentration in the cluster center is relatively low. Nevertheless, they stand out clearly from the star background. They differ from the densely packed globular clusters in size, location, age and formation, but above all in the lower star density.

Open star clusters are only found in spiral or irregular galaxies in which star formation is still taking place (for which e.g. elliptical galaxies are too old). The clusters are seldom older than a few hundred million years because they lose members through the proper motion of the stars, their internal processes, or through mutual orbital disturbances . Sometimes they are also destroyed by collisions with other star clusters or gas clouds .

Young open star clusters can still be in the molecular cloud from which they were formed. This is brightened and an ionized H-II area is created . However, the radiation pressure of the young stars leads to the fact that the molecular cloud is gradually dispersed. Usually 10% of the mass of the gas cloud is used for star formation before the radiation pressure blows the rest away.

Open star clusters are very important objects for studying star formation. The reason for this is that all cluster stars are roughly the same age and chemical composition . Small differences in properties are noticed much faster than if one only observes isolated stars. Their common direction of movement ( star current parallax ) can also be used to determine the distance.

Observation history

The most famous open star clusters, such as the Pleiades , have been considered a group of stars since ancient times. Others were observed as spots of light, but could only be identified as star clusters with the invention of the telescope . After further observations, the star clusters were divided into two classes. Some consisted of thousands of stars in a regular, spherical shape and can be found all over the sky. The other group had fewer stars, a more irregular shape, and is found almost exclusively in the galactic plane of the Milky Way . The first group was named globular clusters and the second was called open star clusters or galactic clusters.

It was found that the stars in an open star cluster have similar properties. In 1767 the clergyman John Michel calculated the probability that a group of stars like the Pleiades was merely a random arrangement in the starry sky at 1 in 496,000. As astrometry became more precise, it was found that the stars in the cluster move through the night sky with the same proper motion . The same radial velocity was also determined by spectroscopic observations . From this it was concluded that the stars formed at the same time and are connected as a group.

Although globular clusters and open star clusters form clearly separated groups, the differences between sparse globular clusters and very rich open star clusters can be small. Some astronomers believe that both types of star clusters are based on the same mechanisms with the difference that the causes that lead to the formation of large globular star clusters no longer exist in our galaxy.


All stars arise from multiple star systems , because only a gas cloud with a multiple solar mass is heavy enough to collapse under its own gravity , but such a heavy cloud cannot collapse into a single star.

The formation of an open star cluster begins with the collapse of part of a giant molecular cloud , a gas cloud weighing several thousand solar masses. Many factors can be the trigger for this. As soon as the giant molecular cloud begins to collapse, stars begin to form through the formation of ever smaller fragments, which in the end may turn into several thousand stars. In our galaxy, open star clusters form every few thousand years.

Once the first stars are formed, the largest and hottest stars emit enormous amounts of ultraviolet radiation . This radiation ionizes the gas surrounding the giant molecular cloud, creating an H-II area . Star winds from the heavy stars and the radiation pressure displace the surrounding gas. After a few million years, the first supernova of a star occurs, which ejects more gas from the system. After a few tens of millions of years there is only so much gas left that star formation can no longer occur. Most of the time, only 10% of the initially available gas is used for star formation. The rest will be blown away.

Until then, young star clusters are often still visibly embedded in this molecular cloud or the emission nebula . Well-known examples are the rosette and the cocoon nebula .

As a rule, two or more open star clusters are formed from a molecular cloud. In the large Magellanic Cloud , both Hodge 301 and R136 emerged from gases from the tarantula nebula . An example from our galaxy would be Hyaden and Praesepe . By tracing their motion, it is believed that they formed from the same cloud 600 million years ago.

Sometimes two star clusters that formed at the same time form and form so-called double star clusters . The best-known example in the Milky Way is the double star clusters h Persei and Chi Persei , but ten more are known. Many have been found in the small and large Magellanic Cloud . They are easier to spot in other galaxies, as projection effects in the Milky Way can make stars that do not belong together appear to be close together.

Shape and classification

The number of stars in an open star cluster varies from a few tens of stars to large clusters of a few thousand stars. They usually contain a denser core, which is surrounded by an extensive corona of other stars. The core is usually 3 to 4 light-years in diameter , while the corona extends about 20 light-years from the center. In the core there are around 1.5 stars per cubic light year (the star density in the area around our sun is approx. 0.0035 stars per cubic light year).

Open star clusters are mostly classified according to a scheme developed by Robert Trumpler from 1930. Three pieces of information are required for this. The Roman numerals from I to IV indicate concentration and detachment from the surrounding star field (from strongly to weakly concentrated). The Arabic numerals from 1 to 3 indicate how much the individual stars differ in their brightness (from low to high). The letters p, m, or r indicate whether the cluster has few (poor), average (medium) or many (rich) stars. In addition (optional) three further criteria can be specified: n (nebulosity - nebelig) = interstellar, luminous clouds of matter are embedded in the cluster; e (elongated) = The star cluster appears stretched in one direction; u (unsymmetrical) = The stars in the cluster are scattered in different directions. According to this scheme, for example, the Pleiades are classified as I3rn (highly concentrated with a rich population with clouds of matter), the Hyades are classified as II3m (more scattered and fewer stars).

Number and distribution

There are over 1,000 known open star clusters in our galaxy, but the real number is believed to be up to ten times higher. In spiral galaxies , they are found almost exclusively in the spiral arms. The reason is that, due to the higher gas density, most of the stars are formed here and the star clusters disappear again before they can get beyond the spiral arms. They are concentrated in our galaxy in the galactic plane with an extension of the height of around 180 light years (compared to the radius of the Milky Way of around 100,000 light years)

In irregular galaxies , open star clusters can be found anywhere in the galaxy. Their concentration is greatest where the gas concentration is also greatest. However, they are not found in elliptical galaxies , since the star formation process stopped here many years ago, so that all open star clusters have already dissolved.

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

Composition of the stars

Because open star 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 tens of millions of years. In contrast, older open star clusters have more yellow stars.

Some open star clusters contain hot blue stars that appear younger than the rest of the stars. These blue stragglers are also observed in globular clusters. They are believed to be formed when stars collide and merge, forming a much hotter and heavier star in the process. In any case, the star density is much lower than in globular clusters, so that star collisions cannot explain the number of stragglers. Rather, it is believed that most of them originated in a binary star system. Interactions of the binary star system with other stars then lead to the merging of both stars into one star.

As soon as a star has used up its hydrogen supply and therefore nuclear fusion can no longer take place, it sheds its outer layers and forms a planetary nebula with a white dwarf inside. Most open star clusters, however, disperse before many of their stars reach the white dwarf stage. However, the number of white dwarfs in open star clusters is much lower than expected. One possible explanation is as follows: when a red giant sheds its outer layers and forms a planetary nebula, a small asymmetry in the shed material is enough to give the remaining star a push of a few kilometers per second . This is strong enough to let him escape from the heap.

Fate of the Open Star Clusters

NGC 604 in the Triangle Nebula is a very heavy open star cluster. It is surrounded by an H-II area .

The length of time a star cluster lasts depends mainly on its initial mass. Many open star clusters have been unstable since their formation. Their total mass is so small that the escape speed from this system is less than the average speed of their stars. These star clusters disintegrate within a few million years. As the surrounding gas is blown away by the radiation pressure of the young hot stars, the mass is reduced so that it can be quickly dispersed.

Star clusters with a mass large enough to permanently bind the stars through gravity can exist for tens of millions of years, but here, too, internal and external processes lead to the fact that they are gradually dispersed. If stars come too close inside, this often leads to the fact that the speed of one star is greatly increased, the escape speed of the star cluster is exceeded and it can thus escape it. This leads to the slow dissolution of the star cluster. The time to loss of half of the stars ranges from 150 to 800 million years, depending on the initial density.

On average, every half a million years an open star cluster is destroyed by an external factor such as a collision with a molecular cloud. The tidal forces caused by gravity then lead to the destruction of the structure of the pile. Eventually, the star cluster becomes a band of stars that, although not close enough to be called a cluster, are all connected and moving 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. Such a star association is then also called a motion cluster or motion star cluster . Many of the brightest stars in the Ursa Major 'plow' were formerly an open star cluster, which is now a loose link, the Ursa Major group .

Star formation studies

If you enter the stars of an open star cluster in the Hertzsprung-Russell diagram , then they are mostly on the main sequence . The heaviest stars are a bit off the main sequence and become red giants . The position of these stars can be used to determine the age of the star cluster.

Since all stars in an open star cluster are approximately the same distance from Earth and were formed from the same raw material at approximately the same time, the differences in brightness only depend on the different masses of the stars. This makes open star clusters very useful when studying star evolution . Because if you want to compare two stars of a star cluster, most of the parameters fall out.

The investigation of lithium and beryllium deposits in open star clusters are important clues for the evolution of stars and their internal structures. While hydrogen nuclei cannot fuse to helium below a temperature of 10 million K , lithium and beryllium are destroyed at temperatures of 2.5 million K and 3.5 million K. This means that their occurrence strongly depends on what happens inside the star. The data can be used to infer the age and chemical composition.

Distance measurement

To understand an astronomical object, it is imperative to know its distance. The closer clusters can be measured using two different direct methods. On the one hand, one can determine the parallax , i.e. the apparent displacement of the object in relation to very distant objects, which actually results from the movement of the earth around the sun. The second method is the so-called moving star cluster method (star current parallax, see parallax ). It is based on the fact that the stars in a star cluster move together towards a common vanishing point (vertex). The radial velocity is now determined from the star spectra with the aid of Doppler effect measurements. As soon as one knows the radial velocities, the proper motion and the observed angle from the star cluster to the vanishing point, one can calculate the distance with simple trigonometry. The Hyades are the best known example of this method being used. Their distance is 46.3 parsecs .

As soon as the distance to nearby star clusters is known, techniques can be used for greater distances that build on the data obtained for nearby star clusters. The nearby star clusters are known to be in the main sequence of the Hertzsprung-Russell diagram at a known distance , and so one can easily determine the distance to star clusters that are much farther from Earth.

The closest open star clusters to Earth are the Hyades. However, they are more like a moving star cluster than an open star cluster. The farthest open clusters in the Milky Way is the Berkeley 29 with a distance of around 15,000 Parsec . Open star clusters can be found in many local group galaxies.

The exact distance from open star clusters is important in order to calibrate the period-luminosity relationship of certain sizes of variable stars ( Cepheids and RR Lyrae stars ). These stars are very bright and can still be made out at a very great distance. They are therefore used as a standard candle to calculate the distance to nearby galaxies in the local group.


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