Globular cluster

A globular cluster ( briefly also globular clusters ) is a tight, ball-shaped group of many stars that are gravitationally bound to each other. Typical sizes are some 100,000 stars. Mutual track changes in the densely populated center frequently, which has the spherical shape result. Globular clusters are themselves gravitationally bound to galaxies in their Halo they move widely. They consist mainly of old, red stars, only a few heavy elements contain ("Metal poverty "). This differs significantly from open clusters, which are among the most recent developments in galaxies.

Globular clusters are common. In the halo of the Milky Way about 150 of them have already been found and it is estimated that there are 10-20 more undiscovered. In Halo large galaxies significantly more globular clusters may be present, such as in the halo of the Andromeda galaxy, where there are around 500. The halos giant elliptical galaxies like M87 may contain even 10,000. These globular clusters orbit the galaxy at a distance of 40 kiloparsecs ( approximately 131,000 light years) or more.

In the Local Group all major, massive galaxies have a halo system of globular clusters. The Sagittarius Dwarf Galaxy and the Canis Major dwarf galaxy just seem their globular clusters (such as Palomar 12) of the Milky Way to pass. This shows how galaxies may have received their globular clusters.

The star of such clusters - so-called extreme population II stars - are all about the same age and show no spectral lines of heavier elements. These spectra indicate a high star age, as are the heavy elements in the course of billions of years, eg, by supernovae. Old stars that have emerged in the early universe, may contain such elements in their cases hardly. Young stars, in particular Population I stars, however, are " recycled": they were formed from material that was already partly melted in older stars to heavy elements (see also section metal deposits ).

Although the stars in globular clusters of the first among which were formed in galaxies, their origins and their role in galactic evolution are still unclear. Meanwhile, it is assumed that globular clusters differ significantly from elliptical and dwarf galaxies have formed rather as part of a galaxy rather than as individual separate galaxy.

In Halo some of elliptical galaxies also very young globular clusters can be observed. Of these galaxies, it is believed that they originated from the fusion of two or more original galaxies. Such collisions solve a wave of star formation from ( starburst ), at the latest findings also globular clusters can be formed again, so that several generations of globular clusters are found in the halo of such a galaxy.

• 4.1 radii
• 4.2 brightness
• 4.3 N-body simulations
• 4.4 intermediate forms

Observation history

The first globular cluster, M22, was discovered in 1665 by the German amateur astronomer Johann Abraham Ihle. However, due to the small opening of the early telescopes was the resolution is so low that no individual stars could be observed in the cluster, but only a round fuzzy patch was seen. Nicolas Louis de Lacaille mentioned several such objects in his catalog, published 1751-52, in particular the later as NGC 104, NGC 4833, M55, M69 and NGC 6397 as designated systems. The M before a number stands for the 1781 published in final form catalog of Charles Messier, while NGC refers to the New General Catalogue by Johan Dreyer (1880 ). The first globular clusters could be observed in the individual stars, and also the nearest to us is the cataloged by Messier in 1764 as M4.

William Herschel began in 1782 to make a new survey. With more powerful telescopes, he was able to show individual stars and found 37 more star clusters in all 33 then-known globular clusters. In its second catalog of deep-sky objects, which appeared in 1789, he first used to describe the name of globular clusters.

The number of detected globular clusters grew continuously, from 83 in 1915 to 93 in 1930 and 97 in 1947. Nowadays are known in the halo of the Milky Way 151 globular clusters, and it is believed that there exist a total of 160 to 200 such clusters. This undiscovered globular clusters are probably covered by the gas and dust of the Milky Way. Most globular clusters are visible on the southern sky.

1914 Harlow Shapley began with studies of globular clusters, which he published in 40 works. He examined the Cepheids variable stars of a given type in the star clusters and used their periodic brightness variations for distance determination.

Most globular clusters of the Milky Way are located near the galactic bulge. 1918 Harlow Shapley made ​​the highly asymmetric distribution to use to determine the extent of our galaxy. He went from an approximately uniform spherical distribution of globular clusters from around the galactic bulge and used the position of the star clusters in order to identify the position of the sun relative to the galactic center. Since his results were consistent with former notions contrary, he concluded that the extent of the galaxy was much greater than previously thought. Shapley's estimate is at least of the same order as the now accepted value.

Shapley also found out by the fact that the Sun is very far from the center of the Milky Way away. This went against what was then considered as one perceives in any direction about the same number of stars in the night sky. Stars lie in the galactic disk, an area with a lot of gas and dust, which absorbs the most light. The globular clusters are, however, outside the galactic disk in the galactic halo and therefore can be seen even from a distance.

Henrietta Hill Swope and Helen Hogg also studied star clusters. In the years 1927 to 1929 Shapley and Sawyer began to categorize the clusters according to the concentration of stars in the center of the star cluster. The clusters with the highest concentration were in Class I.. With decreasing concentration of eleven classes were formed up to class XII. These classes have been internationally known as the Shapley - Sawyer Concentration Classes. Sometimes Arabic instead of Roman numerals are used.

Composition

Globular clusters are generally composed of hundreds of thousands of metal-poor stars. Such stars are also found in the bulge of spiral galaxies, but not in this quantity in a volume of a few Kubikparsec. Globular clusters also contain no gas and dust, as from this previously star formation.

Although globular clusters may contain many stars, they are not a suitable place for a planetary system. The planetary orbits are unstable as passing stars disturb the web. A planet that orbited a star at a distance of one astronomical unit would survive only about 100 million years on average in a globular cluster 47 Tucanae like. However, it has detected a planetary system orbiting the pulsar (PSR B 1620-26 ), which belongs to the globular cluster M4.

With few exceptions, you can assign an accurate age each globular clusters. As the stars are mostly all in the same phase of stellar evolution in the cluster, it seems likely that they were formed at the same time. In no known globular clusters still do stars. Consequently, it is in the globular clusters are the oldest objects in the Milky Way that are incurred when the first stars formed.

Some globular cluster Omega Centauri as in the halo of our Milky Way and Mayall II in the halo of the Andromeda galaxy ( M31 ) are compatible with many millions of solar masses particularly heavy and contain multiple stellar populations. In both it is believed that they were the nuclei of dwarf galaxies and were captured by a larger galaxy. It is suspected that many globular clusters with heavy nuclei (such as M15) contain black holes.

Metal deposits

Globular clusters usually consist of Population II stars, which contain little metal compared to Population I stars like the sun. ( For astronomers, the term metal all the elements heavier than helium, such as lithium and carbon, see metallicity )

The Dutch astronomer Pieter Oosterhoff noticed that there is a second population of globular clusters, which received the name of Oosterhoff group. In this group, the periodicity of RR Lyrae stars is longer. Both groups contain only weak lines of metallic elements, but the stars of Oosterhoff type I clusters ( OOI ) are not as severe as those in type II ( ooii ). We are talking about Type I as "metal- rich ", while Type II referred to as " metal-poor ". In the Milky Way can be found the metal-poor star clusters in the outer halo and the metal-rich near the bulge.

These two populations were observed in many galaxies (especially in massive elliptical galaxies). Both groups have approximately the same age ( about as old as the universe itself), but differ in metal deposits. Many scenarios have been proposed to explain the existence of two different types, this includes, for example, the merger of galaxies with high gas deposits, the accumulation of dwarf galaxies, and the existence of multiple phases of star formation in a galaxy.

Since the Milky Way metal-poor star clusters are located in the outer halo, it seems likely that this type II clusters were captured by the Milky Way and are not the oldest objects that were formed in the Milky Way, as previously thought. The differences between the two globular cluster types would then be explained by a temporal difference in their formation.

Unusual star

Globular clusters have very high stellar density, which leads to greater mutual influence and relatively frequent Beinahkollisionen the stars. This exotic stars as blue stragglers, Millisekundenpulsare and light X-ray binaries are found much more frequently. A Blue stragglers formed from two stars, possibly from the collision of a binary system. The resulting star has a higher temperature than similar stars in the cluster with the same brightness, and is therefore outside the main -sequence stars.

Black Holes

Astronomers looking since the 1970s by black holes in globular clusters. For this you need an accuracy, as it is currently only possible with the Hubble Space Telescope. In independent programs in a medium weight black hole of 4,000 solar masses in the globular cluster M15 ( the constellation Pegasus ) and a 20,000 solar masses heavy black hole in the globular cluster Mayall II was discovered in the halo of the Andromeda galaxy.

These are of interest because they were the first black holes, assume an intermediate size between a conventional, resulting from a star, the black hole and the supermassive black holes exist in the centers of galaxies such as the Milky Way. The mass medium black holes is proportional to the mass of the star cluster, which are at the same mass ratio as the supermassive black holes and their host galaxies. The discovery of medium-heavy black holes in globular clusters is controversial and the observations can be explained without the assumption of a central black hole.

Black holes can be indeed in the center of globular clusters are located (see above M15), but need not necessarily be present. The densest objects migrate to the center of the cluster due to mass segregation. These are old globular cluster white dwarfs and neutron stars mainly. In two scientific work under the direction of Holger Baumgart has been shown that as the mass - light ratio can rise sharply even without black holes in the center. This applies to both M15 and for Mayall II

In summer 2012, they discovered with radio telescopes that Messier 22 even has two black holes in the constellation Sagittarius, what had previously been on the grounds of celestial mechanics for excluded. The two radio sources each have 10-20 solar masses.

Hertzsprung -Russell diagram

The Hertzsprung -Russell diagram ( HR diagram ) is a graph showing the stars with their absolute brightness and color. The color index is the difference between the brightness of the star in blue light ( B) and the brightness in yellow to green light ( V). Large positive values ​​indicate a red star with a cold surface temperature, while negative values ​​indicate a blue star with a hot surface.

If one enters star from the solar neighborhood in the graph, then there are many of them on this chart in a sweeping curve, the so-called main sequence. The diagram also includes stars in the later stages of their evolution, which have moved away somewhat from the main sequence.

Since all the stars of a globular cluster have approximately the same distance from the Earth, its absolute brightness for visible or apparent brightness by the same value differs. It is estimated that the main sequence stars in the globular clusters in the graph in the same curve are like the stars in the solar neighborhood. The accuracy of this estimate was confirmed by having compared the brightness of adjacent fast variable stars, such as RR Lyrae stars and Cepheids with those in clusters.

Since these curves coincide in the HR diagram, one can determine the absolute magnitude of the main sequence stars in the cluster. With the help of the apparent brightness of the star is obtained as the distance of the star cluster to Earth. This distance determination is made from the difference between the apparent and the absolute brightness of the distance module.

When the stars of a globular cluster are plotted in an HR diagram, are the most for a well -defined curve. This differs from stars in the solar neighborhood, since no stars of different origin and age were collected. The shape of the curve is characteristic of a group of stars that have formed around the same time with the same material and differ only by their mass. Because different the positions of stars from globular clusters in the HR diagram only by their age, one can conclude on their age.

The heaviest stars in globular clusters are the brightest and the first to become a giant star. Also stars with lower mass will turn into giant Later. One can determine the age of a globular cluster thus also, by holding out by stars who have already reached the stage of a giant star. These form a " bend " in the HR diagram and connect the lower right end of the line with the main sequence. From the absolute magnitude of this " bending " can be directly the age of the globular cluster read, so that one could draw an axis for the age of globular clusters parallel to the lightness axis in an HR diagram. You might as well but also determine the age, by examining the temperature of the coldest white dwarfs in this globular cluster.

The typical age for globular clusters is 12.7 billion years. In comparison, open clusters with only ten million years are much younger.

The age of globular clusters is the age of the entire universe limits. The lower limit brought the cosmology in embarrassment. During the early 1990s, astronomers found globular clusters, which appeared to be older than it allows the cosmological model. However, could be shown by better measurements of the cosmological parameters such as the COBE satellite that previous measurements were in error.

Through studies of the occurrence of metals ( metals in astronomy items that are heavier than helium), one can determine the concentration of the original substances and transmit these values ​​then the entire Milky Way.

Recent studies with space-based satellites and telescopes of the 8 -meter class have shown that all studied in detail globular clusters do not consist of a chemically homogeneous population. Thus, variations have been demonstrated to the abundances of elements such as carbon, oxygen, nitrogen, sodium and aluminum spectroscopy at various globular clusters and photometrically the presence of multiple main sequences. A particular example is Omega Centauri, in which three separate main rows and five distinct red giant branches could be detected. Therefore, it might have come in the development of globular clusters to several phases of star formation.

Shape

In contrast to open clusters most stars her life are gravitationally bound in globular clusters. An exception is made in strong interactions with other massive objects. This leads to the dispersion of the stars.

The formation of globular clusters is a poorly understood phenomenon. Through observations of globular clusters could be shown that they have formed in areas where a strong star formation was in progress and where the interstellar medium had a greater density than in the average star-forming regions. Globular clusters are formed usually in star-forming regions and in interacting galaxies.

After the stars have formed, they begin to interact gravitationally with each other. This constantly change with every star magnitude and direction of the velocity, so that one can not draw any conclusions about their original speed after a short time. This characteristic interval is called the relaxation time. It depends on the length of time it takes for a star to pass through the clusters and the number of stars in the system. The relaxation time varies from cluster to cluster, but is on average one billion years.

Although globular clusters usually have a spherical shape, also elliptical shapes are possible by tidal effects.

Radii

Astronomers characterize the shape of a globular cluster by standard radii. This is the core radius (core radius) (rc), the half- light radius (half -light radius) ( rh ) and the tidal radius ( tidal radius) (rt). The brightness decreases in their entirety with increasing distance from the core and the core radius is the distance at which the surface brightness is dropped to the half. A similar figure is reported as the half- light radius, which marks the distance from the center, up to the half of the total light is collected. This value is typically greater than the core radius.

It must be noted that the half-light radius of stars are counted that are located in the outer part of the cluster if they lie on the line of sight through the core, so that theorists nor the half- mass radius (half -mass radius) (rm) need. This radius is the size of the area that contains half of the mass of the star cluster. If the half- mass radius is very small compared to the overall size, it has a dense core. For example, the globular cluster M3 has a visible extent of 18 arcmin, but only a half- mass radius of 1.12 arc minutes.

The tidal radius is the distance from the center of the core, in which the gravitational influence of the galaxy is larger than that of the other stars in the cluster. This is the distance that can escape the globular clusters in which individual stars. The tidal radius of M3 is about 38 '.

Brightness

Are you the brightness of a globular cluster as a function of radius, so takes in most globular clusters brightness with increasing distance from the nucleus, but falls at some point again. This is usually away for one to two parsecs from the core. However, 20% of globular clusters have experienced the process of Kernkollabierung. For them, the light intensity decreases toward the center to resistant. An example of such a Kernkollabierung can be found at globular cluster M15.

It is believed that it comes to Kernkollabierung when in a globular cluster heavy stars collide with less severe concomitant stars. Thus they lose kinetic energy and begin to move towards the core. Over a prolonged period of time this leads to a mass concentration in the core.

The brightness distribution of the globular clusters in the halo of the Milky Way and in the halo of the Andromeda galaxy ( M31 ) can be thought of as a Gaussian curve. A Gaussian curve can be with the help of two indications, the average brightness of Mv and variance σ2, characterize. This brightness distribution of a globular cluster Globular Cluster Luminosity Function is called ( GCLF ). The GCLF is also used as a standard candle to determine the distance to other galaxies. It does so, however from the presumption that the globular clusters in the halo of the galaxy under study behave exactly as in the halo of the Milky Way.

N-body simulations

To calculate the motions of the stars in the globular cluster, we examined the interactions between stars in globular clusters. Since everyone is at the same time of the N stars in star clusters with N-1 stars in interaction, one has to deal with an N- body problem. Using simple computer algorithms, the time required would be proportional to N2, so that an accurate simulation can take a lot of computing time. To simulate the star to save time, you can combine them dynamically to groups of stars with similar position and velocity. The motion will then be described with the so-called Fokker-Planck equation. This can be solved as an equation or calculated using the Monte Carlo simulation. However, the simulation is difficult, when one adds the effects of binary stars and the gravitational forces of the Milky Way in the model.

The results of N- body simulation show that the stars can take unusual movements by the star cluster. They trace loops or fall directly into the core instead of orbiting it. Because of the interactions with other stars individual stars can not get enough speed to escape the star clusters. Over a longer period of time this leads to the resolution. The resolution usually occurs during a period of 1010 years.

Make binary star systems with up to 50 % of the stars for a significant proportion of a globular cluster. It was shown by simulations that binary star systems to stop the process of Kernkollabierung and can even be reversed. If a binary star interacts with a single star, the double stars the individual star are closely bound to it and transfer kinetic energy. When the massive stars are faster through this process in the cluster, the core expands further and does not collapse so easily.

Intermediate forms

There are between two types of star clusters, the globular clusters and open clusters, no clear dividing line. For example, is located in the southern part of the halo of the Milky Way star cluster BH 176, which is both.

2005 astronomers found a completely new type of star clusters in the halo of the Andromeda galaxy. These objects of the same globular cluster in the number of stars, the age and metallicity. The difference lies in the much larger extent of many hundreds of light-years, so that this large cluster have a significantly lower density. They lie with their size between the globular clusters and the spherical dwarf galaxies.

As these star clusters have formed, is not known. However, they could have originated in a similar way as the globular clusters. It is also unknown why the Andromeda galaxy has such star clusters, while it does not seem to give such an object in the halo of the Milky Way, and whether there are other galaxies with these large cluster.

Tidal effect

If a globular cluster is near a very heavy object, such as the core region of a galaxy, then act on it gravitational forces. The difference between the gravitational force on the location of the cluster, which is the mass of the next, and to the place which is away from the crowd most, tidal power is called. Crossing the object plane of a galaxy, one can speak of a " tidal surge ".

The tidal surge causes of the stars of the cluster that have happened to a lot of kinetic energy, many are torn, so that the pile then drags a stream of stars behind. This can be in small clusters contain a large part of the original stars of the cluster, and it can come in these streams to form lumps.

For example, the small globular cluster Palomar 5 was, as he crossed the galactic disk of the Milky Way, torn apart. He is now close to the apogalaktischen point of its path and was stretched to a length of 13,000 light years. The tidal forces have thrown many stars of Palomar 5 and it is expected that further crossings of the galactic disk of the Milky Way will transform him into a single stream of stars, which will then travel through the halo of the Milky Way.

Tidal effects provide many remaining stars of the heap additional kinetic energy, which heats up the pile and dramatically increases the dissolution rate. A tidal shock also leads to faster core collapse.