Globular cluster

A globular cluster is a cluster of a large number of gravitationally bound stars whose density is spherically symmetric, decreasing equally in all directions from the center, where the stars are very close together, to the edge. Typical globular clusters contain several hundred thousand stars. The high density of stars near the center leads to mutual orbital changes, which results in the spherical appearance.

Globular clusters, for their part, are gravitationally bound to galaxies in whose halo they move over a wide area. They consist mainly of old, red stars, which contain only a few heavy elements ("metal poverty"). This clearly distinguishes them from open star clusters, which are among the youngest formations in galaxies.

Globular clusters are common. In the Milky Way halo, about 150 are known and it is estimated that more are still undiscovered. In the halo of the Andromeda Galaxy, there are about 500 globular clusters. The halos of giant elliptical galaxies like M87 may contain as many as 10,000. These globular clusters orbit the galaxy at a distance of 40 kiloparsecs (about 131,000 light-years) or more.

In the Local Group, all the larger, massive galaxies have a halo system of globular clusters. The Sagittarius dwarf galaxy and the Canis Major dwarf galaxy appear to have just given their globular clusters (such as Palomar 12) to the Milky Way. This shows how galaxies may have preserved their globular clusters.

The stars 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 stellar age, since the heavy elements are formed only in the course of billions of years, e.g. by supernovae. Old stars that were formed in the early universe can therefore hardly contain such elements in their envelopes. Young stars, in particular Population I stars, on the other hand, are "recycled": they were formed from material (including heavy elements), some of which had already been formed in older stars by nuclear fusion (see also the section on metal deposits).

Although globular cluster stars were among the first to form in galaxies, their origins and role in galactic evolution are still unclear. Globular clusters are now thought to be significantly different from elliptical dwarf galaxies and to have formed as part of a galaxy rather than as a single separate galaxy.

Very young globular clusters can also be observed in the halo of some elliptical galaxies. These galaxies are thought to have formed from the merger of two or more parent galaxies. Such collisions trigger a wave of star formation (starburst), during which globular clusters can be formed again according to the latest findings, so that several generations of globular clusters can be found in the halo of such a galaxy.

Observation History

The first globular cluster, M22, was discovered in 1665 by the German amateur astronomer Johann Abraham Ihle. With the telescopes of his time, the resolving power was still so low that only a diffuse, round spot could be seen and not yet individual stars in the cluster.

Nicolas Louis de Lacaille mentioned several such objects in his 1751-1752 catalogue, notably those later named NGC 104, NGC 4833, M55, M69, and NGC 6397. The M before a number here stands for Charles Messier's catalogue published in final form in 1781, while NGC refers to Johan Dreyer's New General Catalogue (1880). The first globular cluster in which individual stars could be observed was catalogued by Messier in 1764 as M4. M4 is the closest globular cluster to Earth.

William Herschel began to make a new survey in 1782. Using more powerful telescopes, he was able to detect individual stars in all 33 globular clusters known at the time, and found 37 more. In his second catalogue of deep-sky objects, published in 1789, he used the term globular cluster to describe them for the first time.

The number of globular clusters discovered increased steadily, from 83 in 1915 to 93 in 1930 and 97 in 1947. Today, 151 globular clusters are known in the Milky Way halo, and another 10 to 50 are thought to lie behind the Milky Way's gas and dust. Most globular clusters can be seen in the southern sky.

In 1914 Harlow Shapley began studies of globular clusters, which he published in 40 papers. He studied the Cepheids, variable stars of a certain type, in the clusters and used their periodic brightness variations to determine distances.

Most globular clusters in the Milky Way are located near the galactic bulge. In 1918, Shapley took advantage of the highly asymmetric distribution to determine the extent of the Milky Way. He assumed a roughly uniform spherical distribution of globular clusters around the galactic bulge and used the position of the clusters to figure out the position of the Sun relative to the galactic center.

Shapley found that the Sun is very far from the center of the Milky Way, and concluded that the extent of the galaxy was much greater than previously thought. His estimate is, after all, of the same order of magnitude as the value accepted today.

This contradicted the then-current model of the universe, since one perceives roughly the same number of stars in each direction in the night sky. In the meantime, we know that there is still a lot of gas and dust between the stars that form the galactic disk, which absorbs most of the light from the galactic center. The globular clusters, on the other hand, are located outside the Galactic disk in the Galactic halo, so they are visible from greater distances. With the assumption of an approximate uniform distribution over the galactic disk, the true location and extent of the Milky Way thus became roughly discernible for the first time.

Henrietta Hill Swope and Helen Hogg also studied star clusters. In 1927 to 1929, Shapley and Sawyer began to categorize star clusters according to the concentration of stars in the center of the cluster. The star clusters with the greatest concentration were assigned to Class I. As the concentration decreased, eleven more classes were formed up to Class XII. These classes became known internationally as the Shapley-Sawyer Concentration Classes. Sometimes Arabic numbers are used instead of Roman numerals.

The globular cluster M75 is a very dense class I globular cluster.

Composition

Globular clusters generally consist 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 cubic parsecs. Globular clusters also do not contain gas and dust, because stars have been formed from them before.

Although globular clusters can contain many stars, they are not a suitable place for a planetary system. The planetary orbits are unstable because passing stars disrupt the orbit. A planet orbiting a star at a distance of one astronomical unit would survive on average only about 100 million years in a globular cluster like 47 Tucanae. However, a planetary system has been found (PSR B1620-26 b) orbiting the pulsar (PSR B 1620-26), which belongs to the globular cluster M4.

With few exceptions, each globular cluster can be assigned an exact age. Since the stars in the cluster are mostly all in the same phase of stellar evolution, it is reasonable to assume that they formed at the same time. No stars are still forming in any known globular cluster. Consequently, globular clusters are the oldest objects in the Milky Way, formed when the first stars formed.

Some globular clusters, such as Omega Centauri in the Milky Way halo and Mayall II in the Andromeda Galaxy halo (M31), are particularly heavy, with many millions of solar masses, and contain multiple populations of stars. Both are thought to have been the cores of dwarf galaxies and to have been captured by a larger galaxy. Many globular clusters with heavy cores (like M15) are thought to contain black holes.

Metal deposits

Globular clusters consist mostly of Population II stars, which contain little metal compared to Population I stars such as the Sun. In astrophysics, the term metal includes all 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 was named the Oosterhoff group. In this group the periodicity of RR Lyrae stars is longer. Both groups contain only faint lines of metallic elements, but the stars in the Oosterhoff type I clusters (OoI) are not as heavy as those in type II (OoII). Thus, Type I is referred to as "metal-rich", while Type II is referred to as "metal-poor". In the Milky Way, the metal-poor clusters are found in the outer halo and the metal-rich ones near the bulge.

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

Since in the Milky Way the metal-poor star clusters lie in the outer halo, the assumption is obvious that these Type II star clusters were captured by the Milky Way and are not the oldest objects formed in the Milky Way, as assumed so far. The differences between the two globular cluster types would then be explained by a temporal difference in their formation.

Unusual stars

Globular clusters have a very high stellar density, which leads to greater mutual interference and relatively frequent near-collisions between stars. As a result, exotic stars such as blue stragglers, millisecond pulsars, and light X-ray binaries are much more common. A blue straggler is formed from two stars, possibly from the collision of a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same brightness and is therefore outside the main sequence stars.

black holes

Astronomers have been searching for black holes in globular clusters since the 1970s. This requires a level of precision that is currently only possible with the Hubble Space Telescope. Independent programs have discovered a medium-gravity black hole of 4,000 solar masses in the globular cluster M15 (constellation Pegasus) and a 20,000 solar mass black hole in the globular cluster Mayall II in the halo of the Andromeda Galaxy. These are of interest because they were the first black holes to occupy an intermediate size between a conventional black hole formed from a star and the supermassive black holes that exist at the centers of galaxies such as the Milky Way. The mass of these intermediate-mass black holes is proportional to the mass of the star cluster, and they have the same mass ratio as the supermassive black holes with their surrounding galaxies. However, the discovery of intermediate-mass black holes in globular clusters is controversial, and the observations can be explained without assuming a central black hole.

Black holes can be found in the center of globular clusters (see M15 above), but they do not necessarily have to be there. The densest objects migrate to the cluster center due to mass separation. In old globular clusters these are mainly white dwarfs and neutron stars. In two scientific papers led by Holger Baumgart it was shown that this way the mass-to-light ratio can increase strongly even without black holes in the center. This is true for M15 as well as for Mayall II.

In the summer of 2012, radio telescopes discovered that Messier 22 in the constellation Sagittarius even contains two black holes, which was previously considered impossible for reasons of celestial mechanics. The two radio sources each have 10-20 solar masses.

The globular cluster M15 has a 4000 solar mass black hole in its core.