A galaxy is a large collection of stars , planetary systems , gas nebulae , dust clouds , dark matter and other astronomical objects with a total mass of typically 10 9 to 10 13 solar masses (M ☉ ) bound by gravity . Their diameter can be several hundred thousand light years . While large galaxies often form the structure of spirals , dwarf galaxies are mostly of an irregular type . There are also other types and forms . The Milky Way , home galaxy of the Solar System , is a barred spiral of around 1.5 trillion M ☉ with about 250 billion stars. With the latest technology, more than 50 billion galaxies can be observed from Earth. Since 2016, research has assumed that there are around one trillion galaxies in the observable universe.
The name comes from the synonymous ancient Greek γαλαξίας galaxías and goes back to an ancient legend , according to which it is the splashed milk ( γάλα gála ) of the goddess Hera when she wanted to breastfeed Heracles . In German, the galaxy ( singular ) is specifically the Milky Way. In English ( galaxy , for the galaxy also Galaxy ) there is a similar distinction. Alexander von Humboldt used the term "world island".
Galaxies vary greatly in appearance (morphology), size and composition. With a diameter of around 100,000 light years, the Milky Way is one of the larger galaxies. Its closest neighboring galaxy of comparable size is the Andromeda Galaxy at a distance of around 2.5 million light years. Together with other galaxies of lower mass, both galaxies form the local group . Galaxies often appear in groups or clusters of up to a few thousand members.
History of exploration
Before the power of astronomical telescopes was sufficient to break up distant galaxies into individual stars, they appeared as " nebulae ". For a long time it was unclear whether these “spiral nebulae” belong to the galaxy or whether they form their own star systems. Immanuel Kant already suspected milky way-like star systems in the “misty stars”, and in 1923 Edwin Hubble succeeded in clarifying this question. He determined the distance to the Andromeda Nebula and found that it is far too far away to belong to the Milky Way, i.e. it is a galaxy of its own.
According to their shape, galaxies are divided into various main groups and subgroups of the so-called Hubble classification (see morphology ). This classification was established by Edwin Hubble and, with some extensions, is still in use today, although it was originally only based on a small sample of nearby and bright galaxies that could then be observed in the optical wavelength range. The Hubble classification is purely empirical and says nothing about the evolution of galaxies. The individual types are:
- Elliptical galaxies do not show any special substructures. The lines of equal brightness have the shape of an ellipse. The elliptical galaxies have a uniform decrease in brightness from the inside out. They contain almost no cold gas, so their star formation rate approaches zero. Their spectrum is dominated by old and therefore red stars. Elliptical galaxies are dividedinto classes E0 (circular) to E7 (highly elliptical)according to their numerical eccentricity . The number after the E indicates the first decimal place of the eccentricity, i.e. a class E7 galaxyhas approximately 0.7 eccentricity. The absolute brightnesses of elliptical galaxies cover a large area. The brightest galaxies are mostly elliptical galaxies and in this case they were probably formed by the merging of several small to medium-sized galaxies. Elliptical galaxies are often found in large galaxy clusters .
- Lenticular (lens-shaped) galaxies belong to class S0 . Such galaxies have properties of both elliptical and spiral galaxies. They have a core that corresponds to that of the spiral galaxies, but their galactic disk does not contain any spiral arms, but is roughly evenly bright (example: M 102 ).
- Spiral galaxies (also known as spiral nebulae out of date).
- with a regular spiral have a spherical core, the so-called bulge , and spiral arms extending from it , which lie in a flat disk component. While the bulge resembles an elliptical galaxy and no longer shows any star formation, the gas and dust present in the disk allow stars to form in the spiral arms. Therefore, the spiral arms appear mostly blue in pictures and the bulge mostly reddish. The spiral arms are further divided into the classes Sa, Sb and Sc . Sa- type galaxies have a very pronounced core and tightly wound spiral arms (example: Sombrero galaxy M 104). The type Sc has a relatively weak galactic core, extremely loosely wound spiral arms and therefore sometimes almost the shape of an intertwined "S" (example: the triangular nebula M 33). Together with the lenticular galaxies, Sa, Sb and Sc are also called disk galaxies .
- with bar spiral ( bar spiral galaxies ) have a long bar starting from the center, to which the spiral arms are connected (example: M 109 ). Just like the spiral galaxies, they are subdivided into the classes SBa, SBb and SBc with increasing development of the core and opening of their spiral arms . The Milky Way is one such bar spiral.
- Irregular (irregular) galaxies have neither spiral arms nor elliptical shapes. On average, they are weaker than elliptical and spiral galaxies. This group mostly includes dwarf galaxies.
In addition to the Hubble classification, there are also other classifications, for example according to Gérard-Henri de Vaucouleurs or the Yerkes classification, which are, however, used less often. The rough classifications often do not do justice to the multitude of galaxy types found, which is why many other characteristics are used to describe galaxies.
More galaxy types
There are other forms of galaxies that cannot be classified in the above scheme or complement it. These include:
- Dwarf galaxies are lower brightness galaxies, they are much more numerous than giant galaxies. In contrast to these there are mainly elliptical (dE), spheroidal (dSph) and irregular (dIrr) dwarf galaxies. The elliptical dwarf galaxies can be divided into compact (cE) and diffuse galaxies. The next compact dwarf elliptical galaxy, which is also the only one in the Local Group , is M32 . Compact elliptical dwarf galaxies are more similar in their morphology to the large elliptical galaxies. They have a more pronounced central region than the diffuse ones, which indicates a different history.
Interacting galaxies are encounters between two or more galaxies. Since different nuclei and also tidal arms can be observed depending on the stage of interaction, these systems cannot be divided into the Hubble classification scheme.
- Tidal dwarf galaxies (TDG) are galaxies that result from the interaction of two gas-rich galaxies in long tidal arms from gas and dust.
- Polar ring galaxies describe very rare results of the merging of two galaxies. As a result of gravitational interaction, two galaxies came so close that the interaction partner with lower mass was often torn apart and its stars, gas and dust were captured in the gravitational field of the other galaxy. Depending on the orientation of the collision, this sometimes results in a ring of stars that surrounds a galaxy like an additional spiral arm. Since this ring is mostly aligned perpendicular to the main plane of the galaxy, one speaks of polar ring galaxies (example: wagon wheel galaxy ). There are indications that the Milky Way also has such a polar ring.
- As active galaxies is called i. General a subgroup of galaxies with a particularly bright nucleus (also called AGN, Active Galactic Nucleus). This high luminosity very likely indicates an active massive black hole in the center of the galaxy. This group includes:
- Radio galaxies emit a great deal of synchrotron radiation in the area of radio waves and are therefore alsoexaminedwith the help of radio astronomy . Often up to two streams of matter, so-called jets, are observed in radio galaxies. Examples of strong radio galaxies are: Centaurus A , Perseus A, Cygnus A and M 87 in the constellation Virgo .
- Seyfert galaxies have a very bright, punctiform core and show prominent emission lines in the area of the visual spectrum. About one percent of the main galaxies belong to this category.
- BL Lacertae objects are active galaxies whose spectrum shows no absorption and emission lines . Although they are sometimes very bright, their redshift can therefore be difficult to determine. Their brightness is highly variable. Along with the quasars, BL-Lac objects are among themost luminous known objects.
- Quasars are the objects with the greatest absolute brightness that are observed. Due to the great distance of these objects, originally only their compact, punctiform core could be observed, hence the name quasar (= quasi stellar object).
- Starburst galaxies are galaxies with a very high rate of star formation and the resulting intense radiation. A well-explored starburst galaxy is M 82 .
- Ultra-diffuse galaxies are low-luminosity galaxies. This broad type includes massive galaxies like Dragonfly 44 in the Coma galaxy cluster , which has an extremely high proportion of dark matter. Its mass is close to that of the Milky Way, but its light emission is lower by a factor of 100. There are also ultra-diffuse galaxies that seem to be almost completely devoid of dark matter. An example of this is the almost transparent galaxy NGC 1052-DF2 . Its size is comparable to that of the Milky Way, but it has around 200 times fewer stars than it.
Origin and development
The microwave background reflects the distribution of matter in the universe 380,000 years after the Big Bang . At that time the universe was still very homogeneous: the density fluctuations were in the order of 1 in 10 5 .
In the context of cosmology , the increase in density fluctuation can be described by gravitational collapse . Above all, dark matter plays a major role, as it gravitationally dominates over baryonic matter. Under the influence of dark matter, the density fluctuations grew until they collapsed into dark halos . Since only gravity plays a role in this process, it can now be calculated with great accuracy (e.g. Millennium simulation ). The gas followed the distribution of dark matter, fell into these halos, condensed and the stars formed. The galaxies began to form. The actual formation of the galaxies is not understood, however, because the stars just created influenced the incoming gas (the so-called feedback), which makes a more precise simulation difficult.
After their formation, the galaxies have evolved. According to the hierarchical model of galaxy formation, galaxies grow mainly by merging with other galaxies. Then the first proto-galaxies, which were still relatively low in mass, formed in the early cosmos under the influence of gravity. Gradually, so the idea, these galaxy precursors merged through collisions to form full-grown specimens such as the Milky Way and even larger galaxies. The relics of such collisions can still be seen in the Milky Way as so-called star currents . These are groups of stars whose common movement pattern points to an origin outside the Milky Way. They are attributed to smaller galaxies that were torn apart and swallowed up by the Milky Way by tidal forces.
A model of galaxy formation assumes that the first gas clouds developed into spiral galaxies through rotation . According to this model, elliptical galaxies only emerged in a second stage through the collision of spiral galaxies. According to this idea, spiral galaxies can grow in that nearby (dwarf) galaxies fall into their disc and dissolve there ( accretion ).
The observation of highly redshifted galaxies makes it possible to understand this development. In particular, deep surveys such as the Hubble Deep Field had great success . Overall, the formation and development of galaxies as a current research subject is not yet complete and therefore cannot yet be explained with sufficient certainty.
The latest studies assume that in the center of every galaxy there is a supermassive black hole that was significantly involved in the formation of the galaxy. This is how galaxies emerged from huge gas clouds (hydrogen), the centers of which collapse into supermassive black holes. These in turn heated the surrounding gas to such an extent that stars and ultimately planets formed through compression. The size of the galaxies and their centers (supermassive black holes) are directly related: the larger a galaxy, the larger the center.
Formation of the spiral arms
Even if in spiral galaxies it looks as if the galaxy only exists within the spiral arms , there are also a relatively large number of stars in less luminous parts of the galaxy disk.
A galaxy does not rotate rigidly like a wheel; rather, the individual stars run out of and into the spiral arms. The spiral arms are visible expressions of standing density waves (like sound waves in air) that run around in the galactic disk. This theory was first proposed by Chia-Chiao Lin and Frank Shu in the 1960s. After that, the density of matter in the spiral arms and in the central bar is increased, so that a relatively large number of bright, blue, short-lived stars emerge from the interstellar medium. This makes these areas appear brighter than their surroundings. These density waves arise from the interaction of all orbits of the stars, because the stars do not move around a fixed center (a black hole in the center of the galaxy) like the planets in the solar system , because the total mass of the galaxy is not concentrated enough for this. Therefore, a star does not return to its starting point after one orbit around the center of the galaxy, so the orbits are not ellipses , but have the shape of rosettes . Density waves are created when many stars move at the same speed. In a barred spiral galaxy, for example, all orbits are aligned in the same way, whereas in a pure spiral galaxy, they are still shifted against each other. The orbits are synchronized by means of gravitational feedback . Using computer simulations that also take interstellar gas into account, the formation of spiral arms can even be modeled. It shows that these are by no means static, but arise and disappear. After that, every galaxy goes through a cycle (duration approx. Ten billion years) of constant conversion from the bar to the spiral shape and back. Furthermore, the spiral arms disrupt the trajectories of the stars, which leads to the so-called Lindblad resonances .
When galaxies collide, gas clouds within the galaxy can become unstable and collapse. This creates new stars. The stars of the interacting galaxies themselves very rarely merge with one another in this process. The merged galaxies shine in the blue light of the newly formed stars. Such an interaction can last hundreds of millions of years. The shapes of the galaxies can change significantly. Interactions between two galaxies are quite common. The stars can be greatly deflected by the effects of the galaxy's gravity. Examples of such colliding galaxies. The systems M 51 - NGC 5195 and the “antenna” galaxies NGC 4038 - NGC 4039 (see illustration) in the constellation of eagles are partially merged .
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