As a galaxy ( ancient Greek γαλαξίας Galaxias, Milky Way ') or sometimes even world island a gravitationally bound large collection of matter, such as stars and planetary systems, nebulae, dust clouds and other objects commonly referred to in astronomy.

As the galaxy in the singular is exclusively referred to in astronomy our own galaxy, the Milky Way. In English, each system is called a " galaxy ". In a dark and clear night, the stars of the crowded galactic disk as a trail of spilled milk look.

  • 3.1 Origin of spiral arms


Galaxies vary widely in appearance (morphology), size and composition. The Milky Way is one of the larger galaxies and has about 300 billion ( 3.1011 ) star with a diameter of about 100,000 light years. In addition to the stars, there is also a galaxy of gas, dust and dark matter. The Andromeda Galaxy is our nearest large neighbor galaxy. The distance between these two galaxies is 2.4-2.7 million light years. Together with other galaxies form both galaxies, the Local Group. Galaxies often occur in groups or clusters of up to several thousand members. Due to the recent "Ultra Deep Field " images of the Hubble Space Telescope in March 2004 can roughly estimate that galaxies could theoretically be observed with today's technology from Earth over 50 billion ( 5.1010 ).

For a long time the exact nature of galaxies was unclear, since the individual stars could not be resolved and only a mist was observed. The question was whether these spiral nebulae part of our galaxy or own star systems. Even Immanuel Kant suggested in the " misty stars " Milky our equal, but only in 1923 succeeded Edwin Hubble to resolve this question. He determined the distance to the Andromeda galaxy and found that the Andromeda galaxy is too far away to be part of our galaxy.

Galaxy types

Classification by Hubble

Galaxies are classified according to their shape into various main and sub- groups of the so-called Hubble classification (see morphology). This classification was established by Edwin Hubble and with some extensions still in use today, although it was originally based only on a small sample of nearby, bright galaxies, which could be observed in the optical wavelength range at the time. The Hubble classification is purely empirical and says nothing about the evolution of galaxies. The different types are:

  • Elliptical galaxies show no particular substructures. The lines of equal brightness in the form of an ellipse. The elliptical galaxies have a uniform brightness decrease from the inside to the outside. They contain almost no gas, therefore, is their star formation rate to zero. Its spectrum is dominated by old and therefore red stars. Elliptical galaxies are classified according to their numerical eccentricity in the classes E0 ( circular) to E7 (highly elliptical). The number after the E indicates the first decimal place of the eccentricity, that is a Galaxy Class E7 has the eccentricity 0.7. The absolute magnitudes Elliptical galaxies span a large range. The brightest galaxies are mostly ellipticals and are probably smaller in this case arose from the fusion of several to medium-sized galaxies. Elliptical galaxies are often found in large clusters of galaxies.
  • Lenticular ( lens-shaped ) galaxies belong to the class S0. You have a core which corresponds to that of spiral galaxies, but their galactic disk contains no spiral arms, but is approximately uniformly bright ( Example: M 102).
  • Spiral galaxies have a spheroidal core, the so-called bulge, and it sparks, spiral arms, which lie in a flat disk component. During the bulge resembles an elliptical galaxy and shows no star formation, which allow the existing disk of gas and dust, the star formation in the spiral arms. Therefore, the spiral arms appear on images usually blue and the Bulge mostly reddish. The spiral arms are further divided into the classes Sa, Sb and Sc. Galaxies of type Sa have a very strong core and tightly wound spiral arms (example: Sombrero M 104). The type Sc has a relatively weak galactic core, extremely loose wound spiral arms and thus sometimes almost the shape of an intertwining "S " (example: the Triangulum Galaxy M 33). Together with the lenticular galaxies are also called disk galaxies Sa, Sb and Sc. An obsolete alternative name is spiral nebulae.
  • Barred spiral galaxies from the center, starting a long bar, to which then the spiral arms connect (example: M 109). Like the spiral galaxies, they are divided with increasing severity of the core and open up its spiral arms in the classes SBa, SBb and SBc. In our galaxy is a barred spiral galaxy.
  • Irregular (irregular ) galaxies have no spiral arms or elliptical shape. They are on average fainter than elliptical and spiral galaxies. To this group belong mostly dwarf galaxies.

In addition to the classification by Hubble there are also other organizations, such as after - Gérard Henri de Vaucouleurs or the Yerkes classification, but are used less frequently. The coarse classifications are the variety of galaxy types found often not fair, which is why one uses many other characteristics to describe galaxies.

Other types of galaxies

There are other forms of galaxies that can not be sorted in the above scheme or supplement this. These include the following:

  • Dwarf galaxies are galaxies of lower luminosity, 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 again be divided into compact ( cE ) and diffuse galaxies. The next compact dwarf elliptical galaxy, which is also the only one in the Local Group, M32. Compact dwarf elliptical galaxies are similar in morphology rather large elliptical galaxies. They have a more pronounced central region than diffuse, indicating a different genesis.
  • Interacting galaxies are meetings of two or more galaxies. As one can observe different nuclei and tidal tails depending on the stage of the interaction, these systems can not be classified in the classification scheme of Hubble. Gezeitenarm galaxies ( tidal dwarf galaxies, TDG) are galaxies that result from the interaction of two gas-rich galaxies in long tidal tails of gas and dust.
  • Polar ring galaxies described quite rare results of the merger of two galaxies. Due to gravitational interaction, two galaxies came here so close that often the lower mass interaction partner was torn and its stars, gas and dust are trapped in the gravitational field of the other galaxy. It follows, depending on the orientation of the collision, sometimes a ring of stars surrounding a galaxy as an additional spiral. Since this ring is mostly oriented perpendicular to the main plane of galaxies, one speaks of polar ring galaxies (example: Wagenradgalaxie ). There is evidence that our galaxy also has such a polar ring.
  • Radio galaxies emit synchrotron radiation from a lot in the field of radio waves, and are therefore also investigated with the aid of radio astronomy. Often observed in the radio galaxies up to two streams of matter, so-called jets. 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, point-like nucleus and show the range of the visual spectrum of prominent emission lines. Approximately one percent of the principal galaxies belong to this category.
  • BL Lac objects are active galaxies whose spectrum has no absorption and emission lines. Although they are sometimes very bright, its redshift can therefore be determined bad. Its brightness is highly variable. BL Lac objects belong next to the quasars to the most luminous known objects.
  • Quasars are the objects with the largest absolute magnitude observed. Due to the large distance of these objects could originally only observe their compact, point-like core, hence the name quasar ( quasi- stellar object = ).

Formation and development

The microwave background is the matter distribution of the universe back 380,000 years after the Big Bang. At that time, the universe was still very homogeneous: the density fluctuations were of the order of 1 to 105 In the context of cosmology, the growth of the density fluctuation can be described by the gravitational collapse. In particular, the dark matter plays a major role as they gravitationally dominates over baryonic matter. Under the influence of the dark matter density fluctuations grow until they collapse into dark halos. Since this process only the gravity plays a role in this process can be calculated today with great accuracy (eg Millennium simulation). The gas follows the distribution of dark matter, falls in this halos, condenses and leads to the formation of stars. The galaxies begin to form. The actual galaxy formation, however, is not understood, because the stars just created affect the infalling gas ( the so-called feedback), which makes a more accurate simulation difficult. After their formation, the galaxies have evolved. According to the hierarchical model of galaxy formation the galaxies grow primarily by merging with other galaxies. After that formed in the early universe under the influence of gravity, the first still quite low-mass proto- galaxies. Gradually, the idea that these galaxies precursor added by collisions to adult forms such as our Milky Way and even larger galaxies together. The relics of such collisions are reflected in the Milky Way today as so-called stellar streams. These are groups of stars whose common pattern of movement has an origin outside the Milky Way. They are assigned to smaller galaxies that were torn apart and swallowed by the Milky Way by tidal forces.

A model of galaxy formation assumes that the first gas clouds have been developed through rotation to spiral galaxies. Elliptical galaxies form according to this model only in a second stage by the collision of spiral galaxies. Spiral galaxies may in turn after this performance increase by that fall close ( dwarf ) galaxies in their disc and there dissolve ( accretion ).

The observation of high-redshift galaxies makes it possible to follow this trend. Great successes were, in particular, deep surveys such as the Hubble Deep Field. Overall, the formation and evolution of galaxies is a current subject of research not yet complete and therefore not yet sufficiently reliable to explain.

Recent studies suggest that the center of every galaxy has a supermassive black hole is located, which is significantly involved in the formation of the galaxy. This is how galaxies from huge clouds of gas (hydrogen), whose collapse centers supermassive black holes, which in turn heat the surrounding gas to such an extent that form due to compaction stars and ultimately the planet. The size of the galaxies and their centers ( supermassive black holes ) are directly related: the bigger the galaxy, the larger the center.

Formation of the spiral arms

Even if it looks in spiral galaxies, as would the galaxy only exist within the spiral arms, as are also less luminous parts of the galaxy disk relatively many stars.

A galaxy does not rotate rigidly like a wheel; Rather, the individual stars of the spiral arms and run out into it. The spiral arms are visible expression standing density waves (such as sound waves in air), running around in the Galactic disk. This theory was first put forward by Chia- Chiao Lin and Frank Shu in the 1960s. Then in the spiral arms and in the central bar, the matter density is increased, so that there arise many relatively bright blue, so short-lived stars from the interstellar medium new. Thus, these areas appear brighter than their surroundings. These density waves caused by the interaction of the star orbits, because the stars do not move like the planets in the solar system uniformly around a fixed center ( a black hole in the galaxy center ) because it the total mass of the galaxy is not concentrated enough. Therefore, a star returns after one round center of the galaxy is not back to its starting point, the orbits are therefore not ellipses, but are in the form of rosettes. Density waves arise when many stars are moving at the same speed. Thus, all paths are equal aligned with each other, on the other hand still moved in a pure spiral galaxy against each other in a barred spiral galaxy. The synchronization of the tracks is done by gravitational feedback. Through computer simulations that take into account interstellar gas, can even be modeled the formation of spiral arms. It turns out that these are not static, but arise and pass away. After each galaxy undergoes a cycle (duration about 10 billion years) of continuous transformation of the beam in the spiral form and back. Further, the spiral arms interfere with the orbits of the star, which leads to the so-called Lindblad resonances.

Interacting Galaxies

When galaxies collide, gas clouds can be unstable within the galaxy and collapse. This new stars are forming. The stars of the interacting galaxies merge themselves in this process very rarely with each other. The merged galaxies emit in the blue light of the newly formed stars. Such interaction can take hundreds of millions of years. In this case, the shapes of the galaxies changes greatly. Interactions between two galaxies are quite common. The stars can be strongly deflected by the gravity of the galaxies. Examples of such colliding galaxies that are already merged some cases, the systems M 51 - NGC 5195 and the " antennae " galaxies NGC 4038 - NGC 4039 (see figure) in the constellation Aquila.