A galaxy is a stellar system composed of stars, nebulae, star clusters, interstellar matter, and dark matter. The galaxies hold the gravitational force together and the individual galactic components orbit the common center. There is some evidence that supermassive black holes with a mass a million times the mass of the Sun are located in the center of galaxies. In our case, the supermassive black hole is Sagittarius A. Other galaxies are, for example, NGC 55, the well-known Andromeda or even M87, from which the first photograph of the black hole was obtained.
Galaxies are formed from so-called protogalaxies (clouds of gases). Assuming that the rate of star formation during this process of galactic evolution determines whether the galaxy will be spiral or elliptical in shape, the slow process of star formation leads to spiral shapes of galaxies.
There are 3 basic types of galaxies and they are elliptical, spiral and irregular. In spiral galaxies, the spiral arms are approximately the shape of a logarithmic spiral. By simulation, it can be changed that this pattern was created by the disruption of a uniform rotating stellar mass. The spiral arms, like the stars, rotate around the center, doing so with a constant angular velocity (describing the rotational motion of the body). The helical arms are believed to be areas of high density or density waves. In 2003, Dr. Michael Drinkwater a new type of galaxy called ultra compact dwarf galaxy.
The space between galaxies is little empty, except for intergalactic clouds, gas, and dark matter. Only a few galaxies exist alone; these are known as field galaxies. Most galaxies are gravitationally connected to various other galaxies. Structures containing up to 50 galaxies are called galactic groups. The larger structures used by thousands of galaxies pushed into a relatively small region are called galactic clusters. Superclusters are gigantic sets containing tens of thousands of galaxies, arranged in clusters, groups, and sometimes alone.
Our galaxy is a member of the Local Group and, together with the Andromeda Galaxy, dominates it; in total, the Local Group contains about 30 galaxies in a space 1 megaparec wide. The Local Group is part of the Local Supercluster, also known as the Virgo Supercluster.
Current research suggests that most of the known universe is unsuitable for the existence of complex life. The places where life is possible are called the galactic habitable zone. The most important thing for her is her distance from the center of the galaxy. As this distance increases:
a. the number of stars in which heavier elements have already formed have been reduced – these are needed to create terrestrial planets
b. decreases the intensity of X-rays and gamma rays from a black hole in the center of the galaxy, or from nearby neutron stars, which is destructive to life
c. the risk of collision with other objects (planets or planetesimals), which is more often close to the center of the galaxy or spiral arms, decreases
So habitable planets cannot form too far from the center of the galaxy (there is nothing to create from them), nor too close (destructive radioactive radiation and a high probability of collision or disruption of the orbit).
Thus, the galactic habitable zone is approximately circular in shape and is surrounded inside and outside by uninhabitable zones.