A galaxy is a system in which gravity is the connecting element. It is composed of stars, interstellar gas, dark matter, and cosmic dust. Each galaxy has a center of mass around which all objects in it revolve. The word "galaxy" itself is translated from the ancient Greek language as the milky way, "gala" means milk.
Instructions
Step 1
Planet Earth is part of the Milky Way galaxy. All galaxies are very far removed from one another. The distance from the Earth to them is measured in various ways. To those that are closer, they calculate the distance in megaparsecs, and the farthest ones are already removed by the amount of redshift z.
Step 2
Due to the fact that the rest of the galaxies are very far away, only three of them can be seen with the naked eye: these are the Andromeda nebula, the Small and Large Magellanic Clouds. The Andromeda Nebula is observed in the Northern Hemisphere, and the Magellanic Clouds in the Southern. For a very long time it was not possible to examine galaxies in such a way as to distinguish individual stars in them; this was done only in the 20th century.
Step 3
At the end of the 20th century, by the 1990s, about 30 galaxies had already been discovered, in some of them it was already possible to distinguish individual stars. These are galaxies of the so-called Local Group, not too far from the Milky Way.
Step 4
The big breakthrough in the study of galaxies came when the Hubble telescope, which is located in space, was created and launched. On Earth, ten-meter telescopes were launched, which made it possible to distinguish individual stars even in distant galaxies.
Step 5
All galaxies are very different, both in shape and in the substances they contain, as well as in mass and size. Usually, they are divided in shape into disk spiral, sphere-like, elliptical, irregular, dwarf, barred galaxies, and there are other types. The masses of galaxies differ quite strongly. The order of mass can range from 10 to 7 to 10 to 12. For example, the mass of the Milky Way is 3 * 10 to the 12th power of solar masses. The diameter of the Milky Way is about 100 thousand light years, other observed formations are 16 to 160 thousand light years in length.
Step 6
Galaxies are not evenly distributed throughout space. In the observed space, there are quite vast voids in which there are no galaxies at all, these are the so-called voids. It is believed that there are approximately one hundred billion galaxies in the observable part of the Universe, although their exact number is unknown.