First of all, a galaxy is practically a mass system that has an animate shape. This is a gravitational force. A galaxy is exactly a lot of bodies growing together. That is, stars, planets, dwarf planets, asteroids and many other crests. In addition, it also contains matte matter and energy, but also many nebulae. Galaxies are also known for the bright spots they have on them. These bright spots actually sound like stars. What is seen in a galaxy are just a few of the stars it has, but in fact a normal galaxy has millions and billions of stars.
The fact that there are so many stars in a galaxy, most galaxies also form solar systems, named after our Solar System. They are just like our system and orbit planets in those solar systems. In addition to this, stellar swarms can also form. These are a kind of system but they are made up of stars. Basically, the star clusters are made up of many stars together. The galaxy has its own gravitational field, so in order not to be affected most galaxies are far apart. Many known galaxies also have satellite galaxies around them or they are satellite galaxies. A satellite galaxy is at an extremely large distance from the galaxy it orbits, but being small, it can be attracted by the gravitational field of the other galaxy, so that it can rotate. In the Universe there are exceptions by which some galaxies live theoretically "separately from others". That is, most galaxies live in groups, like our Galaxy, the Milky Way, which lives in the Local Group of Galaxies. Others in other groups, but there are rarer ones who live alone.
All galaxies and those that live in groups and those that do not live in groups, usually have arms, a kind of arm like the Milky Way, such as the Orion arm, in which the Solar System is located and of course the Sun. But yes, there are exceptions where there are few arms.
As with most celestial bodies, galaxies are grouped like this:
- elliptical, those that are slightly three-dimensional compared to the others and resemble globular or stellar swarms.
- spirals, those that have arms and are flat, like the Milky Way and are huge disks that are made of dust, gas and many stars, in addition, a central core where the bodies orbit.
- lenticular, those that are flat, flat and are also called old, because most are made up of old stars, and in the galaxy can not form other stars, and most, if not all have a nucleus consisting of a yellow star. Edwin Hubble was the one who classified them into these three groups.
These are the galaxies and their definition, but of course mankind has not yet discovered all that is in them, surely in the future more will be known, but no one still knows what a galaxy contains in its entirety and what surprises it can do.