The Milky Way Galaxy/Image Source: Bruno Gilli
A galaxy is an extensive space system of stars, planets, asteroids, dust, interstellar gas, stellar remnants, and dark matter. These all are held together by gravity.
The term ‘Galaxy’ is derived from the Greek word ‘galaxias’, which simply means 'milky', a direct reference to the Milky Way galaxy that contains our Solar System.
It is assumed that galaxies have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A and it has a mass four million times greater than the Sun.
As of now, GN-z11 is the oldest and most distant galaxy ever observed. It is at the distance of 32 billion light-years from Earth and only existed just 400 million years after the Big Bang.
There are three basic types of galaxies on the basis of morphology that we can find in the universe.
Elliptical Galaxies
Elliptical galaxies have an ellipsoidal profile which means they have an elliptical appearance regardless of the viewing angle. They include some of the massive galaxies containing upto a trillion stars. They are formed due to the interaction of galaxies, resulting in a collision and merger.
Spiral Galaxies
Spiral galaxies bear a resemblance to spiraling pinwheels. They have a huge bulge in the centre, which is composed of old stars, and surrounded by a disk of young stars arranged in spiral arms.
Irregular Galaxies
Irregular Galaxies have no particular shape. There are billions of galaxies in the universe, which do not fit in the above two mentioned morphological categories, falls into this category.