A rotating mass of dust, gas and stars, held together by a gravitational force of attraction is called a galaxy. The Milky Way is a barred spiral galaxy, part of the Local Group. Earth’s position in the galaxy is on a dusty side, which makes a large portion of the galaxy unavailable to view. A black hole is considered to be located at the centre of the Milky Way. Four primary arms spiral from the black hole core into four directions. Earth is located in one of them. The core is surrounded by young stars, called population I stars, while the second disk is populated by denser clusters, called population II.
The space between the stars is called interstellar medium and it is filled with sparse matter. Some of these regions contain all the elements needed to form new stars. As more supernovas explode, the stellar wind disperses on the proximity of the Milky Way, which transform into stars that join the galaxy.
The space outside the Milky Way, which is composed of other galaxies or clusters of galaxies, is classified into spiral, elliptical and irregular types. The spiral galaxies move around an orbit, do not contain much dust and are mainly composed of old stars.
The spiral galaxy is a flat disk that rotates from the middle, which leaves bright trails outside its dimensions. These are regions of dust where the massive young stars form. Both Andromeda and the Milky Way are part of this group.
Irregular galaxies have an undefined form due to the gravitational interaction. They are neither spiral nor elliptical in form and they have a chaotic appearance.
Radio galaxies are visible only in the luminous radio spectrum. These short frequency radiations are called Seyfert, Blazars and Quasars. The last is considered to be the most luminous object in the universe. Galaxies are commonly organised in hierarchical groups. They have large voids between them.