Galaxy

A Galaxy is a system of stellar matter including stars, planets, meteors, nebulae and other interstellar debris that is contained within a self-sustaining gravitational field. In short, it's where we keep all of our cool stuff and it doesn't go floating off into nowhere. Galaxies come in all shapes and sizes; the most common of which are elliptical galaxies, spiral galaxies and of course, your oddly-shaped blob-like irregular galaxies.

Galaxies contain varying numbers of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies.