Galaxies

A galaxy is a vast, gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter. Galaxies vary in size and shape, most commonly appearing as spirals, ellipticals, or irregular blobs, and some galaxies form associations with their neighbours, such associations are called galactic clusters. Galaxies are considered the fundamental building blocks of the universe, each containing millions or even trillions of stars, all orbiting a common centre at which is often a supermassive black hole.

Planet Earth sits in an arm of a spiral galaxy which we know as the Milky Way, our nearest galactic neighbour is the much larger Andromeda Galaxy, having a diameter of approximately 2.5 million lightyears, compared to approximately 87 000 lightyears for Milky Way.