Galaxies can be thought of as ‘cities of stars’ that come in a variety of shapes and sizes. It’s reckoned that there may be as many as one hundred billion galaxies in the observable universe!
Our Sun is a fairly insignificant star in the Milky Way galaxy. The Milky Way is about 100,000 light years in diameter and thought to be spiral in shape. We estimate that we are about two thirds of the way out from the centre towards the rim on one of the galaxy’s spiral arms.
Almost all the stars we see in the sky are within the Milky Way. Those nearest to us appear in all directions in the sky, however because the Milky Way is flattened in shape, we see those stars furthest away as a hazy band of light stretching across the sky.
We can however see other galaxies from Earth. The Andromeda Galaxy (around 2.4million light years away), for example is visible with the naked eye. It is the largest of a cluster of about 30 galaxies that together form what scientists call ‘The Local Group’.
Scientists class galaxies depending on their shape and nature. There are spirals, barred spirals, ellipticals, and irregulars.
At the heart of a spiral galaxy is ‘bulge’ of old stars. Branching and curving out from the centre are arms consisting of younger stars, gas clouds and dust.
Barred spirals have a central bar from which the spiral arms extend.
Elliptical galaxies consist of very old stars and have no arms and little gas and dust. These are the largest galaxies, with perhaps ten times as much mass as a galaxy of the Milky Way. It is thought that some of the ellipticals may have been formed by the merger of smaller galaxies.
Irregulars have no defined shape or structure.
The Universe contains some objects that emit as much energy from a area not much bigger than our Solar System, as an entire galaxy does. These are known as quasars and are thought to be galaxies with massive black holes at their centre.