What is a galaxy?

A galaxy is a collection of gas, dust and billions of stars and their solar systems, all held together by gravity. There is often a supermassive black hole at the centre of a galaxy.

The word galaxy is derived from the Greek galaxias, meaning 'milky', in reference to our galaxy the Milky Way. There are billions of galaxies in the Universe, almost all appearing to have formed soon after the Universe began. Galaxies are so far away that they all appear tiny, except for a small number of exceptions.

Galaxies usually exist in clusters, some of which are grouped into larger clusters. These superclusters are separated by nearly empty voids making the structure of the Universe look like a network of sheets and chains of galaxies.

Our solar system is in the galaxy called the Milky Way, which is in group of galaxies called The Local Group - about 50 galaxies, including the 3 largest: Andromeda Galaxy, the Milky Way and the Triangulum Galaxy. The nearest galaxy to the Milky Way is Sagittarius Dwarf galaxy about 24 kiloparsecs (78000 light-years) away. The Large Magellanic Cloud is another close galaxy. The Andromeda Galaxy is the closest major galaxy.

Andromeda galaxy (M31), M110 (above), M32 (below in bottom left side of Andromeda)

Galaxies are categorised according to their shape, which depends on how the systems were formed and subsequently evolved. They also vary in the amount of activity observed - some being sites of vigorous star formation, whereas others have stopped forming new stars.

Spiral galaxies 

• long twisting arms spiral out from their centre where stars are being formed.

• Some stars are very large, causing nearby dust to glow brightly.

• The spirals we see are glowing clouds illuminated by large, hot stars.

Barred spiral galaxies 

• arms spiral out from a straight bar of stars instead of from the centre.

Elliptical galaxies 

• stars are often very old as they don't actively create new ones.

• They are usually smaller galaxies, but can be large.

• The stars are often close together, making the centre look like one giant star.

Irregular galaxies 

• all that are not spiral or elliptical. Many probably used to be.