Active galaxy A galaxy that gives off exceptional amounts of energy for its size. The energy is produced by a large black hole at the galaxy’s centre. This is called an active galactic nucleus. There are several different types of active galaxies. These include: seyferts, blazars, quasars and radio galaxies. Some astronomers believe that these are all the same thing but seen from different angles.

Andromeda Galaxy A barred spiral galaxy found in the constellation Andromeda. It is the closest spiral galaxy to our own and a member of the Local Group of galaxies. It is one of the few galaxies that can be seen from Earth without using a telescope.

Barred spiral A spiral galaxy with a long, thin, bar-shaped centre. About two-thirds of all spiral galaxies are barred, including the Milky Way Galaxy.

Bulge A large ball of tightly packed stars found at the centre of spiral and elliptical galaxies.

Cluster A group of galaxies held together by gravity.

Dwarf galaxy A small galaxy with low brightness. Usually, this means its mass is less than 100 millon times that of our Sun.

Elliptical galaxy An oval-shaped or round galaxy. Elliptical galaxies contain very old stars and little gas or dust.

Filament A long “string” of galaxy superclusters stretching across space. These are linked together like a net made up of strings and knots.

Galactic arms The bright regions of a spiral galaxy that branch out of its centre. Galactic arms are full of bright, young stars.

Galactic halo The spherical region surrounding the centre of a galaxy. It contains dark matter and clusters of stars.

Galaxy A gigantic, rotating collection of gas, dust, planets and millions of stars, all held together by gravity. Some galaxies only contain a few million stars, while some contain over a trillion. Our own galaxy, the Milky Way, is a vast barred spiral of about 200 billion stars measuring 100,000 light years across. There are billions more galaxies, of various shapes and sizes, all making up what we call the Universe.

Hubble, Edwin (1889–1953) American astronomer who proved the existence of other galaxies than our own. Hubble also helped prove that the Universe is expanding.

Hubble Space Telescope A telescope that orbits the Earth as a satellite. The Hubble Space Telescope is unaffected by weather or pollution, so it can receive pictures that are much clearer than those taken with Earth-based telescopes. It has photographed galaxies as far as 10 billion light years away.

Irregular galaxy A galaxy with no obvious shape. Irregular galaxies often contain lots of very young stars.

Lenticular galaxy A flat, disc-shaped galaxy that looks like a spiral galaxy without arms. Lenticular galaxies are made up of old stars near the end of their life cycle.

Local Group A cluster of more than 50 galaxies, including the Milky Way.

Magellanic Clouds A pair of galaxies that orbit the Milky Way. The Large Magellanic Cloud and Small Magellanic Cloud are a pair of irregular galaxies that can both be seen without the aid of a telescope in Southern Hemisphere skies.

Milky Way Galaxy The galaxy that contains our Sun and Solar System. The Milky Way is a barred spiral of stars clustered together in space. It is named after the misty band of stars we can see on a clear night—actually our side-on view of the nearest parts of the Orion Arm.

Quasar A very bright area surrounding a black hole at the centre of an active galaxy. Its brilliant light comes from a disc of hot gas and dust spiralling into the black hole. One quasar gives off more energy than 100 galaxies combined.

Ring galaxy A galaxy shaped like a hoop surrounding a ball. A ring galaxy's shape may be the result of collisions with other galaxies.

Spiral galaxy A spiral-shaped galaxy. Spiral galaxies have a round bulge at their centre, surrounded by a large flat disc, with arms that spiral out from the centre. Their arms contain younger stars and their bulges contains older stars. The Triangulum Galaxy, the second nearest major galaxy to the Milky Way, is a spiral galaxy.

Starburst galaxy A galaxy in which an exceptionally large number of stars are created. Starburst galaxies are thought to result when two galaxies collide with each other, bringing together dense clouds of gas which collapse, creating stars.

Supercluster A group of clusters of galaxies. Superclusters tend to be stretched into long “strings”, called filaments, that stretch across the Universe.

Virgo Cluster A cluster of galaxies within the constellation Virgo. 

Consultant: Mike Goldsmith