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The Sun is one of billions of stars in a galaxy called the Milky Way, yet this is only one of billions of galaxies existing in the Universe. Galaxies are not usually found on their own; instead they are found in groups called ‘clusters’, and those clusters also form bigger groups called ‘superclusters’.

Galaxies do not appear as bright objects in the night sky. Apart from the Milky Way and its companions, the two Magellenic Clouds (visible from the southern hemisphere), only a handful are visible to the naked eye. These usually appear as faint "fuzzy stars" amongst the night time constellations.

One of the early astronomers to study galaxies was Edwin Hubble, who had the Hubble Space Telescope named after him. Hubble’s research showed what galaxies really were. Hubble was using the observatory at Mount Wilson, USA, to observe the patches of light that we now know to be galaxies, and in particular he was looking at Andromeda. The Andromeda galaxy is the most distant object visible to the naked eye, lying over 2 million light years away, but people originally thought that it lay within the Milky Way. Hubble looked at the light from Andromeda and realised that it looked quite different to that coming from clouds of gas called nebulae. Andromeda’s speed was also found to be very high – it was approaching us at about 270 kilometres per second. These clues suggested that Andromeda lay beyond our galaxy.

The most important discovery, however, was made in 1923. Hubble discovered a Cepheid variable star in Andromeda, allowing him to place it at more than a million light years away - much further than the most distant reaches of the Milky Way. Hubble had therefore shown that Andromeda was an external star system - another galaxy. We know today that the Andromeda galaxy is more than two million light years away.

Andromeda is a "companion" to our own galaxy, and, along with at least twenty other nearby galaxies, makes up the ‘local group’. The Andromeda galaxy is the largest within this group. It is estimated to contain as much material as 3,000,000 million Suns - about twice as much mass as the Milky Way. As the image here shows, we see the galaxy "edge on", and so the spiral arms are not visible. However, this point of view allows us instead to see the lanes of dust that run throughout the system.

The Andromeda galaxy is over 160,000 light years in diameter - much larger than our own. The centre of the galaxy is called the nucleus, and is visible in the image as the bright region in the centre. It is actually very small in comparison to the rest of the galaxy, measuring only 26,000 light years across. The image also shows that the galaxy is not alone: there are two small "satellite" galaxies, visible as the bright patches of light above and below. The Andromeda system lets astronomers study many aspects of galaxies and how they relate to the Universe around them.

Hubble's work with galaxies continued, and one of the important results of this research was the Tuning Fork Diagram. Hubble classified individual galaxies according to their physical appearance. The diagram was originally thought to show the sequence of evolution of a typical galaxy, from a relatively featureless ball of gas, to the magnificent spiral galaxies with their swirling arms and bright central regions. It is known today that this is not true, but the diagram is still a useful tool in classifying galaxies into different types according to their shape.

Hubble's Tuning Fork Diagram
Thanks to the University of Texas, McDonald Observatory for this diagram

Each class differs in both appearance and content. Galaxies can be divided into three main areas: Ellipticals, spirals and irregulars.

Click on the links below to find out more about galaxies.

Formation of Galaxies

The Milky Way

Spiral Galaxies

Elliptical Galaxies

Irregular Galaxies

Tuning Fork Diagram

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001