Our own galaxy is called the Milky Way. It contains billions of stars, including our own Sun, yet it is only one of billions of galaxies existing in the Universe. Galaxies are not generally found alone; instead they are found in clusters, and those clusters also form further groups called ‘superclusters’.
Galaxies do not appear as bright objects in the night sky. In fact, apart from the Milky Way and its companions the two Magellanic 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, after whom the Hubble Space Telescope is named. Hubble made the breakthrough that revealed the true nature of galaxies.
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. It was originally believed to lie within the Milky Way. The advent of the technique known as spectroscopy, which allows astronomers to study the chemical make up of an object, as well as the way it moves in space, revealed that the light from Andromeda looked quite different to that from nebulae, which were known to be clouds of gas. The object’s velocity was also found to be much greater than that of nebulae - approaching us at very high speed (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. Andromeda was therefore proven to be an external star system - another galaxy. We know today that the Andromeda galaxy is more than two million light years away.
Andromeda is of great interest to astronomers. It is a "companion" to our own galaxy, and, 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 which run throughout the system.
The Andromeda galaxy is over 160,000 light years in diameter - much larger than our own. The central nucleus, visible in the image as the bright region in the core, is actually very small in comparison - 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 provides astronomers with the opportunity to 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 work was the Tuning Fork Diagram. This diagram was a result of Hubble classifying individual galaxies according to their physical appearance. It was originally thought to show the sequence of evolution of a typical galaxy, from a relatively featureless spherical mass of gas, to the magnificent spiral galaxies with their swirling arms and bright central regions. It is known today that this is not the case, yet the diagram is still a useful tool in classifying galaxies into different types according to their shape. Each class differs not only in appearance, but also in composition.
Hubble's Tuning Fork Diagram
|Thanks to the University of Texas, McDonald Observatory for this diagram|
Galaxies can be divided into three main areas: Ellipticals, spirals and irregulars.
Click on the links below to find out more about galaxies.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001