Gaze upwards on a dark night, and you will see the ‘Milky Way’, a glowing ribbon made up of many individual stars. This is our first clue to the fact that that stars are not uniformly distributed throughout space, but clustered together to form galaxies.
Galaxies can vary in size between a hundred thousand and three thousand billion solar masses, and can be broadly split into different classes, depending on their shape. Our own Milky Way Galaxy contains about two hundred billion stars. It is a spiral shape, which is why, when viewed from our position about two thirds of the way along one of the arms, it appears as a band across the sky.
Gazing out into the Universe reveals thousands upon thousands of galaxies.
These galaxies tend to be grouped together in clusters, and these clusters in turn are grouped into superclusters. It appears the Universe has structure on many different scales.
Spiral Galaxies – are distinctive, not only because of their shape, but because they tend to contain high numbers of brighter, younger stars. The two arms of a spiral galaxy are often rich in star forming regions. They also contain much interstellar material, visible as reddish emission nebulae or darker dusty clouds. In some spiral galaxies, yellow, ‘fossil’ arms of older stars are also visible.
Barred spiral galaxies have disctinctive ‘bars’ which extend between the core and the spiral arms. Many spirals show this feature to some extent, and the distinction between barred and unbarred galaxies is not always a clear one.
Elliptical galaxies – are the most numerous type of galaxy in the Universe. They tend to consist mostly of older stars, and have little interstellar material. Although individual stars within the galaxy may rotate around the galactic core, the motion of the stars as a whole is random, and elliptical galaxies usually have no net angular momentum. The smallest and the largest known galaxies are all eliptical.
Irregular Galaxies – Nearly a quarter of all known galaxies have little or no discernable structure. Many such galaxies have probably become irregular in shape as a result of gravitationl interactions with other galaxies nearby.
Lenticular Galaxies – Often described as ‘spirals without spiral structure’, lenticular galacies are flat discs of stars. They usually consist of older stars, and have few star-forming regions
Radio Galaxies-All galaxies emit radio waves. However, the term radio galaxy is used to describe galaxies that emit particularly strongly (i.e. 1023 Watts or greater) in the radio regions. Radio galaxies are classified as extended or compact, depending on whether the region of radio emission is greater or smaller than the visible region. Many radio galaxies have large jets or streams of radio matter that extend far out into space.