Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light years across. There are hundreds of billions of galaxies in the Universe. Galaxies vary in size, structure, and luminosity, and, like stars, are found alone, in pairs, or in clusters. Galaxies are divided into three basic types: spirals, ellipticals and irregulars.
Examples of the three main types of galaxies: spiral (left), elliptical (middle), and irregular (right)
Spiral galaxies get their name from the shape of their disks, in which stars, gas and dust are concentrated in spiral arms that extend outward from the central nucleus of the galaxies. They are divided into three main types according to how tightly wound the spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb's are between, having moderately wound arms around an average sized nucleus. Our Milky Way galaxy is an example of a spiral galaxy. Spiral galaxies are rich in gas and dust and have a high rate of star formation. Since spirals contain a high fraction of hot, young stars, they are often among the brightest galaxies in the universe.
Elliptical galaxies are elliptical in shape and are divided into eight subgroups: E0-E7 depending on their elongation. E0 ellipticals are nearly circular, while E7s are highly elongated. Elliptical galaxies contain primarily old stars, and do not have much gas and dust. There is very little new star formation in these galaxies.
Irregular galaxies have no particular shape. They are among the smallest galaxies and they contain a vast amount of gas and dust. As a result they have a very high rate of star formation. The Large and Small Magellanic Clouds are examples of irregular galaxies.