The Universe is populated with countless galaxies, assemblages of stars, nebulae and other interstellar material, and these come in different shapes and sizes. Astronomers have estimated that a typical galaxy contains about 100 billion stars and measures about 100,000 light years across (one light year is the distance light travels in a vacuum in one year, about 9.5 million million kilometres). Galaxies can be placed into groups and described as follows:
|Spirals and barred spirals||
About 30% of all known galaxies are spirals and barred spirals. Spiral galaxies are essentially disk-shaped galaxies with bulbous or lens-shaped central regions or nuclei and bright, spiral arms rich in nebulae and interstellar material. Spiral galaxies contain stars of all ages, the youngest and newest of which are located within the spiral arms. Barred spirals are similar to spirals but have central regions or nuclei consisting of stars arranged in the shape of a bar. Spiral arms are attached to the ends of the bars. Both spiral and barred spiral galaxies rotate.
About 60% of all known galaxies are ellipticals. Elliptical galaxies are generally smaller than spirals and barred spirals and have no spiral arms. Elliptical galaxies are virtually devoid of nebulae and interstellar material and consequently contain mostly old stars.
|Irregulars||About 10% of all known galaxies are irregulars. Irregular galaxies are smaller than spirals, barred spirals and ellipticals. It is thought that many irregular galaxies owe their shape, or lack of it, to the gravitational attraction of much larger galaxies nearby pulling them in all directions. Irregular galaxies are rich in nebulae and interstellar material and consequently contain mostly young stars.|
Galaxies are not scattered randomly throughout the Universe, they occur in clusters. A rich cluster may contain hundreds or even thousands of galaxies, a poor cluster (or group) may contain as few as ten. Clusters of galaxies themselves form superclusters which can spread over regions of space up to 100 million light years across. Superclusters are the largest known structures in the Universe.
Some of the most remote and exotic galaxies or parts of galaxies that have so far been discovered are known as quasars.