Random motions (orbits) of stars which support a self-gravitating body against collapse. Motions can be either ordered or random. The most prevalent form of ordered motion is rotation, in which the majority of stars The elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More in the same direction. A Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical • Lenticular • Spiral • Irregular Click on Term to Read More with strong rotation is “rotation supported.” In contract, elliptical galaxies and the stellar halos and bulges of spiral galaxies show little or no rotation (as many stars orbiting in one direction as in another). Such a galaxy is “velocity dispersion” (or pressure) supported. Rotation can be measured by determining the galactic redshift at a number of positions across the galaxy. Measurement of velocity dispersion requires a measure of the velocity broadening of spectral lines.