In 1923 Edwin Hubble showed that the Andromeda Nebula is a galaxy--a distant collection of stars. This resolved a longstanding controversy about whether spiral nebulae were galaxies or objects within the Milky Way.
Galaxies can be classified using the Hubble system. Smooth, spheroidal galaxies are classified as ellipticals. Galaxies with flat disks and spiral arms are classified as spirals. Compared with ellipticals, spiral galaxies have much more interstellar gas and dust. Spirals also have young stars, but ellipticals do not. A small fraction of galaxies lack overall structure and are classified as irregular.
The different appearances of galaxies result from differences in the way in which they formed or from their interactions with other galaxies. It is believed that elliptical galaxies evolved from clumpy protogalaxies and spirals from smooth protogalaxies. Collisions between galaxies can alter their appearances and also can merge galaxies into larger galaxies.
The distribution of matter in a galaxy can be found by measuring the rotation curve of the galaxy. Many spiral galaxies have flat rotation curves, which suggests that they contain a considerable amount of dark matter. Rapid rotation near the centers of some galaxies suggests that massive black holes may reside there.
The distances to galaxies have been determined through a stepwise process that compares nearby objects with the same kind of objects in more distant galaxies. Distances as large as 3000 Mpc have been found through this method.
Galaxies are receding from us with speeds proportional to their distances. The relationship between speed and distance is called Hubble's law and is a result of the expansion of the universe. Hubble's law can be used to find distances to galaxies for which recession velocities can be measured.
