GALAXY

Galaxy large aggregation of stars , gas, and dust, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926-29) by Edwin P. Hubble that indicated the great distances at which this and other galaxies are located. Previously, the galaxies had been classified with the luminous gas clouds, or bright nebulae , within the Milky Way. The sun and its solar system, as well as the visible stars, are all in the Milky Way galaxy. Harlow Shapley encouraged the exclusive use of the term "galaxies." Billions of galaxies are within the optical range of the largest telescopes; in 1996 analysis of photographs taken from the Hubble Space Telescope increased the estimated number of galaxies from 10 billion to 50 billion. A galaxy is held together by the gravitational attraction between its constituent parts (see gravitation ), while its rotational motion prevents it from collapsing on itself. Just as gravitation binds individual stars into galaxies, it also acts to hold clusters of galaxies together.

Although estimates of the age of the universe are controversial (see Hubble's law ), if it is taken as 15 billion years, then it is estimated that the first galaxies were formed 12.8-13.5 billion years ago.