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. Many large galaxies have smaller galaxies, called satellite galaxies, in close proximity. The galaxies nearest the Milky Way form a cluster called the Local Group. The Local Group includes the Andromeda Galaxy, which is a spiral galaxy similar in size and composition to the Milky Way, and the Magellanic Clouds, which are satellite galaxies of the Milky Way. The vast majority of observed galaxies are classified as either spiral or elliptical (football-shaped), with a small minority, e.g., the Magellanic Clouds, classified as irregular according to a scheme originated by Hubble. Although estimates of the age of the universe have varied (see Hubble's law), current estimates place its age at around 13.75 billion years. The earliest galaxies imaged by modern astronomers include several that were formed less than 600 million years after that, and one galaxy has been discovered that was formed some 380 million years after the big bang.
A typical spiral galaxy is shaped like a flat disk, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars (see stellar populations). This type of galaxy is further classified as being either a normal or a barred spiral. In the normal spiral, the arms, at least two in number, join smoothly with the nucleus; in the barred spiral, such as the Milky Way, the arms project from a bank of stars that runs through the nucleus. The elliptical galaxies, lacking spiral arms entirely and containing little or no gas and dust, resemble the nuclei of spiral galaxies. Their shapes vary from nearly spherical to highly flattened ellipsoids. Elliptical galaxies have a much greater variation in size, mass, and luminosity than do spiral galaxies; their sizes range from the largest known galaxies of all, with luminosities about 10 times that of the Andromeda Galaxy, to the small dwarf ellipticals, which can contain as few as a million stars. Irregular galaxies appear structureless and without any nucleus or rotational symmetry; their light comes mostly from young stars.
Spiral galaxies contain a larger number of bluer, younger stars, while elliptical galaxies contain a larger number of redder, older stars. This has led astronomers to believe that stars initially cluster into spiral galaxies and that over time structural changes transform them into elliptical galaxies. Some researchers speculate that the transformation occurs because of gravitational forces exerted by galaxies as they slowly pass each other. Computer simulations suggest another alternative, called "galactic harassment," in which galaxies interact although they remain far apart and pass each other at high speeds. The most widely accepted alternative suggests that the transformation is caused by collisions of galaxies and gravitational tidal interactions between them as they travel through space, causing them to grow and evolve. Several dwarf galaxies are currently colliding with the Milky Way; others are on course to do so over the next 2 to 3 billion years. The collisions are not cataclysmic because galaxies—even though they may contain many billions of stars—are mostly "empty" space and the probability of two stars meeting is very small. However, the "empty" space is not really empty, it is full of gas and dust which can interact when the galaxies collide. There is also friction between the gas and dust in the colliding galaxies, causing shock waves that can trigger some star formation in the galaxies. These processes can radically affect the colliding galaxies, e.g., two spiral galaxies can merge to form an elliptical galaxy.
Many galaxies radiate a large fraction of their energy in forms other than visible light. With the development of radio astronomy, many radio galaxies were discovered. Other galaxies radiate strongly in the infrared, ultraviolet, or X-ray parts of the spectrum.
See R. J. Tayler, Galaxies, Structure and Evolution (1993); N. Henbest and H. Couper, The Guide to the Galaxy (1994); M. S. Longair, Galaxy Formation (1998); M. Merrifield and J. Binney, Galactic Astronomy (1998); L. S. Sparke and J. S. Gallagher 3d, Galaxies in the Universe (2d ed. 2007); H. Mo, F. van den Bosch and S. White, Galaxy Formation and Evolution (2010).
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