Galaxy evolution and clusters
Galaxies are gregarious—they are usually found in clusters that merge together to form superclusters in the large-scale structure of the universe.
Inside the clusters, galaxies continually collide and merge in ways that are thought to explain how the different types of galaxies arise.
Astronomers once thought that galaxies followed a simple evolutionary sequence from elliptical to spiral. Today, the sequence is thought to be much more complex. Studies of galaxy collisions have shown that gas is often stripped away from the galaxies, becoming hot gas that falls toward the center of galaxy clusters. Robbed of their star-forming gas, merged galaxies are thought to become ellipticals, dominated by red and yellow stars as their blue and white ones burn out. However, cool gas pulled in by an elliptical can eventually rejuvenate a star-forming disk, allowing the cycle to repeat itself.
how galaxies develop
The best modern theory for galaxy evolution suggests they go through a series of mergers and collisions against a steadily declining supply of cool "background" gas. Mergers begin to form elliptical galaxies that eventually come to dominate the central regions of galaxy clusters.
early galaxies start life as small ellipticals early galaxies start life as small ellipticals
spiral arms develop around elliptical hub spirals merge, stripping gas away to form a larger elliptical spiral arms reform as background gas pulled in spirals merge again to form giant elliptical dense background gas spiral arms develop around elliptical hub spirals merge, stripping gas away to form a larger elliptical spiral arms reform as background gas pulled in spirals merge again to form giant elliptical george abell
American astronomer George Abell (1927-1983) carried out the first detailed survey of galaxy clusters, and developed the techniques required to distinguish clusters from randomly distributed background galaxies. An avid popularizer of science, Abell did most of his work at Mount Palomar observatory during the 1950s, using the most powerful telescope available at the time (right). Abell's catalog is still the standard reference for galaxy clusters.
Clusters of galaxies can contain anything from a few dozen spirals and irregulars (as in our own Local Group) to thousands of mostly elliptical galaxies, dominated by one or more giant ellipticals at the center. Surprisingly, there is not much variation in size— most galaxy clusters occupy a space a few million light-years across. although clusters frequently merge at their edges, each is distinctive as it is governed by its own local gravity.
Clusters—and their galaxies—merge over time. A cluster's age can be estimated from how many large ellipticals it contains. Abell 1689 (left) is considered to be highly developed.
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