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Formation And Evolution

For many years scientists had no ideas how galaxies formed. According to all observations at the time, galaxies formed during a single epoch very far back in the history of the universe. In the absence of direct evidence, astronomers formed two theories: the theory of accretion, in which blobs of stars came together to form galaxies; and the theory of collapse, in which galaxies were formed in the collapse of an enormous gas cloud.

In late 1996, scientists got their first view of galaxy formation, looking back in time 11 billion years to see clumps of young star clusters gradually banding together into a galaxy. It is too early to fully dismiss the gas collapse theory, however; there may be more than one way to form a galaxy.

When Hubble first devised his classification scheme, he thought that the different types of galaxies represented different evolutionary stages; they started as one type and gradually evolved into another type. We now know that his theory was true, though the phrase gradual evolution is something of a misnomer.

Elliptical galaxies are formed by the collison of two spiral galaxies. The process is slow—scientists estimate that it takes nearly half a billion years for the merging spiral galaxies to smooth into an elliptical galaxy—but can be quite violent. Although galaxies are mostly empty space, gravitational interaction between stars can cause them to explode into supernovas. More important, gravitationally induced collisions between clouds of interstellar hydrogen gas can create intense heat and pressure that can trigger the formation of new stars.

One clue to the evolution of galaxies is the distribution of different types of galaxies at different distances from us. Because light travels at a finite speed, when we look at a distant galaxy, we are seeing the galaxy as it appeared in the distant past when the light left it. Some types of active galaxies, such as quasars and BL Lax objects, occur only at great distances from us. They existed when the universe was much younger, but no longer exist. Many astronomers therefore think that active galaxies are an early stage in the evolution of galaxies. If this idea is correct, an astronomer living now on a distant quasar might see the quasar as a normal galaxy, and the Milky Way in its earlier active stage as a quasar.

However galaxies formed and evolved, the process must have occurred quickly very early in the history of the universe. The age of the oldest galaxies appears to be not much younger than the age of the universe. Though astronomers now have some support for theories of galactic formation and evolution, they are still searching for more evidence and trying to understand the details.

See also Radio astronomy.



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Bartusiak, Marcia. Thursday's Universe. Redmond, WA: Tempus Books, 1988.

Hodge, Paul. Galaxies. Cambridge: Harvard University Press, 1986.

Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell's Exploration of the Universe. 7th ed. Philadelphia: Saunders College Publishing, 1995.

Smolin, Lee. The Life of the Cosmos. Oxford: Oxford University Press, 1999.

Snow, Theodore P. The Dynamic Universe: An Introduction to Astronomy. St. Paul: West Publishing, 1991.


Cowen, Ron. "The Debut of Galaxies." Astronomy 22 (December 1994): 44-45.

Eicher, David J. "The Wonderful World of Galaxies." Astronomy 21 (January 1993): 60-66.

Lake, George. "Understanding the Hubble Sequence." Sky & Telescope 83 (May 1992): 515-21.

Paul A. Heckert


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Active galaxy

—A galaxy that emits more energy than can easily be explained, usually at least 100 times the energy output of the Milky Way.

Barred spiral galaxy

—A spiral galaxy in which the spiral arms start at the end of a central bar structure rather than the nucleus.

Cepheid variable star

—A type of star that varies in brightness as the star pulsates in size. Cepheid variables are important distance yardsticks in establishing the distance to nearby galaxies.


—The flat disk-shaped part of a spiral galaxy that contains the spiral arms.

Elliptical galaxy

—A galaxy having an elliptical shape.


—A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.


—A spherical distribution of older stars and clusters of stars surrounding the nucleus and disk of a spiral galaxy.

Irregular galaxy

—A galaxy that does not fit into Hubble's classification scheme of elliptical and spiral galaxies.

Light year

—The distance light travels in one year, roughly 6 trillion mi, or 9,654 trillion km.

Milky Way

—The galaxy in which we are located.


—The central core of a galaxy.

Spiral arms

—The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy

—A galaxy in which spiral arms wind outward from the nucleus.

Additional topics

Science EncyclopediaScience & Philosophy: Formate to GastropodaGalaxy - Outside Of The Galaxy, Classification Of Galaxies, Elliptical Galaxies, Spiral Galaxies, Irregular Galaxies