Galaxies are agglomerations of stars, nebulae, and other celestial objects separated by vast regions of interstellar space. These objects are gravitationally attracted to and usually orbiting the galactic center, which often contains one or more extremely large black holes. Galaxies are usually considered the "building blocks" of the universe because most appreciable structure of the universe occurs at or above the galaxy level.
The formation of galaxies is currently under debate among cosmologists. One theory states that "hot dark matter" (neutrinos) were responsible for the accumulation of density fluctuations during the early Big Bang. This theory predicted the formation of large structures, such as superclusters, which then led to the formation of smaller structures. However, computer models of this theory have been less than successful, leading theorists to a second theory. This theory states that "cold dark matter" (weakly interacting massive particles, abbreviated WIMPS) was responsible for density fluctuations and thus smaller structures (galaxies) were formed first, followed by larger structures. This theory, although not perfect, was best supported by computer models.
Galaxies come in three main types: spiral, elliptical, and irregular. Spiral galaxies, exemplified by our own Milky Way galaxy, have a spiral shape with a large central "bulge" and long trailing arms. Most star formation occurs within these arms, where our solar system is also located. An ordinary spiral galaxy contains about 100 000 million stars and is about 100 000 - 120 000 light years in diameter. Such galaxies also often have galactic halos, which consist of gas, dust, scattered stars, and star clusters. Elliptical galaxies are generally circular or egg-shaped (ovoid). Elliptical galaxies are usually older than spiral galaxies and contain many red giant stars but few nebulae. In typical elliptical galaxies, star formation has virtually ceased due to the scarcity of nebulae. These galaxies are the most common in the known universe. Irregular galaxies are the smallest and least common of the three types of galaxies. They are often associated with other galaxies, especially spirals. The Large and Small Magellanic Clouds, containing 10 000 million and 500 million stars respectively, are examples of irregular galaxies and are associated with the Milky Way. Finally, an additional category called peculiar galaxies contains those galaxies that are regular spiral or elliptical galaxies except for one feature or detail. Such galaxies are formed when dust particles are gravitationally attracted to regular galaxies or when two regular galaxies collide and exchange material as they pass through one another.
Essentially, galaxy groups are an intermediate level of organization between individual galaxies and much larger galaxy clusters. The size of galaxy groups can vary from the 22 or so in our group, known as the Local Group and also containing Andromeda and the Magellanic Clouds, to the thousand found in the Virgo group. The individual galaxies in galaxy groups, though they travel in space with their group members, are very distant from each other - thousands to millions of light years away in most cases. In addition to their constituent galaxies, galaxy groups contain hot threads of luminous gas that contribute to the formation of galaxy clusters. The collisions of galaxy groups with larger groups results in the formation of galaxy clusters held together largely by the threads of gas.
Galaxy clusters are enormous congregations of galaxies in the same way that galaxies are congregations of stars. The average galaxy cluster is 1000 trillion times as massive as the sun and is hotter than 75 000 000 ° C. Examples of galaxy clusters include Virgo and Coma, though more than 30 000 of them have been cataloged by astronomers. Within a galaxy cluster, the individual galaxies orbit the cluster's center of mass just as planets orbit their sun - yet their speed of rotation is much greater than their collective mass allows. This offers yet more indirect proof of "dark" matter, whose existence is also required for the closed universe mentioned later in this article. Galaxy clusters contain, aside from the galaxies themselves, huge amounts of very hot gas stretched tenuously through the space between the galaxies.
Scientists believe that galaxy clusters were formed in a very distinct way. Based on observations of galaxy clusters at various stages of their "life," scientists have determined that galaxy clusters are formed by the absorption of galaxy groups (smaller collections of galaxies) by other, larger galaxy groups. Mergers of this sort take place when galaxy groups collide and mingle their gases, which mix together. The actual galaxies pass to the opposite end of the cluster unimpeded, but cannot escape due to gravity. Given billions of years, the gases and galaxies will mix together thoroughly, resulting in a smooth cluster with temperature and galaxy distribution uniform.
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Abell Cluster 2256 is in the early stages of a merger. |
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Abell 754 is several hundred million years further along. |
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Abell 1795 has not absorbed another galaxy group in several billion years. |
The galaxy cluster is the highest level of organization held together by gravity. Beyond this organizational level exist superclusters, walls, and voids - all recently discovered phenomena for which cosmologists have only tentative explanations. Superclusters are large aggregations of galaxies not held together by gravity, instead behaving almost like smaller versions of walls. Walls are vast, thin, spread-out "sheets" of galaxies. The longest of these is called the Great Wall and is millions of light years across. Voids are vast holes of "emptiness" interspersed with the walls. Voids seem to contain no galaxies, but were recently found to contain huge hydrogen-gas clouds instead. These clouds have as much or more mass as a galaxy group. Their identity is yet to be confirmed, but the most popular speculations describe them as primordial hydrogen left over from the Big Bang or as the dim halos of undetected galaxies too faint, distant, or small to be seen by today's instruments.
*Note: This information was obtained primarily from "The Evolution of Galaxy Clusters," by J. Patrick Henry, Ulrich G. Briel, and Hans Böhringer; Scientific American, Dec. 1998. The images used here were based on but not copied from the images accompanying the same article.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.