Galaxy clusters, held together by the force of gravity, are among the largest cosmic structures known. Exerting a mutual pull, the far-flung galaxies in a cluster can extend many millions of light-years across space.
The Wide Field Imager at the La Silla Observatory in Chile produced a picture, released last week, of the galaxy cluster known as Abell 496. The image shows an area some 4.5 million light-years wide. It contains many hundreds of individual galaxies, say scientists at the European Southern Observatory (ESO). The most prominent is a large and bright elliptical galaxy, close to the center.
(The two nearly vertical dashes just to the left of center in the image indicate the trail of a relatively nearby asteroid -- within our solar system -- captured in two of the several frames that make up the composite image.)
Galaxies are classified into four major varieties, based on their appearance: Spirals, barred spirals, ellipticals and irregulars. The first two types, combined, comprise about 77 percent of all known galaxies.
The cluster Abell 496 is known for having a broad range of galaxy types, including numerous elliptical galaxies. There are also spiral galaxies, similar to the Milky Way, and other odd shapes. The majority, however, are dwarf galaxies, many of which will eventually merge with larger galaxies over millions and billions of years.
Typically, galaxy clusters are part of a larger grouping known as superclusters, which can contain dozens of clusters spread across as much as 100 million light-years of space. Superclusters, in turn, often align with other superclusters to form the spongy texture of the observable Universe.
Abell 496 is what scientists call a "rich" cluster, because it has so many galaxies. In contrast, our Milky Way is part of a "poor" cluster of only about 30 galaxies, known as the Local Group. Included in the Local Group are the Andromeda Galaxy (with which we are scheduled to merge in about six billion years) and the Large and Small Magellanic Clouds.
More than 90 percent of the mass of the cluster Abell 496 consists of mysterious, unseen "dark matter." Researchers don't know the nature of dark matter, but they suspect it may interact with visible matter only through the force of gravity. Dark matter is suspected to exist because the amount of observed matter is not sufficient to account for the gravity necessary to bind the cluster together.
Abell 496 also contains a diffuse but ubiquitous cloud of very hot gas, which will eventually be used to produce new stars.
ESO scientists say the galaxies in Abell 496 are about 500 million light-years away.
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