Thinkquest Entry 20632

Galaxy M100

Definition : A massive collection of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. The total mass may be from 10^6 to 10^13 times that of the Sun, and it may stretch a region from 2,000 to 60,000 parsecs across. Galaxies appear to be arranged in aggregations called CLUSTERS with a few members to several thousand.

Astronomers estimate that there are about 50 billion galaxies in the universe. Besides stars and planets, galaxies contain clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays.

Early History of the Study of Galaxies

By the middle of the 18th century, three galaxies had been identified. In 1780, the French astronomer Charles Messier published a list that included 32 galaxies. These galaxies are now identified by their Messier (M) numbers; the Andromeda galaxy, for example, is known among astronomers as M31. Since 1900, galaxies have been discovered in large numbers by photographic searches. Galaxies which are far away from earth appear so tiny on a photograph that they can hardly be distinguished from stars.

The largest known galaxy has about 13 times as many stars as the Milky Way. In 1912 the American astronomer Vesto M. Slipher, working at the Lowell Observatory in Arizona, discovered that the lines in the spectrum of all galaxies were shifted toward the red spectral region. This was interpreted by the American astronomer Edwin Hubble as evidence that all galaxies are moving away from one another and led to the conclusion that the universe is expanding.

Andromeda Galaxy

Classification of Galaxies

Hubble classified galaxies into three classes according to their obvious visible features, ELLIPTICAL, SPIRAL and barred SPIRAL and also a group of irregulars.

Elliptical galaxies come in a vast range of sizes, from giant to dwarf. They range from the almost sperical to the very flattened lens-shaped. Typically, they are made up of older POPULATION 2 stars and hot gas but have little gas.

In contrast, spiral galaxies are flattened disk systems containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplace of stars. The barred spirals are similar in most ways to the spirals, but have a bar structure across the nucleus from the ends of which the spiral arms emerge.

Other disklike galaxies, with no overall spiral form, are classified as irregulars. These galaxies also have large amounts of gas, dust, and young stars, but no arrangement of a spiral form. They are usually located near larger galaxies, and their appearance is probably the result of a tidal encounter with the more massive galaxy. Some extremely peculiar galaxies are located in close groups of two or three, and their tidal interactions have caused distortions of spiral arms, producing warped disks and long streamer tails.

Distribution of Galaxies

Galaxies are generally not isolated in space but are often members of small or moderate-sized groups, which in turn form large clusters of galaxies. The earth's galaxy is one of a small group of about 20 galaxies that astronomers call the Local Group.

The earth's galaxy and the Andromeda galaxy are the two largest members, each with hundreds of billions of stars. The Large, Small, and Mini Magellanic Clouds are nearby satellite galaxies, but each is small and faint, with about 100 million stars. The nearest cluster is the Virgo cluster; the Local Group is an outlying member of the cluster, which contains thousands of galaxies of many types.