Galaxy

I

Introduction

Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. Astronomers estimate that there are about 125 billion galaxies in the universe. All the stars visible to the unaided eye from Earth belong to Earth’s galaxy, the Milky Way. The Sun, with its associated planets, is just one star in this galaxy. 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 (see Interstellar Matter).

II

Early History of the Study of Galaxies

A Persian astronomer, al-Sufi, is credited with first describing the spiral galaxy seen in the constellation Andromeda. By the middle of the 18th century, only 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.

Thousands of galaxies were identified and cataloged by the British astronomers Sir William Herschel, Caroline Herschel, and Sir John Herschel, during the early part of the 19th century. Since 1900 galaxies have been discovered in large numbers by photographic searches. Galaxies at enormous distances 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 (see Redshift; Spectroscopy). 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. It is not known if the universe will continue to expand or if it contains sufficient matter to slow down the galaxies gravitationally so they will eventually begin contracting to the point from which they arose. See Cosmology.

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III

Classification of Galaxies

When viewed or photographed with a large telescope, only the nearest galaxies exhibit individual stars. For most galaxies, only the combined light of all the stars is detected. Galaxies exhibit a variety of forms. Some have an overall globular shape, with a bright nucleus. Such galaxies, called ellipticals, contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a vast range of sizes, from giant to dwarf.

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 (see Star). Often the regions containing bright young stars and gas clouds are arranged in long spiral arms that can be observed to wind around the galaxy. Generally a halo of faint older stars surrounds the disk; a smaller nuclear bulge often exists, emitting two jets of energetic matter in opposite directions.

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. Ring galaxies, for example, form when a small galaxy collides with the center of a spiral galaxy. An intense ring of stars forms at the outer edges of the new, combined galaxy. The Hubble Space Telescope (HST) has revealed many more ring galaxies than astronomers expected, suggesting that galactic collisions may be common.

Quasars are objects that appear stellar or almost stellar, but their enormous redshifts identify them as objects at very large distances (see Quasar; Radio Astronomy). They are probably closely related to radio galaxies and to BL Lacertae objects. The Hubble Space Telescope (HST) completed a survey of nearby galaxies in 1996 that revealed that all large galaxies may be homes to quasars early in the galaxy’s life. The HST survey showed that most of the galaxies contain massive black holes, which may be the next stage in galactic evolution.

IV

Determination of Extragalactic Distances

In viewing a galaxy with a telescope, inferring its distance is impossible, for it may be a gigantic galaxy at a large distance or a smaller one closer to Earth. Astronomers estimate distances by comparing the brightness or sizes of objects in the unknown galaxy with those in Earth’s galaxy. The brightest stars, supernovas, star clusters, and gas clouds have been used for this purpose. Cepheid variables, stars the brightness of which varies periodically, are especially valuable because the period of pulsation is related to the intrinsic brightness of the star. By observing periodicity, the true brightness can be computed and compared with the apparent brightness; distance can then be inferred. Astronomers have learned that the speed of the stars as they orbit the center of their galaxy depends on the intrinsic brightness and mass of that galaxy. Rapidly rotating galaxies are extremely luminous; slowly rotating ones are intrinsically faint. If the orbital velocities of stars in a galaxy can be determined, then the distance of that galaxy can be inferred.

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