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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 50 billion galaxies in the universe. All the stars visible to the unaided eye from earth belong to the 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.
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 and 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. 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.
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. 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. Quasars are objects that appear stellar or almost stellar, but their enormous red shifts identify them as objects at very large distances. 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.
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 the 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. Recently 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.
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. They all share a common direction of motion, the cause of which might be a supercluster hidden from view by our own galaxy, since superclusters up to 300 million light-years across are known. Some theorists suggest instead that a cosmic "string" a one-dimensional flaw in the fabric of space-time, could be the cause. Overall, the distribution of clusters and superclusters in the universe is not uniform. Instead, superclusters of tens of thousands of galaxies are arranged in long, stringy, lacelike filaments, arranged around large voids. The Great Wall, a galactic filament discovered in 1989, stretches across more than half a billion light-years of space. Cosmologists theorize that "dark matter," a hypothetical material that neither radiates nor reflects light, has sufficient mass to generate the gravitational fields responsible for the heterogeneous structure of the universe. The most distant galaxies known, near the edge of the observable universe, are blue because of the hot, young stars they contain. Observing these galaxies from earth is difficult because the light and radiation they emit is mostly in the blue, violet, and ultraviolet range, a range that is mostly blocked by the earth's atmosphere. Astronomers have obtained images of young galaxies using the Keck Telescope in Hawaii and the Hubble Space Telescope, which resides in an orbit high above the earth's atmosphere and thus avoids atmospheric interference. Photos from the HST show galaxies that are as far as 13 billion light years away from the earth, which means they formed when the universe was just 10 to 20 percent of its current age. The galaxies appear to be spherical in shape, and may be early precursors of elliptical and spiral galaxies.
Rotation of Spiral Galaxies
Stars and gas clouds orbit about the center of their galaxy. Orbital periods are more than 100 million years. These motions are studied by measuring the positions of lines in the galaxy spectra. In spiral galaxies, the stars move in circular orbits, with velocities that increase with increasing distances from the center. At the edges of spiral disks, velocities of 300 km/sec (about 185 mi/sec) have been measured at distances as great as 150,000 light-years. This increase in velocity with increase in distance is unlike planetary velocities in the solar system, for example, where the velocities of planets decrease with increasing distance from the sun. This difference tells astronomers that the mass of a galaxy is not as centrally concentrated as is the mass in the solar system. A significant portion of galaxy mass is located at large distances from the center of the galaxy, but this mass has so little luminosity that it has only been detected by its gravitational attraction. Studies of velocities of stars in external galaxies have led to the belief that much of the mass in the universe is not visible as stars. Its exact nature is unknown at present.
Radiation from a Galaxy
Knowledge of the appearance of a galaxy is based on optical observations. Knowledge of the composition and motions of the individual stars comes from spectral studies in the optical region also. Because the hydrogen gas in the spiral arms of a galaxy radiates in the radio portion of the electromagnetic spectrum, many details of galactic structure are learned from studies in the radio region. The warm dust in the nucleus and spiral arms of a galaxy radiates in the infrared portion of the spectrum. Some galaxies radiate more energy in the optical region. Recent X-ray observations have confirmed that galactic halos contain hot gas, gas with temperatures of millions of degrees. X-ray emission is also observed from objects as varied as globular clusters, supernova remnants, and hot gas in clusters of galaxies. Observations in the ultraviolet region also reveal the properties of the gas in the halo, as well as details of the evolution of young stars in galaxies.
Carlos Loboguerrero
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