Types Of Galaxies And Galactic Clusters

Type of physical science: Astronomy; Astrophysics

Field of study: Galaxies

Bound together by gravity, stars compose galaxies, of which there are several distinct types, and their mutual gravitational attraction draws galaxies into clusters. Explaining these formations is one goal of cosmology.

Overview

The study of galaxy and cluster types seeks to explain shared structures and characteristics among the arrangements of luminous matter outside Earth's Milky Way galaxy.

The search, astrophysicists believe, will provide essential clues to the origin and development of the universe. This search is almost entirely a twentieth century endeavor. Although some philosophers and scientists had proposed earlier that certain "spiral nebulas" might lie outside the Milky Way, it was not until the 1920's that astronomers accepted the fact that Earth's galaxy is only one of countless "island universes" in the vastness of outer space, each of which comprises billions of stars like the sun that are bound together by gravity. In 1925, Edwin Powell Hubble announced that he had located Cepheid variable stars in the Andromeda galaxy. The intrinsic brightness of these stars was known, so he was able to compare their apparent brightness with their intrinsic brightness, calculate the distance to Andromeda, and prove it was a discrete system lying far outside the Milky Way. Other astronomers soon made similar discoveries about other galaxies.

Hubble also proposed the first morphological classification system for galaxies. He discerned three basic types: elliptical, spiral, and irregular. Elliptical galaxies are spheroid in structure--almost none appears to be a perfect sphere--and are classified by the difference between their length and width in relation to their axis of rotation on a scale of E0 to E7 (E stands for elliptical). E0 galaxies show a nearly circular outline, while the flattened E7 galaxies resemble fat, stubby cigars in profile. Elliptical galaxies largely lack gas or dust clouds or hot bright stars. The only visible internal structures are globular star clusters, which usually proliferate, and the stars of an elliptical circle its galactic center in complex patterns.

Furthermore, analysis of an elliptical's spectrum will suggest that its stars are all old and of moderate size. The largest ellipticals are about five times larger in diameter and fifty times more massive than the Milky Way, which is between 70,000 and 100,000 light-years in diameter; the smallest, called dwarf ellipticals, are about one hundred times smaller and a million times less massive. Astronomers believe that ellipticals account for about 70 percent of all galaxies in the universe, most of which, being dim dwarfs, are difficult to observe.

Spiral galaxies have the basic shape of a discus, as their stars orbit a central bulge, or nucleus, and are classified in three distinct types: S0 galaxies, normal spirals, and barred spirals.

S0 galaxies have little obvious internal structure, showing a uniform disk with a large nucleus, and like ellipticals contain little gas and dust and few hot bright stars. Accordingly, they are considered to be intermediate between spirals and ellipticals. Normal spirals are further classified a to c depending upon how tightly their spiral arms are wound about the nucleus. Sa galaxies have closely wound arms and relatively little dust and gas; Sb galaxies show a definite whirlpool structure, with the ends of the arms loose in intergalactic space, and contain more dust and gas; and Sc galaxies look like pinwheels, and large gas and dust clouds are evident. The Milky Way and the closest spiral to it, Andromeda, are Sb galaxies. Barred spirals are similarly classified as SBa, SBb, and SBc depending on how tightly wound their arms are. They differ from normal spirals in that their nucleus is elongated, so that their arms look like streamers being spun from the ends of a thick central rod. In composition, they otherwise resemble normal spirals. The largest spirals are about one and a half times larger and slightly more massive than the Milky Way; the smallest are about five times smaller and have about 1 percent of its mass. Spiral galaxies are thought to include about 15 percent of all galaxies.

Irregular galaxies have little or no evidence of spiral arms, nuclei, or overall symmetrical shape; instead, they look like dense, chaotic patches of stars. Their most prominent feature is the presence of large clouds of gas and dust, in which are embedded both young and old stars. Irregulars are small--between 5 and 25 percent of the Milky Way's diameter--and account for about 15 percent of all galaxies. The Milky Way's closest neighbors, the Magellanic Clouds, are irregulars and are visible to the naked eye in the Southern Hemisphere.

Hubble's system has been the basic morphological schema since he introduced it, but since World War II, astronomers have observed an increasing array of bizarre galactic phenomena that suggest classifying by appearance alone is insufficient. Consequently, they also distinguish "normal" galaxies, classified by the Hubble system, from "peculiar" or Arp galaxies (if they are contained in Halton C. Arp's ATLAS OF PECULIAR GALAXIES, 1966), which either emit intense energy or have novel structures. Energy emitters are also known as "active" galaxies.

Those with strange structures are relatively rare and may be the result of interactions between galaxies. For example, some extremely large, apparently elliptical galaxies are now designated "cD." The D indicates in astronomical notation that the central sphere is surrounded by an envelope of stars, and the c denotes unusual size. Some contain multiple nuclei, so that cD's are suspected to be mergers of two or more galaxies, as if an elliptical has swallowed but not completely digested smaller neighbors, a process called galactic cannibalism. Other structural peculiarities include ring galaxies, which either show no nucleus or have an off-center nucleus; polar-ring galaxies; and galactic arcs. These may result from a close encounter with another galaxy or be the product of a rare phenomenon called a gravitation lens, in which a distant galaxy's image is distorted when a nearer galaxy's gravitational field bends the former's light.

When astronomers began using radio telescopes in the 1950's, they found that some galaxies broadcast very high levels of energy. These were dubbed "radio galaxies," and subsequent observations found galaxies that similarly emit intense amounts of ultraviolet, X ray, and infrared radiation. The discoveries prompted a host of new, sometimes overlapping designations. Radio galaxies include those that emit tens of thousands of times more radio radiation than normal galaxies. Most are giant ellipticals, in which a central object shoots out beams of high-energy particles hundreds of thousands of light-years beyond the border of their visible stars. These beams often terminate in pear-shaped lobes that contain regions of intense radio emissions.

Megamaser galaxies produce strong emissions because their interstellar gases amplify the radiation from their stars in the same way a maser does (maser, a predecessor of the laser, stands for microwave amplification by the stimulated emission of radiation). Seyfert galaxies are spirals with very small cores that fluctuate in brightness and can be radio or X-ray sources; many show disturbances in the spiral structure, perhaps caused by the gravity of a nearby galaxy. Markarian galaxies have abnormal amounts of blue light and strong continuous ultraviolet radiation. By 1986, astronomers had cataloged about fifteen hundred of these galaxies. Others with unusual visible light characteristics, suggesting intense activity in the nucleus, include liner galaxies (an acronym for low-ionization narrow emission-line region) and starburst galaxies.

Finally, astronomers identify two galaxy-like phenomena that may represent early stages in galactic evolution. The first is the protogalaxy--that is, a galaxy in the process of forming. They are believed to have been common in the distant past, but it is disputed whether any exist now. The second is the quasar, a blend of "quasi-stellar object," so called because the first were mistakenly thought to be starlike objects inside the Milky Way. Although a controversial subject, most astronomers accept them as the most distant visible objects in the universe; the farthest are as much as 15.5 billion light-years away and, judging from the redshifts in their spectra, speeding away from Earth at more than half the speed of light.

Not only does gravity gather stars into galaxies but also it gathers galaxies into clusters.

Those containing fewer than a thousand galaxies are called poor clusters or groups. Their resident galaxies are loosely associated, there is little intergalactic gas, and they have a large proportion of spirals. The Milky Way, Andromeda, and twenty-seven other galaxies belong to a poor cluster called the Local Group, which is about 1 megaparsec in diameter and is probably only a suburb of the much larger Vega cluster. Clusters that contain more than a thousand galaxies are called rich clusters. Their galaxies tend to condense toward a central point, often occupied by a cD galaxy; regions of hot, sometimes X-ray-emitting gases lie between galaxies; and ellipticals predominate. Rich clusters range up to 10 megaparsecs in diameter.

Applications

The investigation into galaxy and cluster types has helped scientists to understand their evolution and mass. Although fundamental questions remain unanswered about these matters, the latter half of the twentieth century has seen startling developments because of computer simulations and new types of ground-based and space-borne telescopes that greatly increase the range and accuracy of extragalactic observations.

Hubble proposed that galaxies have evolved in a uniform pattern during the history of the universe: He theorized that spheroid ellipticals flatten until they become tight spirals, which gradually unwind until they become irregular galaxies. While the evidence has proved this schema to be incorrect, Hubble did stimulate astronomers to explain the relation between the various types. They have approached the problem by first observing the properties of structure and composition in galaxies and then preparing mathematical models and computer programs based on the information to test various theories of evolution. Furthermore, computer graphic displays have helped astronomers visualize evolutionary processes in minutes that require millions of years in actuality.

From these techniques, two basic theories have emerged. The first assumes that galaxies originally formed when clouds of gas collapsed as a result of gravity; however, astronomers now believe that this theory alone cannot account for the variety of galaxy types.

The second theory proposed that collisions, mergers, and gravitational interactions among galaxies have determined their structures. It may seem unlikely that galaxies ever come close enough to affect one another, much less collide, but actually collisions and mergers are relatively common. The average distance between any two galaxies is only twenty times their diameter, not very much more than the average distance between cars on an interstate highway, and observations, in fact, have detected galaxies that have already collided and others that appear destined to collide. Computer simulations suggest that a head-on, high-speed collision should produce a ring galaxy, and a near miss or glancing collision can start spiral structures in elliptical galaxies. When a small galaxy passes through a larger one, the complex gravitational forces derange the former's structure, sending its stars into random motions; the result is an irregular galaxy.

Computer simulations also suggest that often galaxies never entirely escape each other's gravitational field after they collide, especially when they approach at low velocities; instead, they slow, fall back, and pass through each other again and again until they finally merge into a single large galaxy. This is the case especially if one galaxy is much larger than the other.

Mergers are particularly common in rich, dense clusters, which often have giant ellipticals at their center, but even though the Milky Way is in a poor cluster, astronomers believe it, too, has benefited from this galactic cannibalism, having partially digested the Magellanic Clouds as they passed through millions of years ago.

Clusters are believed to evolve--as poor clusters converge into rich clusters because of mutual gravitational attraction--and gravitationally related structures larger than clusters have been identified: superclusters, or clusters of clusters. Computer simulations and large-scale surveys indicate that superclusters, in turn, may outline bubbles connected by long, narrow filaments of galaxies, between which are immense voids, as if the universe were structured like a sponge.

Because their redshifted spectra indicate that galaxies are speeding away from one another, astronomers have theorized that there is a relation between a galaxy's distance from Earth and its evolutionary stage. Since light travels at a constant value, once a galaxy's distance is estimated, its age relative to Earth is apparent. For example, when a galaxy is observed a million light-years away, one is actually seeing light that was produced a million years ago. So, galaxies at the limit of observation may represent the structures assumed shortly after the origin of the universe. The most distant known objects are quasars; they may therefore be either intensely active galactic nuclei, suggesting that such activity is normal in young galaxies, or interacting galaxies, similarly suggesting that galactic interaction has been a feature of the universe from early epochs.

Attempts to calculate the masses of galaxies have raised unexpectedly daunting problems since the 1970's. The most astonishing is that visible matter accounts for only about 10 percent of the mass needed to make galaxies and clusters gravitationally stable. The rest of the mass, astrophysicists have hypothesized, is invisible to current telescopes. This phenomenon is called the dark matter, or missing mass, problem.

Context

The primary goals of cosmology are to explain the origin, evolution, and structure of space-time, and for this reason the structures and composition of galaxies and clusters have been studied intensively to yield data upon which to base a unified theory. A fundamental assumption behind this effort is the cosmological principle. It postulates that the universe should look the same in all directions from any vantage point (or isotropy) and that matter should be evenly distributed (or homogeneity). The fact that galaxies have been detected in every direction and as far as instruments can detect supports isotropy, but homogeneity has been more difficult to reconcile with observation.

Most cosmologists subscribe to variations of the big bang theory, first proposed by Georges Lemaitre in 1927 to explain why galaxies are hurtling away from one another at high velocities. The theory states that the universe began with the explosion of a "primal atom," and about 1 million years later, its radiation began cooling enough to congeal into the present galaxies. After radical revisions, the big bang theory has succeeded in accounting for structures the size of clusters, but complexes of superclusters and voids makes it appear that matter is not evenly distributed throughout the universe, despite what the theory predicts. Accordingly, cosmologists have supposed that as yet undetected phenomena exist whose forces lie behind large-scale structure. For example, some have suggested that one-dimensional faults exist in space-time, remnants of the big bang. These "cosmic strings" are thought to be either infinitely long or looped and to have gravitational fields strong enough to draw matter into galaxies and clusters.

Another unresolved question is whether--given the assumptions about the big bang--the universe will continue to expand forever or gradually slow to a stop and then reverse direction until it squeezes back into a single object. To answer the question, cosmologists need to know the amount of matter in the universe; the studies of galaxies that have indicated that only about 10 percent of the matter is visible greatly complicate the problem. Combining subatomic particle theories with cosmological theories in grand unified theories (GUTs), cosmologists have predicted many exotic particles within and between galaxies that could constitute the missing mass, but experiments designed to detect them have proved ambiguous or negative.

Galaxies and clusters are likely to remain a focus of investigations into the nature of the universe for some time. In the meantime, their magnificent forms testify to the rich diversity of space and the great depth of time.

Bibliography

Bartusiak, Marcia. THURSDAY'S UNIVERSE. New York: Times Books, 1986. This highly readable overview of modern astronomical discoveries relates the general classifications of galaxies and clusters to theories of the large-scale structure and origin of the universe. Nevertheless, the details of galaxy and cluster variation are superficially described.

Ferris, Timothy. COMING OF AGE IN THE MILKY WAY. New York: William Morrow, 1988. Ferris presents a history of the ideas and observations that led to modern cosmological theories, including both astronomical and subatomic phenomena. Although only two of the twenty chapters pertain directly to galaxies, the book is particularly valuable and entertaining for anecdotal information about the controversies that shaped modern knowledge.

Hodge, Paul W. GALAXIES. Cambridge, Mass.: Harvard University Press, 1986. The single best book for general readers coming to the subject for the first time. Through orderly, lucid explanations and a delightful style, Hodge discusses the composition, structures, and formation of galaxies and clusters. Photographs and diagrams are well placed to help the reader visualize the information in the text.

Hubble, Edwin. THE REALM OF THE NEBULAE. New Haven, Conn.: Yale University Press, 1936. Now largely of historical interest, the book--collecting a series of lectures delivered at Yale University in 1935--is Hubble's first thorough presentation to a general audience of his foundation-laying observations and theories. Many of Hubble's ideas still underlie astronomical measurement and morphology, although in revised form.

Schorn, Ronald A. "The Extragalactic Zoo." 4 parts. SKY AND TELESCOPE 75 (January, 1988): 23-27; 75 (April, 1988): 376-388; 76 (July, 1988): 36-37; 76 (October, 1988): 344-345. This well-titled article attempts to sort out the often duplicative terminology relating to phenomena that astronomers are trying to explain, such as radio galaxies and quasars. Well illustrated with photographs and diagrams.

Seeds, Michael A. FOUNDATIONS OF ASTRONOMY. 2d ed. Belmont, Calif.: Wadsworth, 1990. This large introductory-level college textbook devotes three chapters to galaxies and their modern astronomical theory: "Galaxies," "Peculiar Galaxies," and "Cosmology." Contains clear, simple explanations of complicated phenomena, and includes a wealth of photographs, diagrams, graphs, and drawings.

Principal terms

The Evolution of the Universe

The Expansion of the Universe

Large-Scale Structure in the Universe

ASTROPHYSICS: the study of the physics and chemistry of celestial objects and forces

COSMOLOGY: the study of the origin and structure of the universe

GRAVITY: a fundamental force of nature defined, in accordance with the theory of general relativity, as the curvature of space caused by a mass

LIGHT-YEAR: the distance light travels in one year at 300,000 kilometers per second, or about 10 trillion kilometers

MEGAPARSEC: a unit of measurement equaling 3.26 million light years

REDSHIFT: shifting in the emission lines of a light source's spectrum caused by its outward velocity

SPECTRUM: the distribution of light emissions by wavelength, which provides information about the chemical composition of the light source