Structure Of Galaxies

Type of physical science: Astronomy; Astrophysics

Field of study: Galaxies

The study of the structure of galaxies helps astronomers understand the processes of galaxy formation, which, in turn, gives insights into star formation and the structure of the universe as well as its origin and future.

Overview

Nothing represents the essence of deep space astronomy as much as the majestic form of a spiral galaxy. Like a celestial pinwheel, star-studded arms dusted with interstellar material sweep out from a central core. The form belies its stately motion, which occurs on so vast a scale as to be humanly imperceptible. Although the spiral galaxy is the archetype, there are other types of galaxies with differing structures. To understand the dynamics that form the galaxies is to understand the very basic processes of the universe. This understanding develops a picture of how the universe began, how it evolved, and where these same processes will lead it in the distant future.

Galaxies are the largest individual objects in the universe, vast groupings of stars, gas, and dust, held together by gravitational attraction and their rotation around the nucleus. There are hundreds of billions of galaxies in the universe, each containing from hundreds of thousands to billions of stars. In their groupings, as well as in their structure, they are diverse. They are found in enormous clusters of tens of thousands of galaxies, held together by mutual gravitation, in groupings of only a few, or single galaxies that float solitarily in space. In turn, the clusters of galaxies form larger structures called superclusters. In their activity, galaxies are equally disparate. While some are quiet, others are engaged in inconceivably energetic activity.

The most familiar type of galaxy is the spiral galaxy. Earth's galaxy--the Milky Way--is a spiral galaxy. Other galaxies appear more spherical in shape and are termed ellipticals. Irregular galaxies have no particular symmetrical structure. For these galaxies, it appears that their structure is determined by wrenching gravitational and energetic forces. It was once thought that three-quarters of all galaxies were spirals, including barred spirals, with their arms originating from straight barlike structures running through the central core. In fact, if only the brightest galaxies are studied, this view appears to be true. Considering the number of small elliptical galaxies, however, the percentages are more likely to be 20 percent spirals, 10 percent barred spirals, 60 percent ellipticals, and 10 percent irregulars.

Spiral galaxies vary greatly in size, ranging from giant galaxies such as the Andromeda galaxy, containing at least 2 billion stars, to spirals only a tenth as large. While the details of the spiral structure can vary, the basic design of a spiral galaxy remains consistent. A spiral galaxy has a central core of stars called the bulge, which is spherical or slightly elliptical in shape.

Spreading outward from the core and revolving around the bulge is a pancake-shaped disk of material consisting of stars, gas, and dust. The disk of a spiral galaxy is marked by alternating lanes of bright and dark material that form the distinctive spiral arms. Here, new stars are continually being formed from gas and dust. The bright arms of the galaxy indicate the areas of stellar birth. The arms are often fragmented and indistinct, with smaller spurs projecting from the various arms.

A broader structure called the halo is defined by the orbits of both individual stars and spherical clusters of stars called globular clusters. The halo is centered on the central bulge and encompasses the entire disk of the galaxy. It can be as large as 400,000 light-years in diameter.

The globular clusters within the halo range in size from 15 to 300 light-years in diameter and contain from tens of thousands to a few million stars.

The motion of spiral galaxies indicates that beyond the visible portion of the galaxy lies a region of dark matter, referred to as the dark halo, which is undetectable by astronomical instruments. Apparently featureless, the dark halo extends another 100,000 light-years beyond the visible halo marked by the orbits of the globular clusters. Although it has never been observed directly, its gravitational effects are manifested in the orbits of stars within spiral galaxies. These effects can be measured, but the actual nature of the dark material remains a mystery.

The proportionate sizes of the galactic disk and the central bulge vary greatly between spiral galaxies. The bulge of some galaxies encompasses a major portion of the disk, extending 100,000 light-years in diameter. Other galaxies are dominated by the disk with its structure of spiral arms, while the central bulge appears a minor protuberance at the center. In a typical spiral galaxy, the disk is only one one-hundredth as thick as it is wide. The bulge of an average spiral galaxy contains about a billion stars. Here, the dust and gas found in the disk are virtually absent, and no new stars are formed. As opposed to the stars in the disk that revolve around the core in nearly circular orbits, the stars in the bulge revolve around the center of the nucleus in highly elliptical orbits. The orbits do not lie parallel to the disk of the galaxy, but are highly inclined.

The oldest stars in the galaxy are located in the bulge, at an age of perhaps 10 billion years.

These stars are formed in the very early stages of the formation of the galaxy.

While spiral galaxies are common in the universe, their spiral structure is not understood completely. According to the theories of motion, the spiral structure should not exist in galaxies in the numbers it does. While the rotation of a galaxy would be expected to create a spiral structure, it would also be expected to disappear within only a few rotations. A galactic "density wave" has been theorized as a mechanism to explain the enduring shape of spiral galaxies. According to the hypothesis, the density wave is a disturbance that moves through the material in the disk of a galaxy. Spiral in structure, the wave moves through the galaxy at a lower rate than the stars, gas, and dust. Periodically, material in the galaxy catches up with and falls into the density wave, where it slows down and bunches up. The gas that is slowed by the wave compresses, which triggers a burst of star formation. The young, bright stars formed at the boundaries of the density wave define the spiral structure of the galaxy.

Elliptical galaxies have no spiral structure at all. Their three-dimensional shape would appear as a flattened sphere. As opposed to spiral galaxies, they contain little or no interstellar dust and gas. The density of stars in elliptical galaxies increases toward their centers, with a corresponding increase in brightness. The stars in elliptical galaxies are older stars, similar to the stars found in the central bulges of spiral galaxies and globular clusters. Although they have relatively little structure, the largest and most massive galaxies in the universe tend to be elliptical. Ranging to the other extreme as well, some dwarf ellipticals contain fewer than a million stars.

Often, the structure of a galaxy is not symmetrical. These irregular galaxies have undergone some violent process that has wrenched the galaxy out of a symmetrical structure.

Many times, the irregularities are caused by interaction with other galaxies, either by the gravitational effects of passing closely by another galaxy or by colliding with one. The likelihood that a galaxy will suffer a collision with another galaxy sometime in its lifetime is relatively high.

When galaxies are in a cluster, they generally are grouped many times closer relative to their sizes than are the stars within the galaxies. Therefore, as they orbit around a shared center of gravity, the chances of collision or near-collision are high.

Since the relative distance between stars is great, individual stars are not likely to collide, even though one entire galaxy passes through another. Though individual stars do not interact, nebulas within the galaxies are likely to collide, producing clouds of dense, unstable gas and shock waves. Inside the galaxies, the orbits of stars would be warped and any planetary systems that existed would disintegrate. The primary force between galaxies, however, would be gravitational. The mutual attraction disturbs the arrangement of stars and gas in each galaxy, deforming their structure. The closer the galaxies approach, the greater the gravitational force and resulting deformation. Sometimes galaxies will merge together, or, in other cases, a smaller galaxy will punch a hole through a larger galaxy, carrying away material along with it.

Among the most intriguing objects in the universe are the active galaxies. In a normal galaxy, everything seems to be stable and in equilibrium. In active galaxies, there are strong emissions of energy, or the emissions vary rapidly. There are many types of active galaxies, but they represent different stages in the evolution of a single type of galaxy, which has a black hole at its center. They may indicate galaxies at the early stages of formation. Quasars are the smallest, brightest, and most distant active galaxies. They radiate extremely high levels of energy in the form of visible light, infrared radiation, and X rays. N galaxies are elliptical in shape and have very bright, small nuclei. BL Lacertae objects are a certain type of N galaxy that varies in luminosity. Radio galaxies are distinguished by two enormous regions that emit jets of radio waves from a very compact central source. The likeliest explanation for the source of the radiation is a rotating supermassive black hole at the core of the galaxy. Seyfert galaxies are disk-shaped and are similar to other active galaxies in that they have an extremely bright core.

Analysis of the spectra of these galaxies indicates that they are undergoing violent activity in their centers.

Galaxies are fascinating objects to study, mainly because their appearances are deceiving. Analyzing the galaxies at wavelengths ranging from radio through infrared, visible light, ultraviolet, X rays, and γ rays reveals that even some of the most ordinary of galaxies have extraordinary features.

Applications

The study of the galaxies is the key to the study of the universe. By studying galaxies and clusters of galaxies, deeper insights can be gained into the dynamics and structure of the universe. As the structure of galaxies is studied, astronomers can look at the processes that created these structures.

Once it was understood that the galaxies were moving away from one another, it was natural to look backward in time, when they must have been closer together. Taking this view to its obvious conclusion, it is assumed that at one time the galaxies were merged at some beginning point in time. It was this speculation on the origin of the galaxies that prompted the first serious study of the origin of the first moments of the universe. The most widely accepted theory is that everything that exists began with a colossal explosion some 20 billion years ago.

Matter was thrown out in all directions, creating a universe of incredibly hot subatomic particles.

As the matter expanded and cooled, it began to clump together into clouds of condensed gas that would eventually become galaxies. Stars began as small turbulences in these clouds, contracting until their mass and density allowed nuclear reactions to begin in their cores. The most massive stars gravitated toward the center to form a galactic nucleus. Gravitational interaction with other forming galaxies causes the galaxy to spin, and as it continues to contract on itself, the rate of its rotation increases. If star formation in the galaxy is very rapid, the gas and dust that make up the stars will be consumed almost completely, and an elliptical galaxy will result. In spiral galaxies, star formation slows once the core of the galaxy has formed. Unused material swirls in random motion around the nucleus, colliding and merging, with the end result as a central disk of the spiral galaxy. The disk represents the most stable condition resulting from the pull of gravity and the centifugal force of the galaxy's motion around the core. The origin of the spiral pattern that forms in the disk is as yet unknown. From the force of the original explosion, or the big bang, the galaxies and the universe continue to expand in the present day. This expansion of galaxies was discovered by Edwin Powell Hubble in the early 1920's.

Most of what is known about the structure of the Milky Way has come from observations at nonoptical wavelengths. Because of the dust and gas that is prevalent in spiral galaxies, much information is hidden from view. Nevertheless, other types of radiation, such as radio waves, infrared radiation, X rays, and γ rays, do penetrate the clouds of dust and gas and help to reveal the nature of the galaxy. The Milky Way measures 100,000 light-years in diameter and contains at least 100 billion stars. The Sun is located between two major spiral arms about 30,000 light-years from the nucleus of the galaxy. As seen from Earth, the center of the galaxy lies in the direction of the constellation Sagittarius.

Radio studies of the Milky Way have revealed surprising structures in the core of the galaxy. Aside from the disk of the galaxy in which the spiral arms lie, astronomers have found a second disk in the nucleus of the galaxy, which is tilted 20 degrees with respect to the main disk.

It is about 8,000 light-years across and rests completely within the central bulge of the galaxy. It seems to be rotating much faster than the material in the spiral arms and is expanding outward.

Five powerful radio sources have been discovered in this inner disk and have been named Sagittarius A through E, in order of their discovery.

Within this inner disk lies yet another disk, which is tilted with respect to both the inner and main disks. Inside this disk, at the very core of the galaxy, a supermassive black hole is suspected to reside. It encompasses an area about the size of a large star, yet it contains the mass of at least 4 million suns. The gravity of the black hole pulls in surrounding matter. As the material accelerates into the black hole, its temperature soars, causing it to emit radiation from all ranges of the electromagnetic spectrum. Evidence for black holes has been found in other galaxies. Although they cannot be observed directly and their existence remains theoretical, black holes are thought to be a major mechanism behind the tremendous energy radiated by some galaxies.

Context

Although the galaxies have been observed since the time of Galileo in the 1600's, their nature and the distance of the galaxies was not known until after the first quarter of the twentieth century. Early observers knew them as spiral "nebulas," for the Latin word meaning cloud.

Nebulas were believed to be part of the Milky Way. Until astronomers had the tools to measure the distances of these objects they could only guess at their nature.

Galaxies were first cataloged by comet hunters, who would catalog their positions in the sky so that they would not be mistaken for comets, which also appeared as dim, fuzzy patches of light. In 1784, Charles Messier made a list of one hundred of these objects that were not to be confused with comets. Some of the objects in the list were star clusters or glowing clouds of interstellar dust and gas. About a third of the list included galaxies. Sir William Herschel and his son, Sir John Frederick Herschel, continued Messier's work. By 1864, THE GENERAL CATALOGUE OF NEBULAE was published with 5,079 objects listed. By 1888, Johan Ludwig Emil Dreyer updated the list to include 7,840 objects and published THE NEW GENERAL CATALOGUE OF NEBULAE AND CLUSTERS OF STARS. In 1895, two volumes of the INDEX CATALOGUE were published, listing a total of fifteen thousand objects.

These listings are still in use today. Galaxies, clusters, and nebulas are listed by their "M,"

"NGC," or "IC" numbers, relating back to these important catalogs.

In the 1700's, Sir William Herschel first proposed that instead of the Sun being only one of a random scattering of stars in space, it was part of a disk-shaped system of stars. He drew the first maps of what this system of stars might resemble, and it became known as the galaxy, or the island universe.

Until the 1920's, the accepted cosmology of the universe was that the Milky Way galaxy was the only sizable object that existed. In 1923, Hubble discovered a method of measuring the distances to the galaxies, proving they were much farther away than anyone had seriously imagined before. He discovered that the Andromeda galaxy was a full-fledged star system similar to the Milky Way. By 1930, Hubble's work had changed the perceived face of the universe. In addition to determining the nature of the spiral nebulas, Hubble discovered that rather than being unchanging (as was the theory for hundreds of years), the galaxies were rushing apart from one another, and the universe was expanding. With this concept, the modern theories of cosmology were born. Modern astronomers believe that the universe is infinite and that there is nothing unique about Earth's position in space and time. The universe is basically the same everywhere when viewed on the largest scale. The Milky Way is merely one of billions of galaxies scattered across the cosmos.

Principal terms

ACTIVE GALAXY: a galaxy that contains a compact, highly energetic nucleus

BLACK HOLE: a theoretical object with such tremendous gravitational force that no radiation can escape from it

COSMOLOGY: the study of the nature of the universe as a whole, including its large-scale structure, motion, origin, and possible future

ELECTROMAGNETIC RADIATION: waves of electrical and magnetic energy that travel through space at the speed of light; the entire range of energy states from radio waves through infrared, visible light, ultraviolet, and X-ray and γ-ray radiation is known as the electromagnetic spectrum

LIGHT-YEAR: a unit of distance equal to the distance that light travels in a vacuum in one year; at a speed of approximately 300,000 kilometers per second, a light-year is about 10 trillion kilometers

MASS: the total amount of material in any object, determined by its gravity or inertia

NUCLEUS: in galaxies, the central region or core; in spiral galaxies, sometimes referred to as the bulge

SPIRAL ARM: a dense region of heavy star formation in the disk of a spiral galaxy

Bibliography

Editors of Time-Life Books. GALAXIES. Alexandria, Va.: Time-Life Books, 1989. One volume of a series examining different aspects of the universe. Comprehensively covers theories about galactic structure and formation, active galaxies, the Milky Way, and cosmological theories. Highly illustrated and suitable for the general reader with an interest in astronomy.

Henbest, Nigel. MYSTERIES OF THE UNIVERSE. New York: Van Nostrand Reinhold, 1981. Explores the limits of what is known about the universe. Ranges from theories about the origin of the solar system, galaxies, and the universe; exotic astronomy; and astronomy at invisible wavelengths.

Henbest, Nigel, and Michael Marten. THE NEW ASTRONOMY. New York: Cambridge University Press, 1983. Compares optical, infrared, ultraviolet, radio, and X-ray observations of well-known astronomical objects. Highly visual, and written specifically for general readers.

Kaufmann, William J. GALAXIES AND QUASARS. San Francisco: W. H. Freeman, 1979. A comprehensive and easy-to-read book that explores the subject of normal and active galaxies. Discusses the history of the discovery of galaxies, the structure of various galaxies including the Milky Way, galactic classification, and cosmological theories. Nontechnical language is geared toward the general reader.

Marten, Michael, and John Chesterman. THE RADIANT UNIVERSE. New York: Macmillan, 1980. An overview of imaging that is not accessible in visible wavelengths. Includes electronic processing, as well as infrared and ultraviolet wavelength imaging. Beautiful pictures along with easy-to-read and informative text.

Types of Galaxies and Galactic Clusters

The Evolution of the Universe

The Expansion of the Universe

Large-Scale Structure in the Universe