Key Concepts
One of the large self-gravitating aggregates of stars, gas, dust, planets, rocks, and dark matter that occurs by the hundreds of billions in the universe, collectively containing most of the cosmos' visible matter. Everywhere we look beyond our own galaxy into the broader cosmos, the objects we see are, likewise, galaxies (Fig. 1). These “islands in the void” often occur in groups or clusters containing from a few to as many as thousands of individual galaxies, with the clusters ranging from a few hundred thousand to tens of millions of light-years in expanse. See also: Light-year; Matter (physics); Star
The nearest undisputed galaxy to the Milky Way Galaxy, the Sagittarius Dwarf Galaxy, is about 80,000 light-years (470 quadrillion mi or 760 quadrillion km) away; the farthest, more than 13 billion light-years, having formed in the initial billion years after the big bang. Galaxies are the landmarks by which cosmologists survey the large-scale structure and evolution of the universe. See also: Cosmology; Milky Way Galaxy; Universe
Typical large galaxies have symmetric and regular forms, are about 50,000 light-years in diameter, and are roughly 30 billion times more luminous than the Sun. The stars and other material within a galaxy move through it, often in regular rotation, with periods of a few hundred million years. The characteristic mass associated with a large galaxy is a few trillion times the mass of the Sun, or solar masses. (The solar mass is 4.4 × 1030 lb or 2 × 1030 kg.) See also: Mass; Sun
Composition
Galaxies consist of stars, gas, dust, planets, rocks, and a mysterious substance called dark matter. Astronomers can observe all of these things, at least for the nearest galaxies, except the dark matter, which is detected indirectly by its gravitational influence on visible matter. See also: Dark matter
Modern space telescopes and giant telescopes on the ground make it possible to determine the kinds of stars in a galaxy, the amount and composition of its gas, and the optical properties of its dust. Given the immense distances to galaxies, our instruments cannot detect the faint stars or—in rare exceptions of gravitational microlensing—the planets around extragalactic stars. Only bright blue supergiant stars and luminous red giants can be identified in nearby galaxies. Nevertheless, it is possible to determine the nature of the faint material by its collective contribution to the total light of the galaxy. Based on analyzing this aggregate light, the composition for even very distant galaxies can be described. See also: Gravitational lens; Stellar evolution
The mixture of star types varies depending on the evolutionary history of a particular galaxy. In the Milky Way Galaxy, there are two general types of stellar populations: One type (Population I) is characterized by the presence of young stars and by ongoing star formation. It is usually associated with the presence of gas. The second type (Population II) shows an absence of gas and young stars as well as other indications that star formation ceased long ago. The Sun is a Population I star. See also: Star; Stellar population
Galaxies contain gas (mostly un-ionized hydrogen) in amounts varying from essentially zero up to a considerable fraction of their total mass. Dust in galaxies, although small in mass (typically 1% of the gas mass), is often dramatic in appearance because it obscures the starlight. See also: Interstellar matter
Most, if not all, galaxies are dominated by dark matter, a form of matter whose nature is still unclear and whose existence has been confirmed only by gravitational effects on the surrounding visible matter. Even the outermost stars in a galaxy are influenced by the dark matter that lies farther from the center. The smallest galaxies, called dwarf galaxies, can contain overwhelming quantities of dark matter, exceeding 99%, especially the nearby low-surface-brightness dwarf galaxies such as those in the constellations Leo and Draco. See also: Constellation
Form and size
Scientists recognize three broad varieties of galaxies: spiral, elliptical, and irregular.
Spiral galaxies
A common form for a galaxy is to have is a disk with a central bulge. The disk is typically 100,000 light-years in diameter and only about 1,000 light-years thick. Its appearance is characterized by radially decreasing brightness with a superimposed spiral or bar pattern or both (Fig. 2 and Fig. 3). The central bulge may vary in size from hundreds to many thousands of light-years. Such galaxies are classified as spirals (S) and subclassified a b or c (for example Sa) to distinguish increasingly open spiral structure and small bulge size. The disks of these galaxies are dominated by Population I stars while their bulges contain mainly Population II stars. The Milky Way Galaxy is an Sb type. See also: Milky Way Galaxy
Many spiral and irregular galaxies have a nearly linear feature in their central regions. Called barred galaxies, these objects can otherwise fit into the general scheme of galaxy types. The letter B is added after the S in the classification of spiral galaxies that contain conspicuous barlike features (for example, M61 is an SBc galaxy). It is likely that almost all spiral galaxies have at least some stars moving in barlike orbits.
Elliptical galaxies
Another common type of galaxy is an ellipsoid with radially decreasing brightness. These galaxies are classified as ellipticals (E) and subclassified according to their axial ratios by a number from 0 (E0 = round; Fig. 4) to 7 (E7 = 3-to-1 axial ratio). Additionally, ellipticals are subclassified according to the shape of their lines of constant brightness on images, so-called isophotes. Very luminous E galaxies show isophotes with boxlike shapes (boxy E) while less luminous E galaxies show isophotes with disk-like shapes (disky E). Ellipticals may vary in size from thousands to several hundred thousand light-years. They are most commonly found in clusters of galaxies and often contain a hot gas halo and in about half of all cases appreciable amounts of dust. The brightest galaxies are usually ellipticals. They are dominated by Population II stars.
Irregular galaxies
Other, rarer forms of galaxies include a transition class called S0 that has a disk superimposed on an otherwise elliptical type of light distribution, and an irregular (Irr) class composed of galaxies with chaotic forms (Fig. 5) and generally low total luminosity. Some irregular galaxies appear to be small spiral galaxies that had their ordered shapes gravitationally scrambled by interactions with other galaxies. The Small Magellanic Cloud, classified as a dwarf irregular galaxy, likely lost its formerly barred, spiral shape from perturbations by its much larger neighbor, the Milky Way.
Exotic galaxy types
Some galaxies lie outside the normal range of morphologies. Most of these galaxies have suffered some disturbing event, such as a gravitational encounter, a merger, or violent nuclear activity. Other such galaxies seem to have started out with anomalous characteristics, such as the low-surface-brightness galaxies. Those with activity in their nuclei are described below.
Starbursters
One of the more spectacular examples of exotic galaxies is the starbursters, galaxies that are presently manufacturing stars at an unusually vigorous rate. It is now known that some gravitational impulse has triggered the unusual star formation activity in most cases. The burst is a temporary condition and the galaxies now bursting must have spent most of their lives in a quieter state. See also: Starburst galaxy
Low-surface-brightness galaxies
Another type of exotic galaxy is the low-surface-brightness galaxy, which has such a low spatial density of stars that it is almost invisible. The Sculptor dwarfs have many characteristics similar to the globular star clusters (very old stars, cluster-type variable stars, and smooth stellar distribution), but are millions of times less dense and dominated by dark matter. Other low-surface-brightness galaxies have a composite population of stars. For example, at least two star-forming events occurred in the Carina dwarf, one about 13 billion years ago and one about 7 billion years ago. See also: Star clusters
Another type of low-surface-brightness galaxy includes extreme irregular and spiral galaxies, which have some of the structural properties of the normal examples of these types but are so faint that it is difficult to detect them against the sky's ambient brightness. A significant fraction of the mass of the universe may be in the form of these nearly invisible galaxies.
Internal motions
The stars and interstellar matter in a galaxy revolve around the center of mass of the galaxy, which is often a bright nucleus. In a spiral galaxy such as the Milky Way, there are generally two types of stellar orbits. The stars in the flat plane tend to have nearly circular orbits, while the stars in the bulge and halo have more highly elliptical orbits. All of the motions are the result of the stars responding to the gravitational field of the galaxy. The galaxies are supported against gravitational collapse by these motions in the same sense in which the planets of the solar system are kept from falling into the Sun by their orbital motions. Usually, the inner regions in the plane of a spiral galaxy undergo nearly solid-body rotation (velocity proportional to radius), while the outer regions rotate differentially (velocity constant). The spread in velocity of the material within galaxies varies from one to several hundred miles per second. Typical orbital periods for stars are several hundred million years. See also: Orbital motion; Solar system
The distribution of kinetic energy into randomly oriented and circular rotational motions varies with the galaxy type. The disk of a spiral galaxy may have only about 1% of its total kinetic energy in random motions, while boxy E galaxies in contrast to disky E galaxies may have most of their kinetic energy in their random component. See also: Energy
Luminosities
The number of galaxies with total luminosities L is roughly proportional to L−y, where y is between 1 and 1.5 for luminosities less than about 30 billion solar luminosities. The number is exponentially cut off for higher luminosities. The brightest observed galaxies are fainter than 200 billion solar luminosities; the faintest, brighter than about 1 million. The Milky Way Galaxy's total luminosity is roughly 10 billion times greater than the Sun's.
The distribution of luminosities is such that while there are very many faint galaxies, they do not contribute a large fraction of the total light given off by galaxies. Only the brighter galaxies, visible for great distances through space, can be observed easily and in great numbers. It is generally found that the masses of galaxies are roughly proportional to their luminosities. See also: Mass-luminosity relation
At each luminosity, the fraction of different galaxy types strongly varies with luminosity. The most luminous galaxies are mainly ellipticals and the less luminous mainly spirals. Additionally the number of elliptical galaxies at all luminosities increases in environments where the density of galaxies is high.
Clustering and origin of cosmic structure
Although galaxies are scattered through space in all directions for as far as they can be observed, their distribution is not uniform or random. Most galaxies are found in associations (Fig. 6) containing from two to hundreds of individual bright galaxies and at least 10 times as many fainter dwarf galaxies. The E and S0 galaxies tend to be concentrated in large clusters more strongly than spirals. The Milky Way Galaxy and the Andromeda Galaxy (Fig. 2) are members of a cluster called the Local Group. See also: Andromeda Galaxy; Local group
Like the stars within a galaxy, the galaxies within a cluster move about under the influence of their mutual gravitational attraction. The motions are generally randomly oriented and the cluster shows little evidence of rotation. Typical velocities range from about a hundred up to a thousand miles per second or more.
On scales larger than individual small groups and rich clusters, the distribution of galaxies through space is still not random. This very large-scale structure in the galaxy distribution is usually referred to as superclustering to indicate that it involves the higher-order clustering of the individual first-order associations of galaxies. The largest structures generally do not have the shape of spherical clusters, but rather the concentrations of galaxies tend to be filamentary in nature. Between filaments there are often large-scale voids, in which few galaxies are found. This overall architecture of the observable universe, expressed in galaxies and intergalactic matter associated with them, is called the Cosmic Web.
The universe's large-scale structure is generally attributed to the hierarchical clustering that resulted from the gravitational instability from primordial fluctuations immediately following the big bang. These fluctuations created over- and underdense regions of mass. The overdense regions would have systematically formed small galaxies first, followed by larger and larger concentrations of more massive galaxies as the smaller galaxies merged. Evidence for the existence of the density fluctuations that are inferred to have seeded structure formation has come from the discovery and mapping of the relic temperature fluctuations in the cosmic microwave background. Indeed, computer simulations of conditions in the early universe indicate that the observed large-scale distribution of galaxies and galaxy clusters is a natural result of the evolution of the universe after the big bang, when the earliest structures began to form in what was then a much denser universe. See also: Big bang theory; Cosmic background radiation; Galaxy formation and evolution
Active nuclei
In the very central regions of galaxies, violent astrophysical behavior is often observed. In the most extreme cases, the energy in the nuclear activity surpasses that in the rest of the galaxy combined. The objects that exhibit this behavior are called active galactic nuclei.
A characteristic feature of active galactic nuclei is the ejection of large masses of high-temperature gas at great velocities. Temperatures and velocities are in the range of tens to hundreds of thousands of kelvins and thousands to over 10,000 mi/s (16,000 km/s). Total gas masses exceeding 1 million times that of the Sun may be involved. These powerful gas flows reveal themselves as bright and broad spectral lines.
A second feature of active galactic nuclei is the emission of radiation over a wide range of different wavelengths, from the radio to gamma rays. An ordinary star emits radiation, because it is hot, in a characteristic wavelength band. The typical active galactic nucleus instead emits nonthermal radiation, implying that it is produced by a quite different mechanism. See also: Heat radiation; Radio astronomy
Another indication that the radiation has a nonthermal origin is the dramatic brightness changes of active galactic nuclei over relatively (by astronomical standards) short periods of time. Most active galactic nuclei show moderate changes over time scales of months to years, and some show dramatic variations (by factors of 2 to 10 or more) over times ranging down to a few hours. Since sources of radiation cannot generally change their brightnesses in times much shorter than that required for light to travel across them, these variations imply that the radiation from an active galactic nucleus arises in a very small region, in extreme cases no larger than the solar system. This is a fantastically small volume, considering that the total radiative power output can rival or exceed that of an entire galaxy many tens of thousands of light-years across.
Another important and peculiar phenomenon exhibited by active galactic nuclei is the emission of highly directional jets of relativistic plasmas and magnetic fields. These jets contain elementary particles, particularly electrons, moving at velocities near the speed of light, and often extend far outside the nucleus and even beyond the whole body of the galaxy. See also: Electron; Elementary particle; Plasma (physics)
There is a variety of classes of active galactic nuclei. Seyfert galaxies display the broad emission lines produced by the rapid outflow of hot gas but frequently do not exhibit much radio-wavelength emission. Another complementary class shows strong radio emission but weak or absent emission lines. Yet another class (BL Lac objects, often referred to as blazars) also shows only weak emission lines but is often extremely variable. Quasars are the most extreme sort of active galactic nuclei, with emission so intense that the ordinary galaxy in which they exist is entirely lost in the glare of the nuclear emission. It appears that active galactic nuclei in general, and certainly quasars, were much more common during the early history of the universe than they are at present. If so, most or all large galaxies may contain the burned-out remnant of an active nucleus in their centers, although only about 1% still show detectable activity. See also: Quasar
The energy source that drives all of these diverse phenomena is released when matter falls into a supermassive black hole occupying the center of a galaxy (Fig. 7). These black holes are found to have masses in the rough range of a million to 10 billion solar masses. The origin of supermassive black holes continues to be explored, though part of their aggregate bulk owes to the high density of material expected to accumulate at the center of a galaxy due to its gravitational field. Such a black hole will continue to accrete any gas that finds its way into the vicinity. See also: Black hole
As this gas falls toward the black hole, its angular momentum causes it to take up a nearly circular orbit in a disk of material surrounding the black hole. This disk (called an accretion disk) slowly injects gas into the black hole. The black hole's enormous gravitational field compresses and heats the gas to very high temperatures, causing it to radiate. Intense jet radio emission, powered by energy released during infall onto the central black hole, is ejected along the minor axis of the accretion disk. Depending on the viewing angle of the observer, the resulting morphology can account for a wide variety of active galactic nuclei, radio galaxies, and quasars. See also: Angular momentum
A given mass of gas can release 10 or more times as much energy in this way as it could if it were used as nuclear fuel in a star or a reactor. A gas infall rate onto the central engine of several solar masses per year suffices to power the most luminous active galactic nuclei in the universe. There is ample gas available in the interstellar medium to act as a fuel supply. Gas may be driven into the central regions of the galaxy, perhaps following a merger, where it can supply and activate the nucleus. The mechanisms that convert the thermal radiation generated in this way into the nonthermal radiation and relativistic plasmas observed in active galactic nuclei are not well-known. See also: High-energy astrophysics
Virtually all large galaxies have supermassive black holes in their centers, though most are not active enough to qualify as active nuclei. The feedback from these black holes into their wider galactic hosts has a profound influence on a galaxy's overall starmaking and evolution over cosmic history, a mechanism that remains the subject of significant ongoing research.
