Galaxy, external

(astronomy) Any galaxy known to exist, besides the Milky Way.

One of the large self-gravitating aggregates of stars, gas, and dust that contain nearly all of the observed matter in the universe. Typical large galaxies have symmetric and regular forms, are about 50,000 light-years (3 × 10¹⁷ mi or 5 × 10¹⁷ km) in diameter, and are roughly 3 × 10¹⁰ 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 nearest galaxies to the Milky Way Galaxy, the Magellanic Clouds, are about 150,000 light-years (9 × 10¹⁷ mi or 1.5 × 10¹⁸ km) away; the farthest, almost 1 × 10¹⁰ light-years (6 × 10²² mi or 1 × 10²³ km). See also Magellanic Clouds.

The hundreds of billions of stars making up a galaxy are not generally individually observable with current telescope technology because they are too faint and distant. Only the brightest stars in the nearest galaxies can be observed directly with large telescopes. Two general types of stellar populations are distinguished: 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.

Galaxies generally display strikingly regular forms. The most common form is a disk with a central bulge. The disk is typically 100,000 light-years (6 × 10¹⁷ mi or 1 × 10¹⁸ km) in diameter and only about 1000 light-years (6 × 10¹⁵ mi or 1 × 10¹⁶ km) thick. Its appearance is characterized by radially decreasing brightness with a superposed spiral or bar pattern, or both (see illustration). 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 letter B is added after the S in the classification of spiral galaxies that contain conspicuous barlike features. The Milky Way Galaxy is an Sb type. See also Milky Way Galaxy.

“Whirlpool” galaxy (NGC 5194), type Sc, and a companion irregular satellite (NGC 5195)

Another common type of galaxy is a featureless 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) to 7 (E7 = 3-to-1 axial ratio). They may vary in size from thousands to several hundred thousand light-years. They are most commonly found in clusters of galaxies and rarely contain much gas or dust. The brightest galaxies are usually ellipticals.

Some galaxies lie outside the normal range of morphologies. One of the more spectacular examples of such 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 at least most cases. Another type of exotic galaxy is the low-surface-brightness galaxies, star systems that have such a low spatial density of stars that they are almost invisible. A significant fraction of the mass of the universe may be in the form of these nearly invisible galaxies.

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 containing from two to hundreds of individual bright galaxies and at least 10 times as many fainter dwarf galaxies. The Milky Way Galaxy and the Andromeda Nebula are members of a cluster called the Local Group. See also Andromeda Galaxy; Local Group.

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. Neither empirical nor theoretical understanding of this very large scale structure is well established or generally agreed upon. See also Universe.

In the very central regions (sizes at least as small as a light-year, 6 × 10¹² mi or 1 × 10¹³ km) of galaxies, violent and apparently explosive behavior is often observed. This activity is manifested in many ways, including the high-velocity outflow of gas, strong nonthermal radio emission (implying relativistic particles and magnetic fields), intense and often polarized and highly variable radiation at infrared, optical, ultraviolet, and x-ray wavelengths, and ejection of jets of relativistic material. In the most extreme cases the energy in the nuclear activity surpasses that in the rest of the galaxy combined. These phenomena are generically referred to as nuclear activity, and the objects that exhibit them are called active galactic nuclei.

There are a variety of classes of active galactic nuclei. The 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. When active galactic nuclei achieve such great luminosities that they dominate that of the rest of the galaxy, they are sometimes referred to as AGNs. Quasars are widely believed to be the most extreme sort of active galactic nuclei, having emission so intense that the ordinary galaxy in which they exist is entirely lost in the glare of the nuclear emission. See also Quasar.

Perhaps the most intriguing question concerning active galactic nuclei is that of the nature of the energy source that drives all of their diverse phenomena. According to a widely accepted best guess or consensus model, active galactic nuclei are powered by the energy released when matter falls into a massive black hole occupying the center of a galaxy. These black holes are imagined to have masses in the rough range of 10⁶−10⁹ solar masses and to have formed because of 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. As such gas falls toward the black hole, its angular momentum will cause it to take up a nearly circular orbit in a disk of material surrounding the black hole. This disk (called an accretion disk) will slowly inject gas into the black hole. As the gas approaches the black hole, the latter's enormous gravitational field will compress and heat the gas to very high temperatures, causing it to radiate. 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. See also Astrophysics, high-energy; Black hole.