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large aggregation of starsstar,
hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.
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, gas, dust, and usually dark matterdark matter,
material that is believed to make up nearly 27% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiation, such as light or radio signals.
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, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky WayMilky Way,
the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
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 came from a study of the Andromeda GalaxyAndromeda Galaxy,
cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.
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 (1926–29) by Edwin P. HubbleHubble, Edwin Powell,
1889–1953, American astronomer, b. Marshfield, Mo. He did research (1914–17) at Yerkes Observatory, and joined (1919) the staff of Mt. Wilson Observatory, Pasadena, Calif., of which he became director. Building on V. M.
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 that indicated the great distances at which this and other galaxies are located. Previously, the galaxies had been classified with the luminous gas clouds, or bright nebulaenebula
[Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g.
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, within the Milky Way. The sun and its solar system, as well as the visible stars, are all in the Milky Way galaxy. Harlow ShapleyShapley, Harlow
, 1885–1972, American astronomer, b. Nashville, Mo., grad. Univ. of Missouri, 1910, Ph.D. Princeton, 1913. He was astronomer at Mt. Wilson Observatory from 1914 to 1921, when he became director of Harvard Observatory.
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 encouraged the exclusive use of the term "galaxies." Billions of galaxies are within the optical range of the largest telescopes. In 1996, analysis of photographs taken from the Hubble Space TelescopeHubble Space Telescope
(HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km)
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 increased the estimated number of galaxies from 10 billion to 50 billion, and subsequent observations after improvements to Hubble led astronomers to revise this upward to at least 100 billion. A subsequent estimate (2016) of 2 trillion includes many relatively small galaxies (some with as few as 1 million stars), most of which have merged during the life of the universe to form larger galaxies. Research with the Hubble Space Telescrope has also indicated that supermassive black holes are found at the center of large galaxy. A galaxy is held together by the gravitational attraction between its constituent parts (see gravitationgravitation,
the attractive force existing between any two particles of matter. The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called
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), while its rotational motion prevents it from collapsing on itself. Just as gravitation binds individual stars into galaxies, it also acts to coalesce galaxies together in clusters and cluster into superclusters, which are among the large structures of the universe and may span 150 million light-years or more. Computer simulations of the development of the universe further suggest that most galaxies are clustered along filamentlike structures that consist of dark matter, with large voids among the filaments, and dark matter also forms the framework about which galaxies themselves arise (most galaxies contain roughly five times more dark matter than normal matter). Many large galaxies have smaller galaxies, called satellite galaxies, in close proximity. The galaxies nearest the Milky Way form a cluster called the Local GroupLocal Group,
in astronomy, loose cluster of at least 40 nearby galaxies, including our own Milky Way galaxy, the Andromeda Galaxy, and the Magellanic Clouds. The Local Group is spread over an ellipsoidal region of space with a major axis of approximately 3 million light-years.
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. The Local Group includes the Andromeda Galaxy, which is a spiral galaxy similar in size and composition to the Milky Way, and the Magellanic Clouds, which are satellite galaxies of the Milky Way. The vast majority of observed galaxies are classified as either spiral or elliptical (football-shaped), with a small minority, e.g., the Magellanic Clouds, classified as irregular according to a scheme originated by E. Hubble. The Local Group is part of a supercluster, known as the Laniakea Supercluster, that spans some 500 million light-years. Although estimates of the age of the universe have varied (see Hubble's lawHubble's law,
in astronomy, statement that the distances between galaxies (see galaxy) or clusters of galaxies are continuously increasing and that therefore the universe is expanding.
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), current estimates place its age at around 13.75 billion years. The earliest galaxies imaged by modern astronomers include several that were formed less than 600 million years after that.

A typical spiral galaxy is shaped like a flat disk, about 100,000 light-yearslight-year,
in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 1012 km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant.
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 in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars (see stellar populationsstellar populations,
two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. Population I stars are young, recently formed stars, whereas population II stars are old and highly evolved.
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). This type of galaxy is further classified as being either a normal or a barred spiral. In the normal spiral, the arms, at least two in number, join smoothly with the nucleus; in the barred spiral, such as the Milky Way, the arms project from a bank of stars that runs through the nucleus. The elliptical galaxies, lacking spiral arms entirely and containing little or no gas and dust, resemble the nuclei of spiral galaxies. Their shapes vary from nearly spherical to highly flattened ellipsoids. Elliptical galaxies have a much greater variation in size, mass, and luminosity than do spiral galaxies; their sizes range from the largest known galaxies of all, with luminosities about 10 times that of the Andromeda Galaxy, to the small dwarf ellipticals, which can contain as few as a million stars. Irregular galaxies appear structureless and without any nucleus or rotational symmetry; their light comes mostly from young stars. Ultra diffuse galaxies can be as large as the Milky Way in extent but with only 1% of the stars.

Spiral galaxies contain a larger number of bluer, younger stars, while elliptical galaxies contain a larger number of redder, older stars. This has led astronomers to believe that stars initially cluster into spiral galaxies and that over time structural changes transform them into elliptical galaxies. Some researchers speculate that the transformation occurs because of gravitational forces exerted by galaxies as they slowly pass each other. Computer simulations suggest another alternative, called "galactic harassment," in which galaxies interact although they remain far apart and pass each other at high speeds. The most widely accepted alternative suggests that the transformation is caused by collisions of galaxies and gravitational tidal interactions between them as they travel through space, causing them to grow and evolve. Several dwarf galaxies are currently colliding with the Milky Way; others are on course to do so over the next 2 to 3 billion years. The collisions are not cataclysmic because galaxies—even though they may contain many billions of stars—are mostly "empty" space and the probability of two stars meeting is very small. However, the "empty" space is not really empty, it is full of gas and dust which can interact when the galaxies collide. There is also friction between the gas and dust in the colliding galaxies, causing shock waves that can trigger some star formation in the galaxies. These processes can radically affect the colliding galaxies, e.g., two spiral galaxies can merge to form an elliptical galaxy.

Many galaxies radiate a large fraction of their energy in forms other than visible light. With the development of radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
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, many radio galaxies were discovered. Other galaxies radiate strongly in the infrared, ultraviolet, or X-ray parts of the spectrum.


See R. J. Tayler, Galaxies, Structure and Evolution (1993); N. Henbest and H. Couper, The Guide to the Galaxy (1994); M. S. Longair, Galaxy Formation (1998); M. Merrifield and J. Binney, Galactic Astronomy (1998); L. S. Sparke and J. S. Gallagher 3d, Galaxies in the Universe (2d ed. 2007); H. Mo, F. van den Bosch and S. White, Galaxy Formation and Evolution (2010).

Schematic views of the Galaxy, (a) from above (b) from the sideclick for a larger image
Schematic views of the Galaxy, (a) from above (b) from the side


(gal-ăks-ee) (Milky Way System) The giant star system to which the Sun belongs. The Galaxy has a spiral structure and, like other spiral galaxies, is highly flattened. It is estimated to contain of the order of 100 billion (1011) stars, the bulk of which are organized into a relatively thin disk with an ellipsoidal bulge, or nucleus, at its center. This system is embedded in an approximately spherical halo of stars and globular clusters (see illustration). The radius of the disk is approximately 20 kiloparsecs and its maximum thickness (at the center) is about 4 kpc. The halo is more sparsely populated than the disk and its full extent is uncertain, although its radius is known to be greater than that of the disk. There may also be a very much larger dark halo, or corona, of unseen matter stretching out to a radius of 100 kpc or more (see below). The characteristic spiral arms, which contain many of the brightest stars in the Galaxy, wind outward from the nuclear region, in or close to the central plane of the disk.

The Sun is situated only a few parsecs north of the central plane, near the inner edge of one of the spiral arms. Our distance from the center is nominally taken as 8.5 kpc (IAU, 1985).

The entire Galaxy is rotating about an axis through the center, the disk rotating fairly rapidly, the halo more slowly (see galactic rotation). At the Sun's distance from the center, the systematic rotation of the disk stars is about 220 km s–1, whereas the halo system is rotating with a speed of only about 50 km s–1.

Many astronomers now believe that our Galaxy (and others) is enveloped in a huge dark halo. Its presence is inferred only from the rotation curve of the Galaxy and by its gravitational effect on the more distant globular clusters in the halo and on nearby dwarf galaxies. It contains ten times as much mass as the stars of the Galaxy. Most of the matter, like other dark matter, is likely to consist of some kind of elementary particle.

The objects in the halo are old stars or clusters of old stars, i.e. globular clusters, that belong to population II. They increase in density toward the center of the Galaxy but show little concentration toward the galactic plane. Stars in this system are believed to have condensed early in the life of the Galaxy, maybe 15 billion years ago, when the gas cloud from which it formed was still almost spherical. The bulk of the stars in the disk, and probably in the nucleus also, are stars of intermediate age (3–5 billion years) belonging to the disk population. The young population I stars are mainly confined to a layer about 500 pc thick along the center of the disk.

Most of the interstellar gas and dust detected in the Galaxy lies in or close to the galactic plane, and about half of it is concentrated into very dense molecular clouds distributed along the spiral arms. The youngest stars in the Galaxy, notably the T Tauri stars and the very bright, short-lived O and B stars, are also largely confined to the spiral arms and it is almost certain that stars are still being formed from the molecular clouds there. Tracing of the spiral structure is complicated by our position in the disk. Three relatively nearby sections of arms have been traced optically – principally by mapping the distribution of O and B stars and associated emission nebulae. These are the Orion arm, in which the Sun is located; the Perseus arm, located about 2 kpc farther out along the plane; and the Sagittarius arm, which lies about 2 kpc nearer the center. Another section, the Carina arm, may be a continuation of the Sagittarius arm, the concatenation being called the Sagittarius-Carina arm. Interstellar extinction makes optical tracing impossible beyond the first few kiloparsecs of the Sun in most directions. More distant spiral features have been traced by radio-astronomy techniques, mainly by mapping the distribution of neutral hydrogen in H I regions and, more recently, the carbon monoxide in molecular clouds. The analysis is not simple, however, and it is even uncertain whether the Galaxy is a two-armed or four-armed spiral.

Interstellar extinction also obscures the central nucleus of the Galaxy at optical wavelengths. Information about this region has been derived mainly from radio, infrared, and X-ray observations. See galactic center.

The total mass of the Galaxy (ignoring the dark halo) is estimated to be a little less than 2 × 1011 solar masses, of which about 10% exists in the form of interstellar matter. Where our Galaxy fits in the Hubble classification is still open to argument. Some observations suggest a type Sb structure, others a type Sc. There is evidence of a central bar extending to a radius of at least 2.4 kpc, with a mass of around 1010 solar masses.


(religion, spiritualism, and occult)

Contrary to what one might anticipate, stars are not evenly distributed throughout the universe. Instead, they cluster together in galaxies (from the Greek gala, meaning “milk”), which are large groupings containing billions of stars. Our galaxy is called the Milky Way.

The astrological effects of the fixed stars were the only influences from outside the solar system considered in traditional astrology. More recently, astrological researchers have begun to explore the potential astrological significance of galactic as well as extragalactic phenomena—phenomena such as the galactic center, black holes, pulsars, and quasars. This area of study is still very much in its infancy, with little information immediately applicable to the interpretation of individual natal charts.


Erlewine, Michael, and Margaret Erlewine. Astrophysical Directions. Ann Arbor, MI: Heart Center School of Astrology, 1977.
Sedgwick, Philip. The Astrology of Deep Space. Birmingham, MI: Seek-It Publications, 1984.


A large-scale aggregate of stars, gas, and dust; the aggregate is a separate system of stars covering a mass range from 107 to 1012 solar masses and ranging in diameter from 1500 to 300,000 light-years.




any of a vast number of star systems held together by gravitational attraction in an asymmetric shape (an irregular galaxy) or, more usually, in a symmetrical shape (a regular galaxy), which is either a spiral or an ellipse


the. the spiral galaxy, approximately 100 000 light years in diameter, that contains the solar system about three fifths of the distance from its centre


An extensible language in the vein of EL/1 and RCC.

["Introduction to the Galaxy Language", Anne F. Beetem et al, IEEE Software 6(3):55-62].
References in periodicals archive ?
(2014) Toward the standard population synthesis model of the X-ray background: Evolution of X-ray luminosity and absorption functions of active galactic nuclei including Compton-thick populations.
Eichler, "High-energy neutrino astronomy--a probe of galactic nuclei," The Astrophysical Journal, vol.
ETH researchers discovered in their extensive data collection of observed active galactic nuclei that about 5 percent appeared to be in a 'switched-off' state.
By sifting through records for 55 active galactic nuclei Alex Markowitz, an astrophysicist at the University of California, San Diego and the Karl Remeis Observatory in Bamberg, Germany and colleagues found a dozen instances when the X-ray signal dimmed for periods of time ranging from hours to years, presumably when a cloud of dense gas passed between the source and satellite.
The astronomers took advantage of three distant active galactic nuclei (AGN) that lie behind the cloud from our perspective.
hydro models of galaxy formation, active galactic nuclei, and cosmic rays.
Some (known as active galactic nuclei) are very bright, emitting intense radiation from superheated gas falling into the black hole.
Sixty-nine papers from the October 2011 conference present research on the physical characteristics of active galactic nuclei (AGN) accretion disk winds.
Astronomers refer to galaxy centers exhibiting such intense emission as active galactic nuclei (AGN).
Giacconi was an early figure in the pioneering use of x- ray observation in order to investigate the role of high-energy phenomena, explosions, high-energy particles, and million-Kelvin plasmas in the formation and evolution of the cosmos and to discover stellar-mass black holes and the process of energy generation through accretion onto collapsed objects, which powers active galactic nuclei and quasars.
This unified model said that the active nuclei in each type of galaxy were similar, but researchers are now proposing that the active galactic nuclei are actually different on a structural and energetic level.