Milky Way

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Milky 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. Although its motion is not readily apparent, the entire galaxy is rotating about the Milky Way's center. Relative to the universe, the galaxy is moving at a speed of c.370 mi per sec (c.590 km per sec) in the same direction that the constellation Leo lies relative to the earth; it is also moving at c.60 mi per sec (c.100 km per sec) relative to the center of mass of 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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 of galaxies. The sun, traveling at a speed of c.150 mi per sec (c.240 km per sec) in a nearly circular orbit, takes some 230 million years to complete one revolution.

Visual Characteristics of the Milky Way

Among the constellations the Milky Way passes through are Carina, Crux (the Southern Cross), Sagittarius (where it is brightest), Scorpius, Aquila, Cygnus, Perseus, Cassiopeia, Auriga, and Gemini. In the direction of Cygnus is the Great Rift, a band of dark matter that lies along the Milky Way, dividing it into two forks. Another dark region is the Coalsack, in Crux. Once believed to be vast empty regions in space, these dark areas are now known to be clouds of dark matter blotting out the light behind them. Such nonluminous clouds of dust and gas, called dark 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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, obscure many parts of the sky from sight; in the direction of the galactic center, the view is almost entirely obscured.

Size and Shape of the Milky Way

The Milky Way is a large barred spiral galaxygalaxy,
large aggregation of stars, gas, dust, and usually dark matter, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
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 comprising an estimated 200 billion stars (some estimates range as high as 400 billion) arrayed in the form of a disk, with a central elliptical bulge (some 12,000 light-years in diameter) of closely packed stars lying in the direction of Sagittarius. A compact radio source found there, Sagittarius A*, is believed to be a supermassive black holeblack hole,
in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
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; such black holes are believed to lie at the center of most spiral and elliptical galaxies. The central bulge is surrounded by a flat disk marked by six spiral arms that project from a dense, elongated concentration of stars, or bar, that runs through the bulge—four major and two minor—which wind out from the nucleus like a giant pinwheel. Our sun is situated in one of the smaller arms, called the Local or Orion Arm, that connect the more substantial next inner arm and the next outer arm. The sun lies roughly 27,000 light-years from the center of the galaxy, and in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way. The diameter of the disk is c.100,000 light-years; its average thickness is 10,000 light-years, increasing to 30,000 light-years at the nucleus.

Certain features of the region near the sun suggested that our galaxy resembles 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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. In 1951 a group led by William Morgan detected evidence of spiral arms in Orion and Perseus. Another bright arm stretches from Sagittarius to Carina in the southern sky. With the development of radio astronomy, scientists have extended a nearly complete map of the spiral structure of the galaxy by tracing regions of hydrogen that dominate the spiral arms. The development of telescopes that could be placed in orbit led by 2005 to confirmation that the Milky Way was a barred spiral galaxy, not a spiral one as had been believed.

Surrounding the galaxy is a large spherical halo of globular star clustersstar cluster,
a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction.
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 that extends to a diameter of about 130,000 light-years; this is called the stellar halo. The galaxy also has a vast outer spherical region called the corona, or dark halo, which is as much as 600,000 light years in diameter and, in addition to dark matter which accounts for most of the Milky Way's mass, includes some distant globular clusters, the two nearby galaxies called the Magellanic clouds, and four smaller galaxies.

Stellar Populations and Galactic Evolution

The stars, gas, and dust that make up the Milky Way can be grouped into two broad 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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 that suggest how the galaxy evolved. The spiral arms and central plane of the Milky Way contain the interstellar gas, cosmic dust, and bright young stars categorized as Population I. The halo, spaces between the spiral arms, and central core of the galaxy contain the older, less spectacular stars that are categorized as Population II. This distribution can be explained by an evolutionary model in which an enormous cloud of gas and dust began to condense to form what are now Population II stars. The remaining gas and dust then collapsed, either suddenly or in stages, into the relatively thin disk in which Population I stars were (and still are being) formed.

Like other galaxies, the Milky Way is growing by absorbing small satellite galaxies. It is currently merging with the Large and Small Magellanic Clouds, a process that will be completed in about 100 million years. In 2003 a previously unknown galaxy was found to be colliding with the Milky Way. Its distinctive red stars are slowly being pulled into the Milky Way, and the dwarf will soon lose all its structure. Called the Canis Major dwarf galaxy after the constellation in which it lies, it is about 25,000 light years away from the solar system and 42,000 light years from the center of the Milky Way. This is closer than the Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the Milky Way. Several other galaxies are also, apparently, on a collision course with the Milky Way. The biggest and most spectacular collision will be with the Andromeda Galaxy. In about 2 billion years, massive tidal gravitational effects will tear spiral arms apart and start to shred the pinwheels from the outside in. The result will be an elliptical rather than a spiral Milky Way.


See E. J. Alfaro and A. J. Delgado, ed., The Formation of the Milky Way (1995); G. L. Vogt, The Milky Way (2002).

Milky Way

A dense band of faint stars that extends right round the celestial sphere, dividing it into roughly equal parts. Its central line marks the central plane of our Galaxy, inclined at about 63° to the celestial equator. Although the vast majority of the stars are too faint to be seen individually, they are collectively visible on a clear moonless night as a diffuse band of light. The Milky Way is seen because the Sun lies close to the central plane of the galactic disk. Because this region of the Galaxy is highly flattened, a much greater depth of stars is visible in directions along the plane (i.e. toward the Milky Way) than in other directions. The Milky Way has a distinctly patchy appearance; it also varies considerably in width and brightness and is noticeably brighter toward Sagittarius (the direction of the galactic center). Many of the apparent gaps are due to dark nebulae, such as the Coalsack, along our line of sight, which prevent stars behind them from being seen.

Milky Way


a faint, luminous, and diffuse whitish band that encircles the stellar sky and whose north pole is in the constellation Coma Berenices. It consists of myriads of faint stars not individually visible by the unaided eye but separately distinguishable in a telescope or on photographs taken at a sufficient resolution. The visual appearance of the Milky Way is a consequence of the perspective from which we view it; we view the large, highly flattened cluster of stars of our galaxy from the inside, from a point situated near the plane of symmetry of the cluster. The brightness of the Milky Way varies with location. The band of the Milky Way is about 5°–30° wide and has the form of a cloud structure. The structure is a consequence of (1) the existence of stellar clouds, or condensations, and (2) the nonuniformity in the distribution of the light-absorbing dark dust nebulae that form sections with an apparent deficit of stars owing to the absorption of the light of these stars. The term “Milky Way” originated from a Greek myth, in which the goddess Hera, while feeding Heracles from her breast, inundated the heavens with maternal milk.


Bok, B., and P. Bok. Mlechnyi put’ Moscow-Leningrad, 1948. (Translated from English.)
Agekian, T. A. Zvezdy, galaktiki, metagalaktika. Moscow, 1966.


Milky Way

[′mil·kē ′wā]
The faint band of light which encircles the sky and results from the combined light of the many stars near the plane of our galaxy.

Milky Way

1. the diffuse band of light stretching across the night sky that consists of millions of faint stars, nebulae, etc., within our Galaxy
2. another name for the Galaxy
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