Milky Way Galaxy The large disk-shaped aggregation of stars, gas, and dust in which the solar system is located. The term "Milky Way" is used to refer to the diffuse band of light visible in the night sky emanating from the Milky Way Galaxy.
Although the two terms are frequently used interchangeably. Milky Way Galaxy, or simply the Galaxy, refers to the physical object rather than its appearance in the night sky.
Structure and contents. The Milky Way Galaxy contains about 2 x 10^11 solar masses of visible matter. Roughly 96% is in the form of stars, and about 4% is in the form of interstellar gas. The gas both inside the stars and in the interstellar medium is primarily hydrogen and helium with a small admixture of all of the heavier atoms.
The mass of dust is about 1 % of the interstellar gas mass and is an insignificant fraction of the total mass of the Galaxy. Its presence, however, limits the view from the Earth in the plane of the Galaxy to a small fraction of the Galaxy's diameter in most directions.
The Milky Way Galaxy contains four major structural subdivisions: the nucleus, the bulge, the disk, and the halo. The Sun is located in the disk about half way between the center and the indistinct outer edge of the disk of stars. The currently accepted value of the distance of the Sun from the galactic center is 8.5 kiloparsecs, although some measurements suggest that the distance may be as small as 7 kpc.
The nucleus of the Milky Way is a region within a few tens of parsecs of the geometric center and is totally obscured at visible wavelengths. The nucleus is the source of very energetic activity detected by means of radio waves and infrared radiation.
At the galactic center, there is a very dense cluster of hot stars observed by means of its infrared radiation. In 1997. astronomers confirmed the existence of a black hole with a mass of about 2.5 million times the mass of the Sun at the position of an unresolved source of radio emission known as Sgr A* in the middle of the central star cluster. The black hole appears to be the dynamical center of the Milky Way.
The bulge is a thick distribution of stars centered on the nucleus which extends to a distance of about 3 kpc from the center. It contains a relatively old population of stars, nearly as old as the Milky Way itself. Direct imaging with infrared satellites has demonstrated that the bulge is actually an elongated barlike structure with a length about two to three times its width. The Milky Way is thus classified as a barred spiral galaxy, a classification that includes about half of all disk-shaped galaxies.
The disk is a thin distribution of stars and gas orbiting the nucleus of the Galaxy. The disk of stars begins near the end of the bar and can be identified to about 16 kpc from the center of the Galaxy; the disk of gas can be identified to about twice this distance, about 35 kpc from the center. The faint low-mass stars make up most of the mass of the disk. There is also a thick disk of stars and gas. The thin disk of stars contains most of the mass and has a thickness relative to its diameter similar to that of a commercial compact disk. The disk is the location of the spiral arms that are characteristic of most disk-shaped galaxies, as well as most of the present-day star formation.
The halo is a rarefied spheroidal distribution of stars nearly devoid of the interstellar gas and dust that surrounds the disk. The stars found in the halo are the oldest stars in the Galaxy. The stars are found individually as "field" stars as well as in globular clusters: spherical clusters of up to about a million stars with very low abundances of elements heavier than helium. The extent of the halo is not well determined, but globular clusters with distances of about 40 kpc from the center have been identified.
Dynamical evidence suggests that the halo contains nonluminous matter in some unknown form, commonly referred to as dark matter. The dark matter contains most of the mass of the Galaxy, dominating even that in the form of stars.
Evidence suggests that the ages of the oldest stars in the Milky Way are within about 10% of the age of the universe as a whole; thus parts of the Milky Way must have formed early in the history of the universe, about 12-16 billion years ago. There is increasing evidence that the Milky Way formed as a result of the coalescence of small galaxies and protogalaxies, objects with the masses of small dwarf galaxies that are thought to have been among the first objects to form in the Univers.
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