Sagittarius A


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Sagittarius A

The brightest member of a complex of radio sources at the galactic center. It has two components. Sagittarius A East is a source of nonthermal emission 8 parsecs across, and is probably a supernova remnant. Sagittarius A West is a spiral-shaped source of thermal emission from ionized gas and is about 2 parsecs across. It also contains an intense, very compact, and variable radio source, Sagittarius A* (pronounced A star), that is probably smaller than 10 AU and lies very close to or at the Galaxy's exact center. This ‘point’ source resembles a scaled-down version of the central radio sources in active galaxies, which are believed to be accretion disks of gases around a massive black hole. X-ray and gamma-ray observations show similar resemblances to an active galaxy's core.

Infrared and radio observations show that Sgr A West contains gas and dust with velocities up to 250 km s–1; there may be a ring of material in orbit about the compact central source, and also streams of gas falling toward the central source. The velocities indicate the gravitational influence of 6 × 106 solar masses within Sgr A West. Infrared observations of a source believed to be a cluster of stars at the Galaxy's core (almost coincident with the compact radio source) are consistent with only 3 × 106 solar masses of stars. The difference, 3 × 106 solar masses, may reside in a central black hole. Many of the properties of Sgr A West could, however, result from activity caused by star formation, as occurs in a starburst galaxy, so that the case for a black hole is not accepted by all astronomers.

Sagittarius A

[‚saj·ə′ter·ē·əs ′ā]
(astronomy)
An intense radio source in the constellation Sagittarius, apparently comprising a gaseous envelope surrounding a small dense core that is believed to constitute the center of the Milky Way Galaxy.