SAGITTARIUS A

strongest source of cosmic radio waves from the direction of the constellation Sagittarius. The source has been identified as the nucleus of the Milky Way Galaxy. Most of the radio radiation is from a synchrotron mechanism, indicating the presence of free electrons and magnetic fields; this compact point source is designated Sagittarius A*. Hard X-ray, infrared, and spectroscopic investigations have indicated the relatively small dimensions of this region. In addition, the region comprises an intense source of infrared radiation that is thought to be emitted partly by stars and partly by circumstellar dust. Observations indicate that the galactic nucleus contains collapsed gas in the form of a black hole with a mass equivalent to 10,000 to 1,000,000 Suns. These properties are suggestive of those of other galaxies with active nuclei (e.g., Seyfert galaxies) but on a very much smaller scale.