Sagittarius A

Strongest source of cosmic radio waves, lying in the direction of the constellation Sagittarius. It was discovered by Karl Jansky in 1932 and has been identified as the nucleus of the Milky Way Galaxy. The region is relatively small and constitutes an intense source of infrared radiation, thought to be emitted partly by stars and partly by the dust around them. Within the region is a much smaller radio source, Sagittarius (Sgr) A*, which appears to mark the centre of the Galaxy. Observations indicate that the galactic nucleus contains a black hole with a mass 4,310,000 times that of the Sun; they also provide a strong case that Sgr A* is that black hole. The activity at the Galaxy's centre resembles that of an active galactic nucleus but on a much smaller scale.

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