It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the centre of the spiral, Sagittarius A*.
Many astronomers believe that there is evidence that there is a supermassive black hole at the centre of the galaxy. Sagittarius A* is agreed to be the most plausible candidate for the location of this supermassive black hole.
An international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics observed the motion of the star S2 near to Sagittarius A* for a period of ten years, and obtained evidence that Sagittarius A* is a highly massive compact object. This is compatible with, and strong evidence in support of, the hypothesis that Sagittarius A* is a black hole.
From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 3.7 ± 1.5 million solar masses, confined in a volume with a radius of 17 light-hours or less.
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