Sagittarius A

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Sagittarius A (or Sgr A) is a complex radio source at the center of our galaxy, the Milky Way. It is located in the sky in the Sagittarius constellation.

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 center 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. 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 (120 AU) or less.

This is compatible with, and strong evidence in support of, the hypothesis that Sagittarius A* is a black hole.

Several teams of researchers, including groups at the National Radio Astronomy Observatory and at the Shanghai Astronomical Observatory, have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry. The images obtained have been consistent with the Saggitarius A* radio emissions being associated with the accretion disc and polar jets of a supermassive black hole.

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References

Schödel, R. et al. "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way." Nature, 419, 694 - 696, (2002).446

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