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Sgr A* (centre) and two light reflections from a recent explosion (circled)

Sagittarius A* (pronounced "A-star", standard abbreviation Sgr A*) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of 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^[1]. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ± 0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU). Later observations ^[2] determined the mass of the object to be about 3.7 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU). By comparison, our sun's mass is approximately 2×10³⁰ kg and Neptune's orbital radius is 30 AU.

This is compatible with, and strong evidence in support of, the hypothesis that Sagittarius A* is a supermassive 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 Sagittarius A* radio emissions being associated with the accretion disc and polar jets of a supermassive black hole.

Sagittarius A* is "associated" with the supermassive black hole; what is seen is not strictly the black hole itself. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. The black hole itself emits only Hawking radiation. In the near future, astronomical interferometers may allow the direct imaging of the event horizon.

Coordinates (J2000): RA 17h 45m 40.045s Dec. −29.00775 degrees (About 10 degrees to the west of the center of the constellation Sagittarius, towards Scorpius)

1. ^ 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
2. ^ http://www.astro.ucla.edu/~jlu/gc/

• Melia, Fulvio, The Black Hole in the Center of Our Galaxy, Princeton U Press, 2003

• UCLA Faculty Research presentation on Sagittarius A* (Video)
• 2004 paper deducing mass of central black hole from orbits of 7 stars (arxiv preprint)
• ESO video clip of orbiting star (533 KB MPEG Video)
• Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours ESO Press Release, October 16, 2002
• The Galactic Center (outdated)
• Data and scientific papers about Sagittarius A*
• Max Planck page on the galactic center, with animation