Supermassive Black Hole
The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center (approx. 18 hrs, -29 deg), and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of our Galaxy. Accretion of gas onto the black hole, probably involving a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments.
A study in 2008 which linked radio telescopes in Hawaii, Arizona and California (Very Long Baseline Interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3 AU). For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers (1.0 AU), while the distance of Mercury from the Sun at closest approach (perihelion) is 46 million kilometers (0.3 AU). Thus the diameter of the radio source is slightly less than the distance from Mercury to the Sun.
Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a supermassive black hole at the Galactic Center, on the order of 4 million solar masses.
Astronomers anticipate an increase of activity in the region of the black hole, giving opportunity for further study, in mid-2013, as it is expected that a large gas cloud will be disrupted by close approach.
Read more about this topic: Galactic Center
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