As of 2015, Sgr A*, or Sagittarius A*, is believed to consist of a supermassive black hole surrounded by the remnants of a supernova. The remnant clouds are designated Sagittarius A East and Sagittarius A West, while the radio source at the center of the structure is Sagittarius A*.
Sagittarius A* was discovered in February 1974 as a powerful radio source at the heart of the Milky Way. It was given the asterisk designation because that is used in chemistry to denote excited atoms, and the astronomers felt its discovery was a particularly exciting astronomical event. By 2002, astronomers had ruled out a number of explanations for its powerful radio signature, and the theory that Sagittarius A* was a black hole became the leading explanation.
The Chandra X-Ray Observatory has provided a wealth of data about the structure of Sagittarius A*, including measuring the speeds of stars orbiting the object at the center of the cloud. Calculations of the gravitational pull of this object place its mass at over 4 million times the mass of the sun, with a radius of only 100 astronomical units. The incredible density of Sagittarius A* makes it unlikely that it could be anything other than a super-massive black hole.
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