This is a PSPC image of the center region of our galaxy. This region has been the subject of much study in many different fields of astronomy. Radio Astronomers found an intense radio source with strings of other radio sources clustered about it in the direction of the galactic center. The intense source was named Sagittarius A (Sgr A)because the center of the galaxy lies in the direction of the constellation Sagittarius. Within Sgr A lies a more "compact" radio source with a size of 80 AU by 150 AU (one AU is the distance from the Earth to the Sun). which was named Sgr A*; Sgr A* seems to mark the exact physical center of the Milky Way.
What about x-ray astronomy? In the 1970's the satellite Uhuru found an x-ray source two degrees in angular size. Later observations, including those made by the Einstein Observatory, turned up pointlike sources; they are most likely x-ray bursters or binary systems producing x-rays by matter being accreted onto a neutron star. One of these sources, with an x-ray luminosity of 10^28 Watts, is coincident with the presumed center of the Sgr A complex. The other sources lie in the same location sources found by astronomers observing Sgr A in the infrared.
Adding up radio, x-ray, and infrared observations has allowed astronomers to deduct that whatever is at the center is very small (astronomically speaking), very massive, emits enormous amounts of energy (perhaps as much as ten percent of the total from the Galaxy), and has material orbiting it at a very fast rate. This has led astronomers to hypothesize that a supermassive black hole may be lurking at the center of our galaxy!
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