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At the center of the circum-nuclear disk we see a LOT of stars.
A million stars, packed into an area smaller than the distance
between the Sun and its nearest Stellar neighbor.
Very careful monitoring of the exact position of the bright, radio source, Sgr A* has been kept, relative to very distant objects for more than two decades. Sgr A*'s position NEVER varies. Everything around it varies, but the radio position of the unresolved source, Sgr A*, (shown as a red cross) does not.
What is Sgr A*?
By looking at the motions of the stars nearby (shown to the right), Astronomers have measured the mass of this unseen object. It appears to have a mass of:
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