Answer to Question 1
Observations show that Sagittarius A (abbreviated as Sgr A) is only a few astronomical units in diameter but is a powerful source of radio energy. The tremendous amount of infrared radiation coming from the central area appears to be produced by crowded stars and by dust warmed by those stars.Since the middle 1990s, astronomers have been able to use large infrared telescopes and adaptive optics to follow the motions of stars orbiting around Sgr A. The size and period of the orbit allows astronomers to calculate the mass of Sgr A using Kepler's third law. The orbital period of the star SO-2, for example, is 15.2 years, and the semimajor axis of its orbit is 950 AU. The combined motions of the observed stars suggest that Sgr A has a mass of 4 million solar masses.At its closest, SO-2 comes within 17 light-hours of Sgr A. The available evidence eliminates alternate hypotheses that Sgr A is a large cluster of normal stars or neutron stars or stellar-mass black holes. Only a single black hole could contain so much mass in so small a region.
Answer to Question 2
According to the modern studies, is clear that significant extra mass lies in the galaxy's halo, extending up to 10 times farther than the edge of the visible disk and containing up to two trillion (2 x 1012 ) solar masses. Some small fraction of this mass is made up of low-luminosity stars and white dwarfs, but most of the matter is not producing any light. Astronomers call it dark matter and conclude that it must be some as yet unknown form of matter.
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