The false-color (left) of the radio wave emission
covers the inner 10 arcseconds (0.3 pc) of the Milky Way.
The compact radio source Sgr A*, most likely a super-massive
black hole, is the bright red spot at the center of the frame.
The diffuse green-red emission strongest to the south of
Sgr A* is from ionized material that may be spiraling inwards.
Why do we think that Sgr A* is a super-massive black hole? Proper motions of stars within 1 arcsecond show that these stars are moving extremely fast, some over 1000 km/s. Indeed, nearly complete stellar orbits have now been measured, clearly indicating a "dark" gravitational mass of nearly 4 million solar masses is contained within a region of about 100 AU centered on the position of Sgr A*.
Can we estimate the mass of the Sgr A* directly?
Just as the motions of stars near Sgr A* indicates the
amount of mass contained within 100 AU of Sgr A*, the
motion of Sgr A* can provide information on its mass.
The plot to the left shows the position of Sgr A*, as
measured against a distant Quasar over an 8 year
period. These measurements were made with the Very
Long Baseline Array operated by the National Radio
Astronomy Observatory, and funded by the National Science
Foundation. The shaded ellipses show the apparent size of
Sgr A* at the wavelength of 7 mm where the observations
were made. Sgr A* moves toward the lower right of the
plot at a rate of about 6 milli-arcseconds per year.
The best fit motion is shown by dashed-red line and the
orientation of the plane of the Milky Way is shown by the
The apparent motion of Sgr A* is almost entirely in the plane of the Milky Way and is a result, not of Sgr A* moving, but of the Sun orbiting the center of the Milky Way. Once the effect of the Sun's orbit is accounted for, we find that Sgr A* is stationary to within about 1 km/s (about 30 times slower than the Earth orbits the Sun!). Were Sgr A* simply an unusual star, with a mass of say 10 solar masses, it would be expected to move at speeds comparable to the stars in that region, probably over 1000 km/s. The low speed of Sgr A* implies it is indeed very massive. The best estimate is that Sgr A* contains over 10% (and probabl most) of the 4 million solar masses known to reside in the inner 100 AU region. This rules out any known type of star, and is strong circumstantial evidence that Sgr A* is a super-massive black hole.
Since Sgr A* is observed at radio wavlengths to be smaller that 1 AU (some giant stars are larger than this), this implies a truly incredible density, approaching within 3 orders of magnitude the "density" of black hole. Were even 0.4 million stars to be placed in such a small volume (comparable to the size of our extended Solar system), they would interact gravitationally, sometimes colliding, and either "evaporate" away or collapse into a black hole very rapidly Thus, the lack of detectable motion for Sgr A*, coupled with the mass required to explain the infrared star orbits, is overwhelming evidence that Sgr A* must be a super-massive black hole.
(References: Reid et al. 1999, ApJ, 524, 816; Schoedel et al, 2002, Nature, 419, 694; Ghez et al, 2005, ApJ, 620, 744; Reid et al. 2003, ApJ, 587, 208)