Sgr A* is also an X-ray source, with an X-ray luminosity of ~ 105 Lsun.
Using radio interferometry, we can study the structure of Sgr A* down to a few milliarcseconds. On these scales it is still unresolved, meaning that it must have a physical size of ~ 20 AU or less -- the size of the solar system!
But they do change. The stellar velocities rise rapidly in the inner few parsecs. What does this mean?
|Over the past 10 years,
two research groups (Ghez etal;
Eckart etal) have been studying the proper motion of stars at
the galactic center. To detect the transverse motion of
stars 8 kiloparsecs away, some of these stars must be
moving very fast: ~ 1500 km/s!
The velocity dispersion of these stars show a Keplerian falloff (v~1/sqrt(r)) from r=0.01 pc to r=0.1 pc:
This argues there is a lot of mass well inside a radius of 0.01 parsecs.
Actual Keplerian orbits can be derived
for individual stars passing w/in a few hundred AU of the
galactic center (Schodel etal
2002; Ghez etal 2002).
More stars have been tracked since then (from the
UCLA Galactic Center Group):
Keplerian orbits -- object is a point mass (or at
least much smaller than the orbit size). In mid-2014, one of the
orbiting stars (G2) passed within 130 AU of the central source
and remained on a Keplerian orbit.
All this data is consistent with a supermassive black hole at the
galactic center with a mass of ~ 4x106 Msun.