The central object

High energy Radiation

 Sgr A* is also an X-ray source, with an X-ray luminosity of ~ 105 Lsun.

(An outburst viewed in the infrared, from the UCLA Galactic Center Group)

Radio Interferometry

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!

Kinematics around the galactic center

Let's look at the velocities of stars around the galactic center. How would we expect the velocities of stars to change?
So we should expect the velocities of stars to not change much as we get close to the galactic center.

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.