What the Hell is that Thing?

Okay, we have a very small but luminous X-ray source at the center of the Galaxy. The radio core size is < 20 AU. We also know that the Keplerian motion of stars indicate there is a very massive object there. But we don't see anything. What is it?

Most likely, a supermassive black hole!

Supermassive Black Holes

If the mass of the supermassive black hole (SBH) at the Galactic Center is 3.7x10⁶ M_sun, what is its Schwarzchild radius? R_s = 2GM/c² = 0.07 AU ~ 14 R_sun

Gas motions around Sgr A*

Again, using radio interferometry we can study the motions of gas clouds close to Sgr A*. We find gas with very high velocities, ~ 800-1000 km/s, streaming away from Sgr A*. It has been suggested that we may be seeing the intense radiation field around Sgr A* blowing gas away from the center.
At the same time, from the motions of the molecular ring, we see evidence of mass accretion onto Sgr A* at the rate of 10^-3 to 10^-2 M_sun/yr.

The Monster at the Galactic Center

The density of stars in the Galactic center is high, and there is almost certainly a supermassive black hole at the center. Imagine a situation where a star strays to close to the SBH and gets tidally disrupted. (How close? About an AU or so...)
When that happens, a lot of mass will get dumped suddenly onto the accretion disk around the black hole. As it falls inwards, it converts gravitational energy into luminosity (optical, UV, and X-rays). This heats the surrounding region, ionizing the gas and blowing some of it outwards.

Once the star's material has been accreted, the luminosity dies down and the black hole sits and waits for its next victim. If this happens every 10,000 to 100,000 years, it would account for the observed properties of the Galactic Center.

The monster is quiet for now...

Do we see these things in other galaxies?

Yes! Using the Hubble Space Telescope, we can study the motions of stars in the centers of nearby galaxies, and find that many of them host supermassive black holes.
Here's evidence for a very massive one (M~10⁹ M_sun) in the giant elliptical galaxy M87:

So what would happen if, instead of eating a star every 100,000 years, the monster had a constant fuel supply -- stuff continually falling onto its accretion disk? Stay tuned....