The stars in the Milky Way rotate around the Galactic center, which is obscured from our view by dust. The Galactic center can however be observed in radio and infrared radiation. The nucleus is marked by a bright radio source, Sagittarius A*. The stars around Sgr A* move with very large velocities, which suggests that they are being pulled by a very large mass at the center. The value of the mass inferred from the motions is 2.6 million solar masses. Observations indicate that Sgr A* is also very compact. All the evidence then points to the presence of a supermassive black hole.
Credit: Right panel - W. Keel (U. Alabama)
Top left - Photo of Milky Wavy Galaxy - Anglo-Australian Observatory (D. Malin). Image width is about 40 degrees.