Milky Way

What is in the nucleus of the Milky Way? If we look with optical telescopes, we see nothing. The interstellar dust obscures the optical light. The center of the Milky Way does, however contain very strong sources of radio waves, infrared light, and x rays. One such source, called Sagittarius A*, appears to lie at the precise center of the galaxy, the point about which the entire system rotates.

The vast energy omitted by Sagittarius A* comes from a region that is less than one light day in diameter (about the size of the solar system) compared to over 120,000 light years for the entire galaxy. There is more energy produced in a very small volume of space than we can easily explain. There is certainly not enough room in this volume to contain enough stars to explain the energy production. What produces so much energy in such a small region of space? Most astronomers think that there is a supermassive black hole with the mass of a million suns, in the core of the Milky Way. Black holes are so highly compressed that a supermassive black hole capable of explaining the energy output of the Milky Way's core would still have a small volume.

Quasars and other active galaxies also emit far more energy than can easily be explained from a small region in their nuclei. An active galaxy is a galaxy with at least 100 times the energy output of the Milky Way. Quasars are among the most energetic and distant types of active galaxy. These galaxies are also thought to contain supermassive black holes in the nucleus, even more energetic than the one in the nucleus of the Milky Way. The nucleus of the Milky Way may be a quieter version of the nucleus of an active galaxy or a quasar.

There are many mysteries concerning the Milky Way, including the antimatter fountains, the nature of the energetic activity at its core, the unknown composition of the dark matter in the halo, and the uncertain process by which it formed.