Astronomers have found that the centers of some galaxies shine with more light than their stars can make and that their brightness changes over time. They speculate that large black holes there are responsible for this unexpected light.

Galactic Nuclei

A galactic nucleus is a galaxy's center. Over time, gravity causes most of the stars and other matter in the galaxy to clump together in its center.

Models

The centers of galaxies are too far away to visit, and things there happen too slowly for people to witness. Fortunately, our knowledge of them lets us model their behavior on computers. We can fine-tune the models until they behave like real galaxies.

Black Holes

At the end of their lifetimes, stars a few times bigger than our Sun may collapse after they expend their fuel. The force of their gravity is so strong that it overcomes the strong nuclear force, which prevents matter from occupying the same point in space. They thus become black holes, massive objects without volume. Inside a black hole's area of influence, known as the event horizon, no matter or light can escape its powerful gravity. We can observe black holes only indirectly, when matter nearby gives off light and X-rays.

Accretion

Once a black hole forms in the center of a galaxy, it continues to pull in stars around it, making it bigger. This process is called accretion. When the stars fall into the black hole, they create intense radiation and light.

Mass

Black holes can draw in an enormous amount of matter. The black hole at the center of our Milky Way galaxy is estimated to have 3.6 million times the mass of our Sun.

Source:

Cornell University: Black Hole Formation

University of Alabama: Active Galactic Nuclei

Max Planck Institute: Galactic Center Research