Cross-sectional illustration of an active galactic nucleus, or AGN. Many, perhaps most large galaxies, are thought to harbour supermassive black holes in their central regions. They can weigh anything from a few hundred thousand to several billion times the mass of a normal star. In some galaxies, such as our own Milky Way, these black holes may be dormant. But in active galaxies such as quasars and Seyfert galaxies, those with unusually bright central regions, the black hole is probably feeding off a vast accretion disc - a donut-shaped gas cloud. The gas in the disc spirals around and is heated to extremely high temperatures by friction, before falling into the hole. In some cases, magnetic fields thread the disc, and lead to the formation of beams of charged particles, which are expelled at great velocities, approaching light speed, along the rotation axis.