Image of the Day: A Galaxy's Supermassive Engine

Astronomers believe that supermassive black holes are standard objects in galaxies with bulges such as the spectacular Seyfert galaxy NGC 4258 shown here. Black holes with masses of a million to a few billion times the mass of the Sun are believed to be the engines that power nuclear activity in galaxies. Some nuclei fire jets of energetic particles millions of light years into space. Almost all astronomers believe that this enormous outpouring of energy comes from the death throes of stars and gas that are falling into the central black hole. A giant black hole in a galactic nucleus exerts a powerful gravitational force on nearby gas and stars, causing them to move at high speeds. 

Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4000 km/s, and are believed to originate near an accretion disk surrounding the central black hole.