Measuring the motions of gas

Because a black hole is both massive and compact, it exerts a strong gravitational pull on the material around it. Astronomers can deduce the presence of a supermassive black hole in the core of a galaxy by measuring the velocities of clouds of gas that orbit the black hole. The gas emits radio waves and other forms of energy. Measuring changes in the speed of the gas clouds toward or away from Earth, astronomers measure their speeds. A more-massive black hole will accelerate nearby stars to greater speeds, so the velocities of gas clouds can reveal not only the presence of a black hole, but its mass as well. Because gas usually follows circular orbits, it is easier to use them to measure the mass of the black hole they orbit.

More about black hole discovery methods »

This document was last modified: March 14, 2012.