Text courtesy of NASA 

Astrophysicists measure how forces of extreme gravity operate near a black hole by mapping the distortions of space-time predicated by Einstein's Theory of General Relativity. There are two classes of black holes: galactic black holes, the remains of massive stars 10-100 times the size of our Sun, and supermassive black holes, the powerhouses in the centers of galaxies that range up to billions of solar masses in size. Our own Galaxy harbors thousands of stellar black holes, and new observations show that most galaxies, including possibly our own, have a supermassive black hole at their core. Within both classes of black holes, space and time as we know it collapse. Thus, black holes are cosmic laboratories, allowing us to explore the ultimate limits of our physical laws and of gravity.

Disk around a black hole in Galaxy NGC 7052. Hubble image courtesy of NASA

A black hole and a companion star orbiting each other; the black hole pulls matter in from the star; this matter moves in a spiral and gives off a characteristic x-ray spectrum; drawing courtesy of NASA

Rotating gas disk in M87; measurements of its rotation rate provide a mass for the central object, which is about three billion solar masses; image courtesy of NASA 

One unavoidable challenge when studying black holes, however, is that they're almost invisible. A black hole is defined by a surface called the event horizon, the point at which gravity is so strong that nothing, not even light, can escape. The stellar matter itself is crushed into a singularity at the center, hidden behind the event horizon. The event horizon of a galactic black hole is only a few miles across. In supermassive black holes, it is only about the size of our Solar System.

Supermassive black holes are likely the power source for Active Galactic Nuclei (AGN), the bulging glow seen in many galactic cores. AGNs might be the result of a huge black hole gobbling up whole stars and pulling dust and gas from the nearby interstellar medium with such fury that the energy produced in this relatively small region (the size of our Solar System) outshines the entire galaxy.

How do we go about observing black holes if they are so compact and emit no visible light? There are a couple of tricks. Stellar black holes are often part of a binary star system, two stars revolving around each other. What we see from Earth is a visible star orbiting around what appears to be nothing. In reality, it is orbiting around the black hole. We can infer the mass of the black hole by the way the visible star is orbiting around it. The larger the black hole, the greater the gravitational pull, and the greater the effect on the visible star.

Another way we can “see” a black hole is by observing X-rays generated around it. Because a black hole has such a powerful gravitational force, a galactic black hole in a binary system can literally tear apart its companion star. Gas from the companion swirls into the black hole like water down a drain. The swirling gas is what we call an accretion disk. As the gas gets closer to the black hole, it heats up from the friction of ever faster moving gas molecules. just outside the black hole's event horizon, the gas heats to temperatures in the range of millions of degrees. Gas heated to these temperatures releases tremendous amounts of energy in the form of X-rays.

Supermassive black holes also have an accretion disk that emits X-rays. This is formed not by a single star, as in a binary system, but by the great amounts of gas present in the regions between stars. In about 10 percent of supermassive black holes, jets of energized matter thousands of light-years in length shoot out in opposite directions. This can be detected in radio, visible, X-ray and gamma-ray wavelengths. These jets accelerate matter to nearly the speed of light through a mechanism not well understood.

From small to large-scale black holes, many questions abound: How is material fed directly into the black hole? How do jets form? Why do some black holes have jets, while many more do not? What keeps the jets powered for millions of years? Why are AGNs more common in the past than today? How do supermassive black holes participate in the formation and evolution of galaxies? What are the masses and spins of black holes?