How Do You Find a Black Hole?

How do you find a black hole if you can't see it? Scientists have found a way out of this dilemma. Black holes exert enormous gravity on nearby objects. Although scientists can't see a black hole, they can see its effects on the surrounding matter.

Binary star systems are the best for finding black holes. Astronomers have detected many binary systems. A binary star system is two stars that orbit around each other. Sometimes astronomers only see one star, orbiting around an unseen companion. It is possible this star is too dim to be seen from Earth. It could also be a black hole.

If a black hole were part of a binary star system, it would pull gaseous material off the surface of the neighboring star. Like water draining out of a bathtub, the gaseous material would spiral into the black hole. Scientists call this an accretion disk.

In 1970 the U.S. satellite Uhuru (see picture on right) was launched off the coast of East Africa. It was made to detect sources of x-rays. Uhuru found more than 100 stars giving out x-ray pulses. One of the most powerful sources it detected was Cygnus X-1, located 6,000 light-years from Earth.

When Cygnus X-1 was examined more closely, scientists found that it was a binary star system. They saw a supergiant star orbiting an unseen companion. By measuring the speed and orbit of the supergiant star, astronomers were able to figure out about how much mass the unseen object has. It had about six solar masses, or six times the mass of our Sun. By 1974 scientists figured out that Cygnus X-1 must be a black hole.

Scientists have found other x-ray sources that could be black holes. They include x-rays in the galaxy Markarian 335, discovered by the Astro Observatory aboard the Columbia in December 1990; LMC X-3 in the Large Magellanic Cloud, discovered in 1983; and A0620-00, discovered in the late 1980's, which may be one of the best chances for a black hole yet.

The Cygnus constellation