We don’t lie nearly close enough to an X-ray source to see this activity, however. So, to make a convincing case for a black hole, astronomers must find an X-ray-emitting binary star in which one star has more than 3 times the mass of the Sun, but remains invisible. And that’s exactly what they’ve done with the binary system known as Cygnus X-1 (so called because it was the first X-ray source discovered in the constellation Cygnus). In this case, the invisible object tips the scale at five times the Sun’s mass, making it almost certainly a black hole.
 

       Another good place to look for a black hole is in the heart of an active galaxy or quasar. These objects radiate tremendous amounts of energy in their central regions. A quasar, for example, can shine as bright as a thousand normal galaxies from a region no larger than our solar system. The same process happens here as in an X-ray binary system, only here entire stars may get swallowed into a supermassive black hole—some of which weigh in at more than a billion solar masses. In just the past two years, observations with the Hubble Space Telescope have detected such supermassive black holes in several galaxies.

       What does general relativity tell us about black holes? First, they seem to possess only three characteristics: mass, charge, and rotation. All other information about the matter that gets swallowed is lost. Second, the event horizon marks the “edge” of the black hole. As the name suggests, this is the boundary between the black hole and the rest of the universe, where a beam of light just fails to escape. Anything that passes inside the event horizon will be lost to the universe forever, even though its mass will continue to exert a gravitational pull in our universe. Likewise, nothing inside the event horizon will ever be able to escape the black hole’s gravitational clutches because nothing can travel faster than the speed of light.
 

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