When the gravity is very high, nothing, not even light, can get out. Such a place is called a black hole. It is called black because no light can escape from it, but we can’t know what effect causes the light trapped down there. Well, things may be attractively well-li on the inside.
Enigmatically indifferent to its surroundings, a black hole a kind of cosmic Cheshire cat. Indeed, when the gravity and density become sufficiently high, the black hole winks out and disappears from our universe. That is why it is called black: no light can escape from it.
However, even if a black hole is invisible from the outside, its gravitational presence is palpable. If, on an interstellar voyage, astronauts are not paying attention, they can find themselves drawn into this stellar corps irrevocably, their bodies stretched into a long, thin thread. But the matter accreting into a disk surrounding the black hole would be sight worth remembering, in the unlikely case that the crew survived the trip.
Black holes were first thought of by the English astronomer John Mitchell in 1783. But the idea seemed so bizarre that it was generally ignored until quite recently. Then, to the astonishment of many, evidence was actually found for the existence of black holes in space: the Earth’s atmosphere is opaque to X-rays.
To determine whether astronomical objects emit such short wavelengths of light, an X-ray telescope must be carried aloft. The first X-ray observatory was an admirably international effort, orbited by the United States from an Italian launch platform in the Indian Ocean off the coast of Kenya and named Uhuru, the Swajili word for “freedom”.
In 1971, Uhuru discovered a remarkably bright X-ray source in the constellation of Cygnus, the Swan, flickering on and off a thousand times a second. The source, called Cygnus X-1, must therefore be very small. Whatever the reason for the flicker, information on when to turn on and off can cross Cyg X-1, no faster than the speed of light, 300, 000 km/sec. Thus Cyg X-1 can be no larger than 300,000 km/sec X 1/10000 sec = 300 kilometers across. Something the size of an asteroid is a brilliant, blinking source of X-rays, visible over interstellar distances.
Cyg X-1, a mysterious brilliant, blinking source of X-rays, visible over interstellar distances. What does it hide from us? Image: © Megan Jorgensen
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