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Airdate: Dec 11, 1997
Scientist: Neil DeGrasse Tyson

BLACK HOLES: Detecting

BLACK HOLES: Detecting
If a black hole is capable of preventing even light from escaping it, how do we know they are there at all?

Transcript:
Imagine hunting for an invisible bear in the woods. You'll never see the bear, but you know it's there because you can see it's footprints. For scientists, that's pretty much what it's like searching for a black hole. I'm Jim Metzner, and this is the Pulse of the Planet, presented by the American Museum of Natural History.

A black hole is an object with a gravity field so strong that not even light can escape it. Well, how do we know they're out there, then? Dr. Neil Tyson, director of the Hayden Planetarium, explains:

"Sure, if a black hole were isolated in space, it would be extremely difficult to deduce its existence. But, if it happened to be in a binary pair, then you have two objects there, one a normal star, and one the black hole, in orbit around each other. Later in that star's life, it will become a red giant, where it swells to bulbous proportions. And then the outer envelope of material around that red giant will be flayed from the outer surface, and spiral down onto this black hole. The material funnels in to the black hole, and heats up the gas, and then it radiates ultraviolet and in some cases X-Rays."

Since stars don't normally give off these types of radiation in large amounts, when astronomers find a star that does, they consider it the "footprint" of a black hole. The black hole's incredibly strong gravity leaves us other clues.

"Einstein described space curving in response to the existence of matter. And it's that curvature of space that we interpret as gravity. A black hole would curve space so severely in its environment, that when you're looking at what might be a pattern of stars behind it, that pattern would become distorted. So if you want to know if a black hole is headed your way, and it doesn't have a companion star feeding it material, look for a continuously distorted starfield. Then duck."

Pulse of the Planet is presented by the American Museum of Natural History. I'm Jim Metzner.