13. What causes a black hole to form?

Answer: D (a large star's core collapses to become so dense that its gravitational pull keeps light from escaping)

black hole An extremely dense celestial object that has a gravitational field so strong that nothing can escape, not even light. A black hole is formed by the collapse of a massive star's core in a supernova. See more at star.

Did You Know?

Black Hole
One of the strangest objects in the universe is the burnt-out remnant of a large star, known as a black hole. The name comes from the fact that the star collapses into itself, becoming so dense that its gravitational pull keeps even light from escaping. And if light can't get out, then nothing that ever enters the black hole would ever escape. Rockets to the moon or Mars need to achieve what is called escape velocity, the speed necessary to overcome the Earth's gravity. But since nothing can ever go faster than the speed of light, nothing could ever go fast enough to reach the escape velocity necessary to pull out of a black hole. Here's how dense a black hole is: the sun has a diameter of about 864,000 miles (1,390,000 kilometers); for it to be as dense as a black hole, its entire mass would have to be squeezed down to a ball less than two miles across.

A Closer Look

A star is a giant sphere of gas that produces its own light by making its own energy. It generates this energy through nuclear fusion in its core, using as fuel the elements that make it up. Fusion in a star combines atoms of lighter elements, such as hydrogen, into different, heavier elements, such as helium. A massive star can burn its fuel into elements as heavy as iron. Nuclear fusion begins when a star is fully formed out of a nebula and ends when a smaller star burns out and becomes a white dwarf or when a massive star's iron core collapses in a supernova. Because a star's composition changes as it burns its fuel, it goes through a series of stages during the course of its life, hundreds of milions or billions of years long.


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