Astronomy

Exploring Black Holes



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TWINKLE, TWINKLE LITTLE STAR

Twinkle, twinkle little star. How I wonder what you are...

Have you ever wondered why stars twinkle? Do you gaze into the night sky and think, "How small are those stars"? Have you stared into space, wondering what is out there that can't be seen? The night sky is full of wonderful sights and surprises. The more you look, the more you can learn about the beautiful galaxy and the countless other stars that light the night sky.

Imagine yourself gazing into the sky on a clear night. Stars twinkle everywhere. Or do they? Starlight passes through Earth's atmosphere before reaching your eyes. It encounters some places where the air is warm and some places where the air is cold. This bends the light on its way to your vision. Because the light is bent, only a bit of starlight reaches your eyes, so the star looks dim. Other times, when more starlight reaches your eyes, the star looks brighter. The amount of starlight can change quickly, so the star appears to twinkle.

Picture a beach ball next to a grain of sand. The beach ball is a star, and the grain of sand is Earth. Stars look small. But they are not. Most stars are really huge-much bigger than the earth. At first, every star looks the same. But if you look closely, you can see that some stars look bigger than others. Some stars shine more brightly than others. Those stars much larger than the sun will experience a violent end. Those stars end up in an explosion that blasts them apart.

Now imagine an object resting in the great emptiness of space, totally undetectable except for its gravitational pull. This object is so huge and so tightly packed that not even light can escape from its strong whirlpool of gravity. Imagine yourself approaching and orbiting the object at the point which light rays become so bent, they actually curve all the way around until you can see the back of your head! You fall deeper and deeper toward the black as forces resembling a tornado, stretch your body and your spacecraft into spaghetti! You are experiencing one of nature's most amazing and bizarre objects known as the black hole.

Astronomers believe black holes form after a supernova- the explosion of a giant star. A star is born when matter remains perfectly balanced between exploding and collapsing. A star spends its life burning hydrogen and other fuels. As long as it burns these fuels, it produces an outward pull that balances the inward pull of gravity. The star produces so much heat that it explodes, becoming a supernova shining millions of times brighter than an ordinary star. When no more fuel remains, the star can no longer support its own weight. As a result, the exploding star throws off its outer layers and collapses until it is squeezed into an extremely small space smaller than an atom's nucleus. The death of the star becomes a black hole.

Black holes resemble an onion wrapped in a pumpkin shell. Each spinning layer affects matter and light differently. The outer photonsphere is an area where light is pulled into an unstable orbit. The ergosphere offers a last chance for objects traveling at light speed to escape. Finally, the spherical surface from which nothing can escape is the event horizon. Once an object passes through this layer it disappears from our universe. Anything passing through the event horizon must fall into the ring-shaped singularity- a single point in space where all of its matter is squeezed into its center.

Any object could become a black hole if it were small enough and tightly packed enough. If the earth could be squeezed into a singularity the sphere of its event horizon would be about the size of a marble. It would have enough gravity to cause light to return to itself. To escape its pull, a person would have to travel faster than the speed of light.

Scientists believe that our galaxy-the Milky Way-contains millions of black holes. The search is a difficult one because black holes are invisible. Astronomers look for black holes where two stars orbit one another. Such double stars are common in our galaxy and a few of them give off x-rays. The most famous x-ray star is Cyg X-1 in the constellation Cyngus. When astronomers first observed Cyg X-1, they found only one star. From its motion they figured that its partner should have a mass ten times larger than the sun and be easy to see. But it was not. Instead Cyg X-1 seemed to be dancing around an invisible partner.

Most researchers now think that the invisible companion is the collapsed

mass of a large star, which became a black hole. Its gravity has been ripping gas off the outer layers of Cyg X-1. The gas swirls into a flattened disk before being pulled into and swallowed by the black hole. This disk is squeezed and heated by friction between the edges to millions of degrees, producing the intense x-rays that first attracted the attention of astronomers and revealed the presence of the otherwise invisible black hole.

Most evidence comes from using a telescope to observe these x-rays that burst from black holes. In 1994, astronomers used the Hubble Space Telescope to uncover the first real convincing evidence of a black hole in the galaxy M87 in the constellation Virgo. Two more were found in 1995 in other galaxies.

Twinkle, twinkle little star? Definitely not! Although astronomers have already discovered much about such phenomena, and about the birth, life, and death of stars, many secrets about the universe are still hidden far out in space for another generation of astronomers to discover. When your imagination cruises through space, and you discover a bunch of matter swirling around a small area, steer clear of the black hole before it's too late.

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More about this author: Bonnie Lippard

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