How many things can you see in the night sky? A lot! On a clear night you might see the Moon, some planets, and thousands of shining stars.
You can see even more with the help of new inventions. You might see stars where before you only saw dark space. You might see that many stars look larger than others. You might see that some stars that look white are really red or blue. With bigger and bigger telescopes you can see more and more objects in the sky. You can find out more about those objects.
But scientists believe there are some things in the sky that we will never see. We won't see them with the biggest telescope in the world, on the clearest night of the year.
That's because they can't be seen. They're the mysterious（神秘的）dead stars called black holes.
You might find it hard to imagine that stars die. But stars do burn out and die after billions of years.
As a star burns, it gives off light and heat. But when the star burns out, it begins to die.
As the star cools, the outer parts of the star pull in toward the center. The star squashes into a smaller and smaller ball. If the star was very small, the star ends up as a cold, dark ball called a black dwarf（黑矮星）. If the star was very big, it keeps squashing inward until it's packed together tighter（紧的）than anything in the universe.
Imagine if the Earth were squashed until it was the size of a tiny ball. That's how tightly this dead star, a black hole, is packed. What pulls the star in toward its center with such power? It's the same force（力量）that pulls you down when you jump—the force called gravity（重力）. A black hole is so tightly packed that its gravity sucks（吸收）in everything—even light. The light from a black hole can never come back to your eyes. That's why you see nothing but blackness.
So the next time you look up into the night sky, remember: there's more in the sky! All over in the darkness are black holes—the great mystery of space.

From the passage we know a black hole is                .

A．a huge hole in the universe

B．a group of stars packed together

C．a dead star that is packed tightly

D．complete silent darkness in space

When a star squashes it will                .

A．turn into a ball	B．suck in everything
C．get smaller and smaller	D．become darker and darker

The passage is mainly about               .

A．how stars burn out

B．what dead stars are

C．how we can see more stars

D．why stars give heat and light