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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're invisible (看不见的). They're the dead stars called black holes.

   You might find it hard to imagine that stars die. After all, our Sun is a star. Year after year, we see it up in the sky, burning brightly, giving us heat and light. The Sun certainly doesn't seem to be getting old or weak. But stars do burn out and die after billions of years.

   As a star's gases bum, they give off light and heat. But when the gas runs out, the star stops burning and begins to die.

   As the star cools, the outer part of the star is  pulled in toward the center. The star turns into a smaller and smaller ball. If the star is very small, the star ends up as a cold, dark ball. If the star is very big, it keeps being pulled inward until it's packed together tighter than anything in the universe.

   What pulls the star in toward its center with such power? It's the same force 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) 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.