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 sparkling stars. You can see even more with a telescope. 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 telescope you can see more and more objects in the sky. And you can see those objects in more and more detail. 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're invisible. They're the mysterious 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 burn, 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 layers 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. If the star was very big, it keeps squashing inward until it's packed together tighter than anything in the universe. Imagine if the Earth were crushed  until it was the size of a tiny marble . That's how tightly this dead star, a black hole, is packed. What pulls the star in towards its center with such power? It's the same force that pulls you down when you jump - the force called gravity. A black hole is so tightly packed that its gravity sucks 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 stare up at the night sky, remember: there's more in the sky than meets the eyes! Scattered  in the silent darkness are black holes - the great mystery of space. Why can't you see light when you look at a black hole?
Most black holes are so far away. Because the gravity of a black hole is so strong that it sucks the light inward. Because as the star's gases burn, it stops giving off heat and light. Because as a star cools, its outer layers pull in toward its center.