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 telescopes you can see more and more objects in the sky. And you can see those objects in more and more details. But scientists believe there are some things in the sky that we will never see. We wont see them with the biggest telescope in the world, on the clearest night of the year. Thats because theyre invisible. Theyre 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 doesnt seem to be getting old or weak. But stars do burn out and die after billions of years. As a stars 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 its packed together tighter than anything in the universe. Imagine if the each were crushed until it was the size of a tiny marble. Thats how tightly this dead star, a black hole is packed. What pulls the star in toward its center with such power？ Its 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 black hole can never come back to your eyes. Thats why you see nothing but blackness. So the next time you stare up at the night sky,remember: theres more in the sky than meets the eyes! Scattered in the silent darkness are black holes—the great mystery of space.
According to the article,what causes a star to die？

As its gases run out.it cools down.
B. It collides with other stars.
C. It can only live for about a million years.
D. As it gets honer and hotter, it explodes.