30 A2+ the science of darkness

CHAPTER 1 The Dark WHAT DOES "DARK" MEAN? Jenny told her roommate Isabel, "Turn on the light. It's dark in here!" Isabel said. "Why? I like the dark. I like to sit and think in a dark room." "OK, but we're not living in the Dark Ages," Jenny said. "So turn on the TV!" "No, I think people use too much electricity," Isabel said. "You don't have to be in a dark mood about it," Jenny answered. "I'm not," Isabel said. "I'm just worried. Are you in the dark about what's going on in the world? We use too much energy! It's terrible." "I think you worry too much. You have dark circles under your eyes."

"Well, I think it's important." Isabel said. "I guess you're right, but, actually, I'm afraid of the dark!" Jenny said. "Afraid of the dark?" Isabel was surprised. "Come on... come outside with me." It was a pitch-dark night. The girls couldn't see in front of them. Finally, they got to the middle of the back yard. "Look up," Isabel said. The sky was completely black except for some stars. "Isn't it amazing?" "Well, yes, it is kind of beautiful," Jenny said. The word "dark" is used in many ways in English. It can describe something that doesn't have light. It can mean something is bad or evil. But what is darkness?

CHAPTER 2 Dark Matters WHAT IS IN THE NIGHT SKY? Have you ever looked at the sky and thought about darkness? Is it nothing? Or is there something in the dark? Cosmologists try to answer questions about things in space. They even study the darkness in space. They say the universe is about 5 percent "normal" matter— things we can see, like Earth and other planets, and the sun and other stars. But about 70 percent of the universe is something called dark energy. Cosmologists are still studying what dark energy is. They have more questions than answers. Cosmologists think this dark energy fills space and shows that the universe is expanding. There is more dark energy in the universe now than there was before.

If about 70 percent of the universe is dark energy and about 5 percent is "normal" matter, what is the rest of the universe made of? Cosmologists say the other 25 percent of the universe is dark matter. Dark matter has some weight, it's heavy so we know it's there. But it's invisible, we can't see it. It is very hard to study something that you can't see! One possible kind of dark matter is a brown dwarf. A brown dwarf is too big to be a planet and too small to be a star. Brown dwarfs are colder than stars, and they don't give off light that people can see. Cosmologists found them by using a telescope that finds heat. They found the first brown dwarf in 1995.

Cosmologists do know a lot about one kind of dark matter black holes. In 1783, English writer and astronomer, John Michell, guessed that there were black holes. He called them "invisible stars." But cosmologists didn't find any black holes until 1971, almost 200 years later. Since then, they've found many black holes in space. It's very dark in a black hole. There's a lot of gravity there, too. There is so much gravity in a black hole that nothing can get out, not even light. This is why we can't see them. Cosmologists find black holes by looking at things around them. For example, stars and gases near a black hole move very fast.

Black holes can be very big or very small. Cosmologists believe the biggest black hole is as big as ten billion of our suns. They think there is a large black hole at the center of every galaxy. The black hole at the center of Earth's galaxy is called Sagittarius A. Cosmologists believe some black holes are as old as the universe. They think others are made when a large star dies. Our sun is a star, but when it dies, it won't become a black hole. It is not big enough. Things in space can fall into a black hole, and when they do, they can never get out. But don't worry... Earth will never fall into a black hole because there is not one close enough.