Q: We have clouds on the earth. Do all planets have clouds?

　　Q: How does a star die and how old are stars when they die?

　　Q: How many stars are there?

　　Q: Mercury is really close to the sun. Is it always hot there?

　　A: On the earth, the clouds are made of small drops of water and ice that float* up in the atmosphere. On the other planets, the clouds aren not made of water. They are made of other things. Jupiter*, Saturn*, Uranus*, and Neptune* have hydrogen* and helium* clouds. Venus* has very heavy carbon dioxide* cloud cover. In fact, it is so heavy that it hides the planet from our view.Mercury* doesn’t have clouds. Mars* doesn’t have clouds either. The other planets are so far away we don’t know whether or not they have clouds. But if they do, they would be frozen solid because those planets are so cold.

　　A: At any one time, half of Mercury is hot and the other part is colder than freezing. That’s because it doesn’t have an atmosphere to hold the heat. So the part facing the sun is hot, hot, hot and the side in darkness is colder than cold.

　　A: Scientists have calculated that there are over ONE BILLION* to ONE TRILLION* stars in each galaxy. But there isn’t just one galaxy in our universe - there are about TEN BILLION. That means there are one trillion times 10 billion stars in the universe. And that is a number even bigger than what Bill Gates can imagine!

　　A: Some stars just get smaller and colder until they finally die. Others explode* in what is called a supernova*. It depends on how long the stars live — their individual lifespan differs greatly. Scientists think that solar system’s star (the sun) is 5 to 6 billion years old. They also figure it won’t die for another 10 billion years. (SD-Agencies)(Difficulty: senior)