Why is the Sun so hot?

The sun is a massive fusion engine, which gives off its heat by radiating it into space. We call the fusion processes stellar nucleosynthesis. The nuclear fusion reactions produce zillions of watts of heat. It's about 383 billion billion megawatts, which is 3.83×1026 W, and it does that every second. That's a lot of energy. Use the link below to read about the sun in the Wikipedia post. And if you're feeling brave, use the other one to the post on the fusion process itself. You'll be measurably smarter if you read all of that article!