The sun is a star, just like the other millions of stars you see when you look at the night sky. In fact, the sun is a relatively normal star. Like all stars, it is a large ball of gas that produces huge amounts of energy. Stars form when particles floating in space are drawn closer together by gravity, until the cloud of space dust is round and dense. Inside that dense center, hydrogen atoms are under so much pressure that they fuse together into helium atoms. This process is called nuclear fusion, and it releases a lot of extra energy in the forms of heat and light. Nuclear fusion is what keeps stars burning.
4.5 billion years ago, when the sun formed, it was not the only clump of gas and dust swirling around space. As the sun's particles pulled together, other particles and clouds farther away began circling around it, too. Those clouds started condensing into planets. The process was dramatic. Clumps of space dust slammed into each other, breaking apart and reforming, over millions of years.

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