A star is a huge ball of hot glowing gas, like the Sun, and a planet is a world, like Earth. Stars produce their own light; they undergo nuclear reactions that burn hydrogen in their cores. But planets are lit by light from the Sun. When you look up into the night sky, it is hard to tell planets and stars apart. However, early astronomers were able to tell the difference because planets in our solar system appear to move in complicated paths across the sky, but stars don’t. There are also other observational differences: planets almost never twinkle, but stars do.