Our sun is a star. Right now in its center, complicated reactions are happening that make it shine really bright! These reactions burn hydrogen, the lightest element on the periodic table, for fuel.
All of the stars in the night sky work a lot like the sun. They start their lives by burning hydrogen, and if they're really massive, they can even burn other elements as well! Stars aren't all the same size. The smallest ones are only a tenth of the mass of the sun, while the largest are almost 200 times as heavy! The mass of a star determines almost everything about it, including its color, its size and even how long it lives.
Different things happen to stars at the ends of their lives, depending on how massive they are. The smaller a star is, the longer it lasts. The smallest stars live so long that none of them have burned out yet! Stars like the sun live for billions of years; that's so long that it's hard for humans like us to understand. The sun is around halfway through its life, and will live for about another five billion years! That's a very, very long time, so none of us need to worry. At the end of its life, the sun will be nearly ten billion years old. That's more years than there are people in the world!
