Hydrogen is the simplest kind of atom
Hydrogen turns into other kinds of atoms
Inside these stars, there was a lot of heat and gravity. The strong gravity pressed the hydrogen atoms together, and four hydrogen atoms squashed together to form a new atom, with two protons, two neutrons, and two electrons. This new atom is helium. Hydrogen and helium atoms together make up almost all of the atoms in a star.
Hydrogen becomes planets
Not all the hydrogen stayed inside stars, though. Some of it began spinning around the outside of the star, orbiting the new star. This hydrogen mixed with other kinds of atoms. Gradually gravity pulled them together into planets. There are many hydrogen atoms on Earth (though most of them have joined up with other atoms to make molecules).
Hydrogen atom and water
Because hydrogen atoms are so simple, they are very light. They are lighter than air, so if you fill a balloon with hydrogen, it will float up to the sky (like a helium balloon that you can get at the store).
We get light and heat from hydrogen turning into helium. But there are two other important things about hydrogen for us. One is that two hydrogen atoms combine with one oxygen atom to make a molecule of water. All our water is made of hydrogen. And the other one is that hydrogen atoms combine with carbon atoms to make hydrocarbons, which all living things are made of.