To understand why stars shine, we must first understand the tiny particles that make up matter. Scientists have studied matter in their laboratories for many, many years. Each kind of atom has a certain unique number of particles called protons, neutrons, and electrons in it. The protons and neutrons cluster together in the center of the atom in what is called the nucleus. The electrons orbit around the nucleus.
Atoms are very, very small.
The simplest atom is hydrogen. The nucleus of a hydrogen atom consists of a single proton. Around this proton orbits a single electron. There is more hydrogen in the Universe than any other kind of atom. It consists of a nucleus containing 2 protons and two neutrons.