Stars differ in their size, temperature, mass, color and age. Stars change constantly, and some stars even orbit other stars.
A star is nothing more than a ball of gases. Many stars, like Earth's sun, are made of helium and hydrogen. The sun and other stars convert hydrogen to helium in a process known as nuclear fusion. When a star converts all of its hydrogen into helium, it begins to die. Stars with plenty of hydrogen appear very bright in the sky and the more hydrogen a star uses, the more dim it appears.
Stars form in a gas and dust cloud. Gravity plays a major role in star formation, causing the gas to form near the center of the star. As the pressure from the gases increase, the star grows larger and hotter.