This means that they can generate their own light. Stars are composed primarily of hydrogen, which undergoes a fusion process and becomes helium. This fusion of elements provides the energy for the self-luminosity. Planets are large objects in space that are composed of heavier elements such as carbon, iron, and oxygen. Planets do not provide light and none of the material on the planet undergoes fusion.
Some stars are bigger, more massive, and brighter.
Others are smaller, less massive, and less bright. Some are much older than our Sun, and others are much younger. In stars with masses comparable to and greater than that of our Sun, the helium in the core ignites and fuses into heavier elements, like carbon and oxygen, releasing energy in the process. Very massive stars continue this process, fusing the carbon into even heavier elements, such as iron and nickel. The most massive stars eventually end their lives with an explosion called a supernova.