A star is huge, spherical mass of mostly hydrogen gas that shines brilliantly due to nuclear fusion occurring at its core. A star represents a fine balance between the crushing weight of its outer material and the outward pressure produced by the nuclear reactions in its center. In fusion reactions, atoms of one element are squeezed together to form a heavier element, a process that releases enormous amounts of energy. Stars come in a range of sizes and colors (from white to red to yellow to blue) that depend on the amount of mass they contain and their age. More massive stars live for only a few million years, while smaller, less massive stars (like our sun) can live for billions of years.