Stars are fueled by the nuclear fusion of **hydrogen** to form **helium** deep in their interiors. Nuclear fusion releases energy, which heats the star and prevents it from further collapsing under the force of gravity. Astronomers call stars that are stably undergoing nuclear fusion of hydrogen into helium **main sequence stars**. This is the longest phase of a star’s life.
For example, our Sun is a main sequence star that is currently fusing hydrogen into helium in its core. The energy released by this process keeps the Sun from collapsing under its own weight and provides the pressure necessary to keep it stable.
