What distinguishes the function of a star from that of a nebula?

A star generates light and heat through fusion, whereas a nebula contains or creates star matter.

The distinction between these two entities lies in their roles within the cosmic life cycle. A star serves as an active agent; it is a finished product capable of producing its own light and heat through the continuous process of nuclear fusion in its core. Conversely, a nebula acts as the environment or the stage. It is a vast reservoir of material—gas and dust—that serves either as a nursery for the birth of stars or as a graveyard that collects the debris left behind after stars have died. While stars are the actors producing energy, nebulae are the supply depots that store and recycle the raw matter required for these actors to exist.