What physical property of a star makes it a self-sustaining entity compared to a nebula?

It possesses a cohesive, self-sustaining gravitational core.

A star is defined by its ability to maintain a dense, stable structure through its own gravity. This gravity is powerful enough to compress the core material to the extreme temperatures and pressures required for nuclear fusion. This creates a self-sustaining feedback loop where the energy produced by fusion counteracts the crushing force of gravity, keeping the star in a stable state for millions or billions of years. A nebula lacks this central, cohesive gravitational core. Because it is a diffuse cloud, it does not have the density or the internal pressure necessary to initiate fusion, meaning it cannot become a self-sustaining entity like a star.