What is the primary physical difference between a star and a nebula?

A star is a dense, hot sphere of plasma held together by gravity, while a nebula is a vast, diffuse cloud of gas and dust.

A star is defined by its extreme density and the presence of a core where nuclear fusion occurs. This process of fusing atoms creates the intense heat and light associated with stellar bodies, which are held in a spherical shape by the star's own massive gravitational pull. In contrast, a nebula lacks the concentrated density and the central fusion engine of a star. Instead, it acts as an expansive, low-density region of ionized gases, hydrogen, helium, and interstellar dust. While nebulae may contain the raw ingredients for future stars, they do not possess the self-sustaining, high-energy core required to be classified as stars themselves.