What physical process distinguishes the Sun as a star from large planets like Jupiter?

Sustained nuclear fusion converting hydrogen into helium deep within its core.

The absolute defining characteristic separating a true star from massive, non-fusing objects like gas giants is the initiation and maintenance of sustained nuclear fusion in its core. For the Sun, this involves the proton-proton chain reaction, where lighter elements, predominantly hydrogen, are continuously converted into heavier elements, primarily helium. This thermonuclear reaction releases staggering amounts of energy in the form of light and heat. Objects like Jupiter, regardless of their internal heat or size, cannot achieve the necessary critical mass, density, and temperature threshold required in their cores to ignite and sustain this self-fueling energy generation process, thus preventing them from qualifying as stars.