How is a protostar different from a star?

Protostars are simply stars that are not hot enough to fuse hydrogen, which makes them different from main sequence stars like our sun. Fusion is the opposite idea to the atomic bombs dropped on Japan, which split atoms (specifically U-235? ), whereas in fusion, atoms are fused together. A protostar is the stage in a star's life before it is hot enough to fuse hydrogen (13 million K).

Hydro-what? HE, as I'll call it, is the state in which the fusion in a star's core counteracts the force of gravity trying to compress the star. Main sequence stars are stars that fuse hydrogen and exhibit a state of "Hydrostatic Equilibrium".

Instead of displaying this feature, protostars are in a state of collapse until they are compressed to a point where the temperature is high enough to fuse hydrogen.

Don't worry about protostars fusing protons to deuterium, an isotope of helium.

In theory, protostars are stars, but I believe you were more interested in learning how they differ from other stars.

A star is a fully formed celestial body that has undergone nuclear fusion in its core and is in a stable phase of its lifecycle, generating energy through the fusion of hydrogen into helium. A protostar is a young, pre-main-sequence star that is still in the process of forming.