How is the protoplanetary disk (stage 2) different from

the nebula (stage 1) ?

open ended question

A protoplanetary disk is round rotating object which is made of dense gas. This surrounds the newly formed star, Herbig star and T tauri star.

It is also considered as a accreation because the remnants like gases and other materials fall from them on to the star. The extremely illuminating protoplanetary disk are known as the proplyds.

Whereas the nebula can be defined as the densely packed hydrogen and other gases that collapse under the force of gravity. The Nebula starts becoming smaller and smaller because the particles of gases collapse and get closer to each other.

Upon force of gravity their friction and heat increases which slowly collapse the star.