Detailed high-resolution photographs have revealed the presence of protostellar and protoplanetary disks in star-forming regions of nebulae.
All of the pieces of evidence listed here contribute to this conclusion.
Groups of stars nearest to molecular clouds tend to be younger than those farther away from such clouds.
It is completely a matter of conjecture, as none of these steps can be observed experimentally.
Emission nebulae have been shown to contain young stars.