When does a nebula become a star?
When the giant clouds of gas and dust begin to clump together and nuclear fusion occurs.
When gravity pulls the clouds of gas together, it begins to heat up, a Protostar is formed before Nucleosynthesis and it grows by acquiring mass from its surrounding envelope of interstellar dust and gas. It then becomes a T-Tauri star, which is a pre-main-sequence star in the process of contracting to the main sequence along the Hayashi track. Pre-main-sequence stars are stars that haven't yet become main sequence. Main Sequence Stars fuse hydrogen to form Helium, creating energy in the process.