Systematic trends are seen on the main sequence. Hot stars are more massive. Giants are less obvious, although supergiants have masses greater than 5M o . White Dwarfs have masses of 0.5 to 1.2M o . Most stellar evolution occurs over timescales which are much longer than human experience, so models are needed to determine how stars change.
Stellar evolution is driven by the fight against gravity.
At different stages different forces hold the star up. Initially, the thermal motions of the gas support the cloud. If gravity becomes too strong, collapse begins. The transition occurs when the orbital speed becomes greater than the thermal gas velocity. This slows the collapse down, but it doesn't stop it.