4.2 Winds, or mass loss, from young stars are important for halting the infall of matter onto the protostar. Winds and disks interact to produce bipolar outflows which provide astronomers with important views of the star formation process. At latter stages winds can sweep away all the matter around the star including the circumstellar disk. Circumstellar disks are believed to provide at least part of the collimation of the wind from young stars that produce bipolar jets of material flowing out along the star's polar axis. All stars can generate energy from gravitational collapse.
Fusion reactions are another way they generate energy.
The proton-proton chain is the dominant process for solar mass or smaller stars while on the main sequence. The carbon cycle occurs in the cores of main sequence stars slightly more massive than the sun. The triple alpha reaction takes place in stars at least as massive as the sun after the hydrogen in their cores has been consumed. More massive stars can also fuse carbon, oxygen, and heavier elements, up to iron, which is the absolute limit. Note that at any particular position in a star only one fusion reaction will dominate, which is due to the strong temperature dependence of fusion reactions.