What powers the stars? The easy answer is: Hydrogen fusion. But as we have just seen, it is gravity that provides the energy that makes the diffuse collection of cold atoms hot enough to make fusion begin. So gravity matters, too. When the Hydrogen is used up (or sufficiently depleted) so that no more energy can be produced by its fusion, what happens? Gravity becomes important again. The (core of the) star shrinks, again, and gets hotter, again. (The outer parts of the star expand, so at this stage there is simultaneous expansion and contraction.) This time it gets hot enough to fuse Helium nuclei together. (The temperature required to get two He's (or one He and one H) close enough to fuse is higher than that needed to fuse two H's.) When the Helium is depleted, gravity becomes important again, and heavier and heavier nuclei are produced. The production of the heavier nuclei from the raw material of Hydrogen is called nucleosynthesis.