Final answer:
When helium fusion ceases, gravity becomes the dominant force supporting the core of a star as it contracts and converts potential energy to heat.
Step-by-step explanation:
Once helium fusion stops in a star, the core is no longer supported by the energy from nuclear reactions and begins to contract under gravitational forces. In stars with less than twice the mass of the Sun, subsequent nuclear fusion of heavier elements is not possible due to insufficient core temperatures required for the process. Instead, the gravity once again becomes the dominant force supporting the star's core as it contracts, causing the energy from the inward-falling material to be converted into heat. This phase of stellar evolution highlights the ongoing battle against gravitational collapse, wherein a star continually seeks energy sources to resist it.