Three helium nuclei can fuse together, forming one carbon nucleus. The mass of carbon is less than three times the mass of helium. When helium fuses into carbon, which of the following occurs? Three helium nuclei can fuse together, forming one carbon nucleus. The mass of carbon is less than three times the mass of helium. When helium fuses into carbon, which of the following occurs? Some other form of energy is converted into rest-mass energy. Rest-mass energy is converted into some other form of energy. Energy is created out of vacuum.