How does a natural "solar thermostat" keeps the core fusion rate steady in the Sun? How does a natural "solar thermostat" keeps the core fusion rate steady in the Sun? If the Sun’s core were a bit hotter, the fusion rate would increase. This would produce more energy, which would cause the core to expand slightly and cool. The cooling would cause the fusion rate to slow back down until the Sun was back to the original size and temperature and fusion occurred at the original rate.