The rate of nuclear fusion would slow down, first of all. Nuclear fusion is the process by which hydrogen nuclei are fused together to make helium nuclei. Energy in the form of radiation is released and transmitted throughout space. The rate if nuclear fusion is inverse to the amount of mass of hydrogen fuel present. That is why stars with many solar masses spend the majority of their lives as blue stars, which have higher surface temperatures. They consume much more fuel than do stars with less mass, and burn that fuel at a higher rate. Consequently, there are not many blue stars in older galaxies, largely due to this increased rate of fuel consumption. Secondly, if the rate of nuclear fusion slows down, the amount of radiation available from the sun would also slow down. It would be conceivable the planet Earth would be subject to another ice age, where most of the planet was covered in ice.