Scientists estimate the sun to be about 4.5 billion years old. They make this estimate based on the amount of helium that has been produced by the sun in a process known as nuclear fusion. Nuclear fusion combines two hydrogen nuclei together, literally fuses them together, at the atomic level. This releases energy that travels out from the center of the sun and travels to the Earth and the other planets in our solar system as solar radiation. Scientist estimate the sun has enough hydrogen to last another 5 billion years, given the rate it is currently consuming hydrogen as a fuel. The sun is classified as a medium-sized, yellow star. It is currently in the middle of its life-cycle. Stars are classified by the color they give off, which gives an indication of their temperature and age. Blue stars are relatively young stars. Yellow stars are in the middle of their life-cycle, while red stars are nearing the end of their life-cycles.