Illustration of the life cycle of the Sun. Our Sun was created approximately 4.6 million years ago from the gravitational collapse of a giant molecular cloud (GMC, left). As the GMC collapsed it released heat and eventually the Sun became hot enough to begin nuclear fusion of hydrogen. So far the Sun has used 37 percent of this fuel and won't run out of hydrogen for another 5 billion years. When all the hydrogen is used, nuclear reactions involving helium will begin. At this point the Sun will enlarge and become a red giant, engulfing the inner solar system. When all the fuel is used the Sun will collapse and become a white dwarf (far right).