P7

Stars are born in a cloud of dust and gas (which is mostly hydrogen and helium). Gravity causes the denser regions of the cloud to contract very slowy into clumps. When these clumps get dense enough the cloud breaks up into protostars. If the protostar is large and hot enough fusion will occur. Over time because of the fusion process, the centre of the protostar (the core) becomes more and more hotter. (a few million degrees) and hydrogen nuclei start to fuse together to form helium. This releases an enormous amount of energy and creates enough outward pressure to stop the gravitational collapse. The star has now reached the main sequence stage. Over more time the hydrogen fuel will run out and the star will expand and get denser. If the star is large enough it will become a red giant or a super red giant.