The first stage in the life cycle of a star is the Giant Gas Cloud stage, which is also known as the nebula stage. A nebula is a large cloud of gas and dust in space, and stars are formed when gravity pulls some of the gas and dust in a nebula together, forming clumps called protostars. The protostar is the earliest stage of a stars life, and it is formed when the contracting cloud becomes hot enough for nuclear fusion to start. During this stage, the protostar gives off a large amount of energy in the form of heat, which causes the nebula to break up into a cluster of many baby stars. As the nuclear fusion reactions continue, large amounts of heat and light energy are produced, which allows the process of nuclear fusion to continue. The outward pressure caused by the nuclear fusion and the force of gravity keeping the star together are balanced, and during this stage of evolution, the star is stable and is known as a main sequence star. This stage of the evolution can last for several billions of years, and stars with greater mass have hotter temperatures and usually shorter lives.