At which point in the life cycle of a star does nuclear fusion begin?

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The life cycle of a star can follow a few different paths depending on the mass it starts out with. Really massive stars may live for only a few million years before going supernova, while low mass stars such as the Sun may take billions of years to use up their fuel and then die quietly. Birth All stars are formed when a giant cloud of gas and dust, called a molecular cloud or nebula, begins to collapse under the influence of its own gravity This may be triggered by a collision with another molecular cloud, the shockwave from a nearby supernova, or even the collision of galaxies As the cloud contracts, it breaks apart. An individual fragment will condense into a hot, dense sphere known as a protostar A new star is born when the protostar becomes hot enough to begin fusing hydrogen into helium. Now, the star will enter the main sequence, or adult, phase If a star is too low in mass to initiate nuclear fusion it will become a brown dwarf Main sequence A star will remain in this state for most of its lifetime, fusing hydrogen to make helium and releasing energy in the process A star may fall on different points in the main sequence depending on its mass. In general, the more massive the star the shorter its lifespan on the main sequence Red dwarfs are small, dim stars that fuse hydrogen at a slow rate, and may remain in the main sequence for hundreds of billions of years. Medium-sized yellow dwarfs such as our Sun will be in the main sequence for several billion years Large stars may only stay in the main sequence for a few million years Maturity Eventually a star will run out of its hydrogen fuel, and begin fusing helium and other elements instead. At that point it will leave the main sequence phase Low-mass (red dwarf) stars will use up all their hydrogen and collapse directly into white dwarfs Mid-sized stars like our Sun will expand and become red giants. This happens when a star runs out of hydrogen at its core. The core will collapse and begin fusing helium while hydrogen fusion is transferred to the outer layers This causes the star to swell to many times its original size and become cooler as the heat is distributed over a larger area More massive stars will grow into supergiants, which are among the largest stars in the Universe In this stage a star will maintain hydrostatic equilibrium by fusing heavier and heavier elements as the lighter ones run out. The largest stars can produce elements up to iron Death and stellar remnants Medium-sized stars like our Sun will eventually die by shedding their outer layers as a planetary nebula The core will collapse into a white dwarf, which will eventually cool into a black dwarf More massive stars will die in a tremendous explosion called a supernova This happens when a massive stars begins to fuse iron. This absorbs energy and caused the core to violently collapse while the outer layers are ejected The extreme heat produced by supernovae is responsible for the nucleosynthesis of elements heavier than iron, up to uranium After a supernova, the core may compress into a neutron star or a black hole. Neutron stars are much denser than white dwarfs, to the point where protons and electrons combine to form neutrons (hence the name) Black holes are denser still, so much so that they produce an extremely strong gravitational force that even light cannot escape Read more: http://wiki.answers.com/Q/What_is_the_life_cycle_of_a_star#ixzz252rQ3YLD