Stellar evolution is the process by which a star changes over the course of time. The most important factor in stellar evolution is the mass of the star, which determines its luminosity, lifetime, and eventual fate. More massive stars are hotter and brighter than less massive stars, and live shorter lives. The least massive stars may burn for billions of years before finally fading away.
The first stage in stellar evolution is the protostar phase, during which a star forms from a collapsing cloud of gas and dust. As the material in the cloud falls inward under gravity, it heats up and begins to radiate energy. Eventually, enough material has fallen into the center of the protostar that nuclear fusion can begin, and the star enters its main sequence phase.
During its main sequence lifetime, a star fuses hydrogen into helium in its core. The energy released by this nuclear fusion keeps the star stable against gravitational collapse. Main sequence stars span a wide range of masses, from about 0.08 to 100 times that of our Sun. Our Sun will remain on the main sequence for another 5 billion years or so before evolving into a red giant.
As a star uses up its fuel (hydrogen), it expands and cools as gravity pulls more material inward towards the core. When all of the hydrogen in a star’s core has been fused into helium, nuclear fusion stops and gravity takes over again causing further collapse until electrons are forced close together forming degeneracy pressure supporting further collapse is overcome with thermal pressure from heating as more collisions occur between these particles now forced closer together with extra kinetic energy available due to contraction potential energy converted to heat due to adiabatic expansion work done on surroundings external to object contracting – expanding space itself results in no added heat but free expansion would result in cooling). This forces hydrogen shell burning around an inert helium core causing outer layers of expanded cool low-density gas envelope to be shed off leaving behind hot dense remnant white dwarf composed mostly carbon & oxygen if mass < 1½ solar masses or neutron degeneracy pressure prevents further collapse if mass greater than 1½ solar masses but less than 3 solar masses - above 3 solar masses even neutron degeneracy pressure cannot prevent continued collapse until black hole endpoint reached) . If initial mass was great enough iron can't be used as fuel so no new elements heavier than iron can be produced through nuclear fusion processes inside dying star (supernova explosion) . Remnant white dwarf slowly loses remaining heat eventually becoming black dwarf cooled completely unable to emit any more radiation - takes many billions year timeframe for this final endpoint . A supernova occurs when an evolved high-mass star runs out of fuel at its core and collapses under its own weight creating conditions necessary for extremely high temperatures & pressures required for production of heavy elements beyond iron through rapid successive captures called r-process nucleosynthesis followed by slower s-process nucleosynthesis then expulsion outwardly at very high velocities (upwards