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The Life Cycle of a Star t u o ab sa e th more nebula All stars begin as a big cloud of dust and gas (mostly hydrogen) called a nebula. Gravity pulls the particles in the nebula closer together and the cloud gets hotter and denser. protostar The increase in pressure means that particles collide more often. When the particles brush past each other they cause friction. This increases the thermal energy store of the protostar, causing it to emit light. si e m o s a ze n u S ur red giant mass ive th an ou r Sun main sequence star Eventually, hydrogen nuclei gain enough energy that they start to fuse when they collide. This is called nuclear fusion. The star is now known as a main sequence star. The heat from the centre of the star causes it to expand. The expansion is equal to the gravitational pull towards the centre and therefore, this part of the lifecycle is the most stable. The larger the star, the hotter and brighter it will be and the faster it will move through this stage. red supergiant Eventually, the larger nuclei start to fuse to make even heavier elements. All naturally occurring elements are produced in this way. This causes an increase in the thermal energy store at the core of the star which makes the star rapidly expand. Stars about the same size as our sun will form a red giant. Stars much bigger than our sun will form a red supergiant. Nuclei as large as iron can fuse here. white dwarf black dwarf A white dwarf is the leftover core of an average sized star after it has finished its red giant stage and collapsed. Fusion is not occurring at this point. White dwarf stars are very bright and very hot because all of their energy is condensed into a small area. Over time, this energy is emitted into the surroundings. The star gets dimmer and dimmer until it turns into a brown dwarf, and then a black dwarf. neutron star supernova The fusion of large nuclei in the red supergiant causes the core to get denser. When it runs out of fuel, the core is so heavy that its gravitational force pulls the outer layers of the star in towards the centre. The shockwaves resulting from this collapse cause the star to explode in a supernova. Elements heavier than iron are produced and the explosion distributes them throughout the universe. All naturally occurring elements larger than iron have come from a supernova. black hole At the centre of a supernova, the particles form an incredibly dense core. Most supernovas collapse to form neutron stars. Fusion does not occur at this stage. The largest supernovas form black holes. Gravity in a black hole is so strong that not even light can escape.