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Like all other bodies in the universe, stars are born, they evolve and then they die. This section will give you the main stages in stellar evolution. Stars condense from clouds of hydrogen, The gravitational attraction between the hydrogen atoms pull them together and the cloud collapses under its own gravitational forces – a kind of free fall collapse. This heats the dust to low temperatures (3050K) and emits infrared radiation. At this stage, it is called a proto-star. As the atoms get closer the GPE of the individual atoms decreases and the KE increases. KE is directly proportional to temperature, and this rises to around 1000K in the centre of the proto-star. As the hydrogen atoms get closer the temperature increases and the atoms move around faster. This increases the probability of collision between atoms. The Main Sequence is the period during which a star ‘burns’ hydrogen. This is where hydrogen atoms collide and fuse together to form helium. The process is called the p-p chain and there are 3 versions of this. You only need to know PP1. 1H + 1H 2H + e+ + (1.44 MeV) 2H + 1H 3He + (5.49 MeV) 3He + 3He 4He + 1H + 1H (12.9 MeV) The numbers in brackets are the energies released at each stage of the chain. It doesn’t seem like a lot but when you consider how many hydrogen atoms are in a star …) A star spends ~80% of its lifetime on the main sequence. During this time it is stable as the gravitational forces that enable hydrogen burning balance and pull the star in, balance with the gas pressure pushing out. This is much like the gas pressure inside a balloon balancing with the tension in the plastic of the balloon. In the star it is known as hydrostatic equilibrium. Stars with an initial mass of 0.4 - 8 solar masses become red giants as part of their evolution. On the main sequence the star burns H but these nuclear processes stop once there is not enough present for fusion. The core still needs to heat up to fuse helium and there is an shell of unburnt H around the core. As the H in the core stops fusing the core contracts due to the gravitational forces taking over. This heats the envelope to enable H burning to take place here. This then means that there is enough energy in the envelope to cause it to expand which in turn cools it. The diameter of the star increases by about 100 and this is the Red Giant phase. If the star is bigger the core temperature is higher and this means that there is more energy to fuse heavier elements (Be and C). As fusion continues in the core, the temperature keeps rising which enables the creation of even heavier elements in shells. The most massive stars are capable of forming an iron core (the top of the fusion spike, A = 57) These are called Red Supergiants. Due to the size of the star, the gravitational field on the outside envelope is weak so large amounts of matter are lost. This mass loss is 10 million times faster than the mass loss from the sun. Eventually the energy from the collapse of the inner shells is not great enough to trigger the next stage of fusion. •The outside layers continue to fuse while the inactive core region grows. •Eventually the gravitational forces in are greater than the gas pressure pushing the star out so the star collapses in on itself and shrinks. As a Red Giant collapses in on itself, the core starts to heat up again to ‘white hot’. A White Dwarf is the end state for a star of 0.4 – 8 solar masses. The upper limit for the final mass of a White Dwarf is approximately 1.4 solar masses. For masses greater than that, the end state is much different.