Aii a protostar collapses until it gets hot and dense

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a.ii) A protostar collapses until it gets hot and dense enough to start nuclear fusion to begin turning its hydrogen into helium. a.iii) As soon as this nuclear fusion begins, the core stops collapsing and stabilizes--it is now a main sequence star
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b) Some protostars don't have enough mass to ever reach the 10 million Celsius required for nuclear fusion to start. These little protostars end up becoming brown dwarfs instead--too big to be a planet and too small to be a star. c) Summary II: c.i) the main sequence star burns until it's used up all of the hydrogen in its core c.ii) Once the hydrogen in the core has all been converted into a core of helium, the nuclear fusion stops. c.iii) As a result, the core destabilizes and resumes its collapse d) The core of a star maintains its stability only while nuclear fusion is going on to counter the pressure of gravity. If there's no nuclear fusion, then the core is going to keep compressing until it gets hot and dense enough for nuclear fusion to occur. e) Summary III e.i) Now that the star is no longer burning hydrogen, its time as a main sequence star is done. e.ii) While the core is collapsing, the outer envelope is expanding rapidly e.iii) Once the core collapses enough, the pressure is great enough to start up nuclear fusion to change the helium into carbon e.iv) Once this nuclear fusion starts, the core is stable again and now you have a red giant. e.v) eventually, the red giant will burn through all of its helium, and what happens at that point is going to depend on the Mass of the star (e.v.1) low mass stars and high mass stars go through the same steps to become a Red Giant (e.v.1.a) high mass stars – go through the steps much quicker (e.v.2) then their paths diverge (e.v.2.a) low mass stars, the red giant’s core will collapse into a tiny, hot super dense core and shed the outer layers into a planetary nebula and the red giant becomes a White Dwarf (e.v.2.a.i) white dwarfs are one of the densest forms of matter in the universe, surpassed by neutron stars (e.v.2.a.ii) hot and dense not very luminous because it’s so small (e.v.2.a.iii) white dwarf is a dying star (e.v.2.a.iv) it has no source of energy, so it will lose all its heat and cool down and become a dead lump of carbon called Black Dwarf (e.v.2.a.v) Black Dwarfs – unknown if any exist (e.v.3) High mass stars, its core of carbon starts contracting, so massive as it contracts it can cause further nuclear reactions converting carbon into oxygen, neon, silicon, sulphur, and finally to iron (e.v.3.a) It keeps undergoing this process each time forming a heavier element that ends with iron (e.v.3.b) Once the core of a red supergiant is all iron, the core collapses and you get a mighty explosion known as a Supernova (e.v.3.c) Supernova’s are important because they release exotic heavier elements (e.v.3.c.i) They radiate more energy than our sun will over its lifespan (e.v.3.c.ii) They release x-rays and gamma rays (e.v.3.c.iii) And elements necessary for life to exist (e.v.4)
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