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Stars (Magnitude :check: (A difference of 1 on the magnitude scale gives a…

- - - - In very small stars, once the hydrogen runs out in the core, not further fusion will ever take place, and the star will cool and fade away.
      - Red Giant
        Once all the hydrogen runs out in the core the outwards radiation pressure is removed and the star begins to collapse again. This heats the core, and the outer layers. Eventually, a layer of hydrogen around the core becomes hot enough to fuse hydrogen into helium. This holds the collapse of the upper layers, but the core continues to collapse, and heat. This heats the shell, which increases the fusion rate in the shell. This increases the luminosity of the star, increases the radiation pressure, pushing the outer layers outwards, and causes the outer layers to cool, making the star appear redder in colour
        
        Eventually the core becomes hot enough for helium to fuse into carbon and oxygen. This increases the power output and the radiation pressure further, increasing the size, the brightness and the redness of the red giant.
        
        When star is less than 8 solar masses in mass
        
        Once all the helium runs out in the core, the whole star starts to collapse again. A shell around the core becomes hot enough for helium to fuse to carbon and oxygen, and a shell above that fuses hydrogen to helium. The dual shell fusing causes the star to become unstable, and the star starts to pulsate in brightness, and ejects the outer layers. As the core continues to collapse the core and inner layers become hotter. The shells increase fusion rate and the star becomes brighter. The star ejects it's outer, cooling layers to form a planetary nebula. The core never becomes hot enough to fuse carbon and oxygen into heavier elements.
        
        The core collapses until the electron degeneracy pressure between electrons in the plasma provide an outwards pressure to counter the inwards pressure from gravity. At this point a white dwarf forms, which is about the size of the Earth, and is very hot to begin with. The white dwarf will cool over time.
        
        When star is greater than 8 solar masses
        
        Once helium runs out in the core, the core collapses, the layers heat, but this time fusion does start in the core, with carbon and oxygen fusing into heavier elements. This creates a three tiered structure, with a very large power output, a very large swelling of the star and a very red colour.
        
        Once carbon and oxygen in the core runs out the collapse process repeats and all the layers heat up again. A four tiered structure, then five, six etc, form in the star.
        
        This process repeats until Iron formed in the core runs out. At this point fusion in the core becomes energetically unfavourable and so the core collapses. It does this in a fraction of a second, and compresses past the electron degeneracy pressure, as the degeneracy pressure from the electrons cannot withstand the gravitational pressure. The electrons and protons in the core are forced together to form neutrons, and neutrinos. The neutrinos inherit a large kinetic energy due to the rapid loss in GPE of all the matter in the core. These neutrinos rush outwards from the core, and transfer large amounts of energy and momentum to the inflowing matter causing a supernova explosion.
        
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- - - - Low mass stars can 'fizzle out' during this phase as the hydrogen fuel runs out
      - The hydrogen fuel in the core eventually starts to run out, and an insufficient radiation pressure causes the star to collapse again. This heats the matter in the star again, and eventually a shell of matter around the core becomes hot enough for hydrogen nuclei to fuse into helium nuclei.
        This exerts an outwards radiation pressure to hold up the outer layers, but the core continues to collapse. As the core collapses, the core and the shell heat further, and so the rate of fusion in the shell increases. This increases the outwards radiation pressure, pushing the outer layers of the star, and increases the power output of the star, making the star appear brighter. As the outer layers are pushed outwards they cool, and so the star turns redder in colour.
        A red giant forms, in the shell hydrogen burning phase.
        
        Eventually the core becomes hot enough for helium nuclei to fuse into carbon and oxygen nuclei. This exerts an even higher radiation pressure, heating the shell more and increasing the fusion rate. The radiation pressure increases again, the outer layers of the star are pushed out further. The star becomes larger, brighter and redder again. The star remains as a red giant in the core helium burning phase.
        
        Once all the helium fuel runs out in the core the star collapses again. The shell heats and helium starts to fuse in the shell in the shell helium burning phase. During this phase a shell above the helium shell becomes hot enough for hydrogen to fuse into helium, increasing the radiation pressure even further.
        
        When mass of star is less than 8 solar masses
        
        The core of the star collapses, but never becomes hot enough for carbon and oxygen to fuse. The collapsing core heats the shell a large amount, and increases the fusion rate. The dual fusion makes the star unstable and the star pulsate. The star ejects it's outer layers, forming a planetary nebula. Eventually, the core becomes so dense that the electron degeneracy pressure of the plasma counters the inwards gravitational pressure and a white dwarf star forms, at the centre of a large mass of cooling ionised gas in the planetary nebula
        
        White Dwarf
        About Earth sized; very dense; very hot; quite dim; will fade over time as the star cools
        
        When mass of star is greater than 8 solar masses
        
        Eventually the core becomes hot enough for carbon and oxygen nuclei to fuse into heavier nuclei. A three tier system is made, where lighter elements are fused further out from the core. The process repeats itself, with fuel running out in the core, the core heating, and fusion starting again with a heavier nucleus. At each stage the star pushed it's outer layers out further, becoming larger, redder and brighter. A red supergiant forms.
        
        The process repeats up until Iron, where nuclear fusion is no longer energetically favourable. At this point the core collapses. The core collapses very quickly (to around 30km across in 1 second) under the large gravitational pressure of the upper layers. The electron degeneracy pressure cannot resist the gravitational pressure, and so the core collapses to densities beyond a white dwarf. Electrons and protons are forced together, fusing into neutrons, and initially the core is compressed to beyond nuclear densities.
        
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