what causes the outward pressure on the star?
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life cycle of a star
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1.I am really happy you asked that because I am generally good on that field:
in a normal satr, the sun produces energy throgh nuclear FUSION, it stars by fusing hydrogen atoms to form helium (well actually by stuff like detrium but don't worry about that, hydrogen comes after that stuff), this energy and nuclear reactions forms what is known as thermodynamic pressure, this is outwards pressure so prevents the star collapsing under its own gravity.
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if you want to know more, then:
2.when the star runs out of hydrogen to fuse, it momentarily can't produce any energy (not hot enough and not enough pressure to fuse heavier elements) so the star does not have any outwards pressure so it collapses in on itself, at this time, the core becomes very heavy and dense and hot, and so the outer layer expand outwards because heat rises and so on. the core is able to fuse helium. the outer layers are some what cooler. eventually the outer layers "puff" and go away from the star, probably due to lower gravity as the stars main gravity comes from and small dense core (might want to verify), this layer of gas is known as planetary nebula. whats left is the core which is called a white dwarf which when it runs out of energy is called a black/brown dwarf.
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3. a larger star (3 times our own) goes through a different stage after red giant, neutron stars or black holes:
as it is of a greater mass, it can reach pressures and tempreatures great enough to fuse heavier elements through the tripple alpha process, eventually it starts fusing Iron (the most stable element) which instead of producing energy, it uses it, so as a core of Iron builds up, then gravity becomes greater and pressure become less, eventually there is greater gravity and so the star collapses in on itself, the core becomes super dense and large outwards pressure is created (explained below), the outher layers of the star hit the super dense core and explde outwards (a type II supernovae) and density and heat becomes so great at this point that out of whats left, electrons are forced into protons to form neutrons and we have a star made of neutrons, well what keeps the star up?
there are only so many neutrons that can occupy low energy levels (as shown in quantum mechanics), so as there are soo many neutrons, some are forced into high energy states (degenerate neutrons), it is this difference in energy states that creates "neutron degeneracy pressure" which prevents the star from collapsing as there is a number of degenerate neutrons that want to become low energy neutrons but can't, this creates pressure and as it is createdd by quantum mechanics (the pressure) it is not effected by heat so will not collapse when the star becomes cool. note, a neutrons star may be the size of a small town but will have the mass of many of our suns, thats how packed it is!
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the other option applies to only really heavy stars, stars that are way above 3 times our sun's mass.
when they go through a type II super novae, they have such a large gravity field that some of the matter exploding outwards is sucked in, the star has such a large gravity field that even the core collapses in on itself (not even neutron degeneracy pressure can help) the core collapses into a singularity ( a very very small point) whihc bends time and space, this singualrity has such a large gravity field the the velocity needed to escape is greater than the speed of light (which has no mass! photons behave like particles and waves- the fact that they can't escape from a black hole says something!) this is known as a black hole ( the singularity), I can tell more about the event horizion etc. just reply and/or vote.
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so to sum up the answer to the question, in a star like our suns, it creates the pressure needed to keep the star upwards throught nuclear fusion, where convection currents are created etc and this creats outwards pressure to counteract gravity.