# How does a star ignite?

I remember reading that X-Rays are generated by 'braking' electrons in a Coolidge tube.

Is it fundamentally a matter that the extreme gravity immediately before a star ignites is so strong that it affects the hydrogen atoms to the point the velocity of it's components must be let-off in the form of heat & light?

How does a star ignite?

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Just exactly what has the first sentence got to do with the rest of the question? Is there a sequitur in there that I've missed? And even if there is why would you expect a particular means of making x-rays on demand in the lab to have anything to do with the environment in the center of a star? – dmckee Apr 14 '13 at 21:15
@dmckee: It does appear to be a non-sequitur... My thoughts were something like - "Electron sheds it's velocity and photoons are generated. Hang on, what happens if an electron can not move at it's rated velocity? It would take a lot of energy to slow an electron down ... what about gravity? Say in the core of a star cloud?" I wasn't consciously thinking along these lines - it was sort-of like jumping to conclusions. )+: – Everyone Apr 22 '13 at 9:35

The nuclear fusion that powers stars has little to nothing to do with electrons. In the cores of stars, temperatures are high enough that all the electrons are stripped from the nuclei, leaving a pure plasma.

As stars contract and condense out of interstellar dust, their gravitational potential energy is converted to heat faster than this heat can be radiated away. Once the temperature reaches roughly $10^7\ \mathrm{K}$, protons (hydrogen nuclei, stripped of their electrons) have a nonnegligible chance of sticking together when they colide, with one of them converting to a neutron along the way: $${}^1H + {}^1H \to {}^2H + e^+ + \nu_e.$$ This is the first step of the PP chain, and it releases energy. There are more steps that ultimately turn four protons into a helium-4 nucleus. In more massive stars than the Sun, there are other ways (e.g. the CNO cycle) to catalyze this process with the help of carbon, nitrogen, and oxygen.

In any event, there is nothing extreme about the gravity. It just happened to pull matter from a huge distance close together. If you took infinitely spread apart particles totaling mass $M$ and formed a uniformly dense sphere of radius $R$, the gravitational potential energy released would be $$\frac{3GM^2}{5R},$$ about half of which you expect to go into heating the material. Once hot, hydrogen naturally forms helium in exothermic processes.

Stellar reactions are self-regulating in the sense that if the rate of fusion increases, the additional luminosity would push the outer layers of the star, causing the star to expand and cool, thus reducing the reaction rate. Thus as long as there is hydrogen in the core, stars more or less burn at a steady rate once ignited.

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BTW The temperature at the core of the sun is estimated to be an order of magnitude lower than the $10^8$ you suggest. That's reflected in the low estimated power density, $<300 W/m^3$. – Dan Piponi Apr 14 '13 at 21:55
@DanPiponi Good catch, thanks. – Chris White Apr 14 '13 at 21:58
Does it need to be 10 MK at some given pressure (what pressure?) – Nick T Nov 4 '15 at 19:58

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