Quick question about black holes

  1. I'm quite new to the world of physics/cosmology and their neighboring subjects. I've just begun learning about black holes, and so far, I've understood everything I've come across except for one term: Fuel.

    I've read several articles that pertain to black holes. In each of these articles, a certain "fuel" is mentioned. To quote one of these articles: "- when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole."

    The lack of explanation leaves me flummoxed. What is this "feul" the star runs out of? Any information regarding the above would be greatly appreciated. Thanks!
     
  2. jcsd
  3. -Job-

    -Job- 1,132
    Science Advisor

    Nuclear reactions in a star's core convert hydrogen into helium and into heavier elements. Once it runs out of hydrogen the nuclear reactions stop and the outward force traditionally produced by this process vanishes and the star is unable to hold its own against its own gravity and collapses. So i'd say hydrogen.
     
    Last edited: Oct 31, 2005
  4. Garth

    Garth 3,529
    Science Advisor
    Gold Member

    That's just for starters. Once the core runs out of hydrogen it then collapses until temperatures and pressures increase by about an order of magnitude when helium fusion begins. This is the stage when a Main Sequence star becomes a Red Giant. Helium then fuses into beryllium etc. A large Red Giant has a core like an onion with each shell fusing a 'heavier' element than the one outside it. The process is basically adding alpha particles i.e. helium nucleii, so the even atomic number elements are made more readily. The odd atomic numbers have to be created by other processes and their relative abundance is observed around us to be an OOM less than their even 'neighbours'.

    When the process reaches iron it stalls as any further nucleosynthesis is endothermic rather than exothermic. It requires the energy input of a super nova. This is fortuituously (?) provided by the iron core that under goes the 'Iron Catastrophe' and reverts back into alpha particles. This is when it has finally run out of fuel and collapses into a neutron star or if the core mass is great enough, ~ 3 Msolar, a black hole.

    Niode your 'fuel' is iron!

    Garth
     
  5. Labguy

    Labguy 733
    Science Advisor

    There is a good scematic of the elements in the "onion" at this site.
     
  6. -Job-

    -Job- 1,132
    Science Advisor

    That's pretty interesting, i think i understand the whole process now. Very cool.
     
  7. Wow... thanks, guys! :biggrin:
     
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