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When did nuclear fusion in the sun really start ?

  1. Feb 24, 2013 #1
    - possible sun timeline

    - 3,130,000 years for molecular gas to condense into protostar
    - 40,000,000 for core to condense and heat up to 15 million K

    - 43,130,000 total

    - sun has only enough matter to 'burn' by gravitational pressure for 100/300 million yrs

    - first stage of proton-proton chain takes 1 Billion years to take place !!!

    - a gap of at least 700 million yrs

    - doesn't seem to add up ... does first stage of p-p chain happen randomly earlier ?

    ....... just curious ........
  2. jcsd
  3. Feb 25, 2013 #2


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    Where did you get this information from?
  4. Feb 25, 2013 #3
  5. Feb 25, 2013 #4
    The page you got this from was referring to earlier theories about what powered the sun, not present day understanding.

    This happening is not dependent on the sun burning by any other method at any time prior to that. All it has to do is get big enough from capturing meteors and such.

    Jupiter radiates 1.6 times a much energy as it receives from external sources already, with a core temp between 13,000-35,000 celsius.

    That doesn't mean it is radiating that much heat because the heat remains trapped for a long time. Much like a thermos bottle that has no heat source of its own.

    There is no need of anything to "burn" before proton-proton fusion kicks in, only that it be compressed by gravity enough to get hot enough.
  6. Feb 26, 2013 #5


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    The statement that the first stage of the proton-proton cycle takes a billion years to take place is incorrect. As soon as the interior of the sun is hot enough, protons begin to fuse into deuterium nuclei. There is no time lag before this begins to take place. The time scale of 10^9 years for this reaction given on one of the web sites you linked means that for a given proton, it takes on the order of 10^9 years before this reaction takes place. However, there are a huge number of protons in the sun, so some protons undergo this reaction right away, as soon as the temperature is high enough. Does this help?
  7. Feb 26, 2013 #6


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    This isn't strictly correct. Compression by gravity doesn't have anything to do with the heat here. Rather, it's due to the fact that Jupiter (like the Sun) collapsed from what was previously a much more diffuse gas. This matters because the atoms in the gas lost a lot of gravitational potential energy as they collapsed to make the planet, and what they lost in gravitational potential energy they gained in kinetic energy, which translates to thermal energy (thermal energy of a gas is made up of the kinetic energy of the atoms/molecules that make up the gas).

    A similar process explains why the core of the Earth is still hot: it's relic energy left over from the gravitational potential energy gained by matter falling to form the Earth. The Earth has been slowly cooling and solidifying over the following billions of years.
  8. Feb 26, 2013 #7


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    No, that is not sufficient to keep earth as hot as it is - in fact, this assumption was used for a (wrong) estimate of the age of earth some time ago. The decay of radioactive elements like uranium provides the energy today.
    Well, compression and heating go hand in hand.
  9. Feb 26, 2013 #8


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    Right, forgot about the radioactive heating element. But this has more slowed the process of cooling than anything.

    Compression doesn't have anything to do with it, though. The initial heating is entirely about a loss of gravitational potential energy.
  10. Feb 26, 2013 #9
    .... thanks to all you guys for taking the time to respond .... it was very helpful ....
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