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Sun running out of Hydrogen.

  1. Feb 29, 2012 #1
    I'm just doing some reading on stars and I've hit a problem which I can't solve.

    According to a book I'm using it claims the amount of hydrogen converted in the Sun over it's lifetime so far is 5.4x10^55. I roughly agree with this figure using the method this author calculates the number. He also says that the sun had about 8.9x10^56 hydrogen atom and so concludes the star has used less than 10% of it's hydrogen.

    However it appears that most people say the sun has about another 5 billion years before it runs out of fuel and yet we've used less than 10& in about 4.7billion years? Can anyone explain the vast differences here?

  2. jcsd
  3. Feb 29, 2012 #2


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    The core of the sun, and stars similar to the sun, cannot bring fresh hydrogen from their outer layers into the core to replace the fused hydrogen. Thus whatever amount is in the fusible area of the core is all that it has to work with. Once this is used up the Sun will turn into a Red giant and begin burning Helium for a while.
  4. Feb 29, 2012 #3


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    It;s interesting to study models of the sun like this Standard Solar Model. The core of the sun is already down to only 33% H, after starting at about 70% H.
  5. Feb 29, 2012 #4


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    Just to contrast, while the Sun is unable to support convection (the process which would bring hydrogen from the outer envelope down into the core), less massive stars are. These stars are able to effectively move the fused helium outside, while bringing fresh hydrogen into the core, which is part of the reason why they can have such ridiculously long lifespans (trillions of years).
  6. Feb 29, 2012 #5


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    And to bounce off your post, these low mass stars also use their fuel much slower than the Sun. A red dwarf that's 49% of the Suns mass only has 3.5% of its luminosity.
  7. Mar 1, 2012 #6
    Trillions? How do you figure that? The entire cosmos is only 13-14 billion years old at this point. What are these Methuselah stars?
  8. Mar 1, 2012 #7
    they WILL exist for trillions of years, they are currently not so old. At most 13.7 billion years old or so
  9. Mar 6, 2012 #8
    A redwood sapling you plant in a two day old park will likely outlive your great, great, great, great, great grandchildren despite the fact that the park is only two days old.

    Similarly, low mass stars burn cool and slowly, live long lives and go out with a whimper while high-mass stars (much like many rock stars) live fast and furious, leave a big bloated corpse, and go out with a bang.
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