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How long before the sun goes red giant?

  1. May 19, 2003 #1

    marcus

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    A french poster (ant284) just came to HomeworkHelp with a question about the lifetime of the sun.

    Some say the sun will go red giant when it has consumed 12 percent of its hydrogen.

    The sun is now 75 percent hydrogen. We know its mass so we can tell how much hydrogen is supposed to be consumed before red giant stage.

    I like trying out natural units (c = G = hbar = 1) which seem
    surprisingly serviceable in a wide range of contexts, so I calculated the lifetime of the sun in those units as a check for ant284. It turned out we both got the same estimate of the lifetime!

    The key thing one must know is the power or "wattage" of the sun. This determines how fast the hydrogen is being consumed,
    and allows one to estimate the life.

    What do you think the wattage of the sun is? If you like using metric units then of course you will wish to express the sun's power as a certain number of watts. What number?

    Ant284 posted a number of watts for the sun, down in Homework section. But why not guess first? It seems that the sun is important to us so shouldn't we have some idea of its power?

    Compared to the natural unit of power c5/G, the sun's power is 1.07E-26. That is just what fraction (about E-26) of the natural power unit it happens to be.
    And in terms of the natural unit of energy, conversion of that much hydrogen (12 percent of 75 percent of the mass of the sun) will release 6.0E34 units of energy.
    So essentially one just divides 6E34 by E-26 to get the life.
    Or more precisely 6E34 by 1.07E-26.
    But you may prefer to make the corresponding calculation in metric...
     
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  3. May 19, 2003 #2

    marcus

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    A question about the power of the sun

    What do you think the power of the sun is?

    1----3.9 x 1021 watts

    2----3.9 x 1023 watts

    3----3.9 x 1026 watts

    4----3.9 x 1029 watts
     
  4. May 19, 2003 #3

    Labguy

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    Re: A question about the power of the sun

    It is 3.86 billion billion megawats, so unless I did the decimal counting wrong, that makes it 3.86 (your 3.9) x 10 26 watts per second.

    Also, your original "Red Giant" question is that the Sun will expand to a red giant in less than 4 billion years, after most of the core hydrogen is used (fused?). But, even before that, swelling and energy output will increase to where life on Earth will be wiped out within 1 billion to 2 billion years from now, long before the final expansion to a red giant. I have seen other models showing we have only about 600,000 years to be toasted, but I'll just have to wait and see which one is correct...

    Edit: Damn, I hate doing "math stuff". I just think and read, and leave the "mathing" to others.
     
    Last edited: May 19, 2003
  5. May 19, 2003 #4

    marcus

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    Re: Re: A question about the power of the sun

    YOU ARE RIGHT! Stop right there, no need to say watts per second :wink:

    Allen's Astrophysical Quantities, a classic reference, says
    3.826 x 1033 ergs per second, which is
    3.826 x 1026 watts
    so you could have said 3.8
    the french student in the other thread had homework data
    of 3.9 so we went with that, no big difference

    Your "billion billion million" might be off but you ended up with
    the right answer.

    You say: "I have seen other models showing we have only about 600,000 years to be toasted, but I'll just have to wait and see which one is correct... "

    It does seem like a good idea to check out some nearby stars for nice-sized watery planets.

    "I just think and read, and leave the "mathing" to others"

    Appreciate very much your taking the trouble to do a little
    "mathing" in this case.
     
    Last edited: May 19, 2003
  6. May 21, 2003 #5
    Re: Re: Re: A question about the power of the sun


    We must realy hurry up then. Just 600.000 years. We need to make a space colony, before humanity goes extinct.
     
  7. May 21, 2003 #6
    Lots of things can change in a half-million years.

    But if the speed of spacecraft doesn't, then 'we' need to leave ahead of time.

    How much ahead of time?
     
  8. May 21, 2003 #7

    marcus

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    Re: Lots of things can change in a half-million years.

    If memory serves the increase in temp during next 600,000 would only require going out to Mars orbit. It is not red giant business, just a little heating up.

    correct me if I have the numbers wrong.

    In 600,000 the moons of jupiter might be a possibility

    a way to pump energy into the earth's orbit gradually has been proposed by a couple of qualified (but a bit optimistic) guys.

    it uses asteroids.

    I dont like any of this. I hope humans get to other stars---much more interesting.

    But strictly speaking it is not necessary to leave the solar system for life to continue for the next half a million-----just move out from the fire a little
     
  9. May 21, 2003 #8

    Labguy

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    Re: Re: Lots of things can change in a half-million years.

    Marcus type guy:
    If we haven't figured out how to travel elsewhere to live happily in 600,000 years, we wouldn't deserve to survive. It is still hard to imagine, but remember that the Wright brothers flew off less than 100 years ago! Also, the two computers aboard the Apollo 11 lander, 1969, each had a total capacity of a whopping 65K.
     
  10. May 21, 2003 #9
    marcus,
    Thanks for the information, I found it quite fastinating. If they (future generations) have to hop over to other 'dirt-clods' in this solar system it could stall off the inevitable…for a time, but like you I hope humans will be able to get out to the stars…

    I hadn’t heard about the possibility of pumping energy into the earth’s orbit until you mentioned it just now. Sounds pretty interesting, how about Rocket Ship Earth? Wouldn’t it be a ‘trip’ if the entire earth was to be relocated into another solar system? Without a sun during the flight is there even the remotest possibility people could survive something like that?
     
  11. May 21, 2003 #10
    all stars are veryable in their output

    ours has been more or less stable for a long time

    but latest data points to a slow increase in output, that is behind global warming, in addition to the co2 counts increasing

    and history of the global temps points to more iceages then warm spells

    but this all will get more interesting as the waters begin to rize
     
  12. May 22, 2003 #11

    marcus

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    Hello Ray and Majin
    Ray did you read about the proposal to steer an asteroid close to the earth every 100 years or so
    for the purpose of enlarging the earth's orbit gradually
    to compensate for this gradual temperature buildup in the sun?

    I saw the article written by the two people who proposed the scheme. It was online. In the past 6 months I believe.

    Majin would probably like to read it. But I do not have the URL.

    It is very much against my intuition and instincts to consider bringing any asteroid or comet into near-collision with the earth-moon system----I shrink from searching for this article. But it was not a silly article. It was a rigorously calculated and knowledgeable proposal by qualified NASA-type people---wacky though it may sound.

    Majin the comets of the Oort cloud contain water and are at a high gravitational potential. One set of thermonuclear explosions could cause a comet to fall 1000 AU to make a tight turn at the sun
    and the most efficient use of propulsion would be to use another
    thermonuclear thrust at perihelion (closest approach to sun where the propellant payoff is greatest) after which the comet would leave the solar system.
    Comets are not as difficult to get to go places. They are not so massive as a planet and they have light elements like hydrogen which can be vaporized or ionized for propulsion.

    I do not like your fantasy of moving the earth. think more elegantly (as I have seen you are very capable of doing!)
     
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