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Red Giant Tip

  1. Jul 20, 2009 #1
    Hi All

    As most of us know, in a qualitative sense, the Sun will eventually become a Red Giant before its core explodes (the Helium Flash) and it's heaved onto the Helium Main Sequence, to shine merrily for about 100 million years. What I want to know, in a quantitative sense, is just how its luminosity behaves with respect to time as it's approaching the Red Giant Tip - its point of maximum luminosity before the Helium Flash. From all the graphs of luminosity against that I've seen that final phase is very rapid compared to its luminosity evolution for the 12 billion years prior. Is there a function that fits that final phase? Any good references?
  2. jcsd
  3. Jul 22, 2009 #2
    I don't know if there is a function of the final phase or if there are good references
    I know that in the RGB phase the star's luminosity is increasing because in this phase the source of photons is only the H shell. In fact you can parametrize the luminosity of the shell by the mass of the core (this is the reason why different masses have the same RGB tip luminosity). In particular you should find that if the mass of the core is increasing also the luminosity of the shell increases. During the RGB phase the shell increases the core's mass, hence luminosity increases.
    I don't remember the reason, but after the RGB Bump you should find that the velocity of the evolution is increased. In particular the change of the slope of the Integrated Luminosity Function is associated to this change of the velocity (the molecular weight changes).

    PS I'm not a native speaker, so I apologize for any possible misunderstanding.

  4. Jul 23, 2009 #3


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    That final phase is indeed very rapid in a massive star. It only takes a matter of hours, even minutes, from the time silicon fusion is initiated until a core collapse event occurs. The sun, however, is not massive enough for silicon fusion to occur. It will merely slowly decay from a red giant into a white, then black dwarf.
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