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Hypothetical question

  1. Apr 24, 2007 #1


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    Low mass stars live WAY longer than even medium mass stars. But there is a limit as to how much mass an object must have before it can burn hydrogen and be considered a star.

    What if I found an brown dwarf that was 1 kilogram shy of becoming a star. So I go there in my spaceship and toss a 1.1 kg rock into the star.

    Would the object ignite into a star, then a fraction of a second later, after it lost 1.1 kg fusing hydrogen into helium, return to being a brown dwarf? Or would I have started a chain reaction that will permit the star to burn for hundreds of billions of years like all the other M dwarfs?
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  3. Apr 24, 2007 #2


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    I rather doubt that stellar (evolution) models are sufficiently accurate to tell!

    In any case, there's an awful lot more going on at the 'kilogram' level that would render any model's (or models') answers moot - brown dwarfs are not perfectly spherical, with perfectly smooth composition gradients; they rotate, they have magnetic fields; they interact with their environment (winds, interstellar dust, comets, etc); ...
  4. Apr 25, 2007 #3


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    The smallest mass stars/largest mass planets have been dubbed from time to time 'hot jupiters'. In reality nature dosn't conform to the boxes that Humans always seem to want to put things in! I suspect there is a smooth transition in stellar systems from those which are orbited by what we call 'planets' and those which are called 'stars'. There is the obvious different between them in terms of nuclear fusion but as your 1kg of additional mass thought experiment shows, in the transition regions between the two things are not so clear.

    This is what disappointed me most about the recent deliberations about planets and pluto at the IAU meeting. They came up with this definition of planets that dealt with the low mass end but ignored the high mass end and the problems there.
  5. Apr 26, 2007 #4
    My guess... the initial fusion reactions will produce enough energy to keep the brown dwarf + 1kg burning as a red dwarf. So It would stay a red dwarf in spite of losing the 1kg
  6. Apr 26, 2007 #5
    yeh...the weight is only needed for the intial energy but the intial fution reactions should keep it as a burning star...
  7. Apr 30, 2007 #6
    Well, there's one thing we should consider first. In my understanding, nuclear reactions (Hydrogen fusion, for instance) in the center of stars occur based on probabilities; there's no strict mass/temperature limit for a single nuclear reaction to occur - it just becomes less and less probable as temperature decreases. What I mean is that even if the proto-star has one half of the required mass to become a star, there can still be nuclear reactions in its center - a tiny amount of them (correct me if I'm wrong).

    The key concept here is that below a certain central temperature, you don't have sustained nuclear reactions (see note below). So, against all probability, a couple of Hydrogen nuclei can fuse into Helium once in a while, but it won't happen again in a long time. So you can produce Helium, but at such a low rate that it's not really perceptible (hence, the object does not shine as a star).

    Think of this as the difference between a controlled nuclear reactor (some reactions occur, with an associated release of energy) and a nuclear explosion (sustain chain reactions occur, with massive amounts of energy released). That's the difference between a proper star and a massive sub-stellar object. The minimum mass for a star (about 80 Jupiter masses) is sort of a "critical mass". (However, because my note below, the transition between controlled and chain reactions is much more violent in a star that it is with nuclear reactors - but still continuous).

    In the light of this, however, your (interesting) question is still valid. So what happens if we're 1kg below the necessary mass to sustain chain reactions in the core of the star? As others have said, I think stellar evolution and nucleosynthesis cannot give answers at 1kg "resolution".

    But personally I don't think the necessary mass for stellar fusion is an precise number (in the same way that the Earth's atmosphere doesn't end at one specific height - it's a continuous thing). So a 1kg difference wouldn't matter, the star would shine.

    Note: when I talk about sustained nuclear reactions, I'm referring to a very specific part of the P-P chain. If, by chance (ie: quantum tunneling), two protons get close enough so that the strong force kicks in, they still haven't made it, since the strong coulomb repulsion makes this "Helium-2" nucleus very unstable. Inverse beta decay must happen at precisely the instant the protons get close enough, so that one of them becomes a neutron and you have stable deuterium (Hydrogen-2). This part of the P-P chain is what regulates Helium production, since it's very unlikely. So if temperature is too low, in practice the P-P chain will never reach its endpoint of producing Helium.
  8. May 1, 2007 #7


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    Thanks for all the replys. Thanks, Meithan, the probability explanation made a lot of sense.
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