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Stellar Evolution

  1. Nov 11, 2009 #1

    Just revising for my Advanced Stellar Evolution exam, was wondering if anyone knew if there was a good detailed Colour Magnitude (HR) diagram on the internet which went in to some detail of different Stellar Mass' and Chemical Compasitions (so more like a poster than a HR diagram)!

    If not would it be of help if I posted one which I am making, its like a complete guide to Stellar Evolution!?
  2. jcsd
  3. Nov 12, 2009 #2
    Hello MattLiverpool
    You got me interested. You mean that shifts in the HR-diagram caused by different chemical conposition can be measured? Or should I conversely say that a HR-diagram has to be converted for chemical composition to get a mass-luminosity relation?
    Sorry my stellar evolution tentamination was a long time ago...
    Greetings Janm
  4. Nov 13, 2009 #3

    Basically features of the HR diagram change due to different initial chemical compasition of the star.

    For example on the RGB if the star's initial chemical compasition has increased Helium abundences (Y) the RGB will have hotter Teff, and if the initial chemical compasition has increased metals the RGB will have a cooler Teff. It also effects time scales and many other areas of the stars evolution.

    The big diagram I am working on shows this and lots of other things like effects of the dredge ups, thermal pulsing, basically its like one big guide to stellar evolution!
  5. Nov 13, 2009 #4
    Hello Mattliverpool
    I do some associations please tell me when I am wrong:
    1 So the star has a little red giant behaviour! is it also bigger?
    2 Metals are dampeners?
    3 What are dredge ups?
    4 This is Cepheidebehaviour?
    greetings Janm
  6. Nov 13, 2009 #5
    1. The chemical compasition doesn't effect the mass of the star, and im not sure what you mean by red giant behavior, I was just giving an example of a stage of evolution where chemical compasition effects the stars evolution.

    2. I dont know if describing them as dampeners would be correct as I am not sure if they always have this effect, but in alot of situations they cause the star to be cooler.

    3. There are three dredge ups in the evolution of a star, basically they are occasions when the convective shell ventures in to regions where CNO cycle burning has happened and 'dredge' different chemicals to the surface. If you would like more details on the dredge ups I would be happy to supply them.

    4. Cepheids are a type of pulsating stars, they happen in the stage of their evolution when crossing a region called the instability strip they pulsate, again if you would like more information on this I can supply it!

    Hope that helps!

    ~ Matt
  7. Nov 13, 2009 #6
    Hello MattLiverpool
    With red giant I mean helium-star, of which hydrogen is exausted. I am mostly interested in the dredge up, because that is something I had not heard until you brought that up. You talk about CNO... Does that mean it is about stars heavier then 10 solar mass or so?
    The convection zone is where energy transfer through radiation is insufficient to give the needed temperature gradient is it not?
    greetings Janm
  8. Nov 14, 2009 #7
    When I say CNO cycle I just mean the type of Hydrogen burning, depending on the stage of evolution and the mass of the star Hydrogen either burns via the pp chain or CNO cycle.

    There are three dredge ups, the first and second happen once mixing more central shells to out layers. The third drege up(s) happen several times during thermal pulsing.

    A convective zone is just where the energy transport mechanism is convection.
  9. Nov 14, 2009 #8
    Hello MattLiverpool
    The pp chain I believe I know. Could you tell more about the CNO cycle?
    What is the difference between the first and the second dredge up?
    What is thermal pulsing?
    greetings Janm
  10. Nov 15, 2009 #9
    Successive protons are fused to, initially, an atom of carbon 12, eventually forming an excited oxygen 16 that spits off a helium nucleus/alpha particle and returns the carbon 12 to the reaction. Thus it's usually called "fusion catalysis". The CNO cycle is more temperature sensitive than the pp-chain and dominates in stars just a bit warmer in the Core than our Sun. It's the main fusion cycle in all larger stars.

    Different times when the inner and outer regions of a star mix. Thus the first dredge up is during the first Red Giant phase and the second is during the Asymptotic Giant phase... I think. Hopefully Matt can clarify.

    Thermal pulsing occurs in unstable stars like Cepheids which periodically warm and cool pretty rapidly over a few days. It's because they've hit a not-quite balanced period of fusion burning and the main fusion reaction is very temperature sensitive. A bit of extra fusion causes the outer layers to warm-up and expand, but that then dampens the fusion reactions a bit, causing it to cool and contract, but then that warms up the fusion... and over and over, until it moves away from the instability.

    Hope that elucidates things for you.
  11. Nov 16, 2009 #10
    Hello qraal
    Yes that elucidated much, thank you. I looked a little into the CNO cycle and saw there is also a "hot CNO" cycle. Do you also know the difference between a usual?-CNO cycle and a hot-CNO cycle?
    greetings Janm
    PS so C, N and O are catalisers in the proces.
    Last edited: Nov 16, 2009
  12. Nov 17, 2009 #11

    Well apparently in large stars the CNO cycle doesn't cycle back to carbon - it can end with just the nitrogen. All sorts of other nucleosynthesis cycles are occurring at the same time and lots of different elements are being forged via fusion. It's quite a complicated suite of fusion reactions and even break-down reactions that occur in the ferment of an intermediate mass star's dying years.
  13. Nov 17, 2009 #12
    Also it gets complicated (and fun) when different bits of physics interact with each other. For example, the CNO cycle is very temperature sensitive, which means that the hot parts of the star get really hot, which means that things start mixing. Once things start mixing, then all of the chemical distributions change, and that affects the fusion reactions.

    When the outer layers of the star start pulsing they throw off some gas, how much gas gets thrown off comes back and changes the reactions.

    And that's just single stars. With double and triple stars, the different stars start throwing mass back and forth.
  14. Nov 18, 2009 #13
    Hello twofish-quant
    You mean like solar-wind? I know that reaches very far and with high velocities, very low pressure, but yet still can fall back to the sun?

    Keeping things a little attached to this thread! How much of these events can be seen on a HR-diagram?

    greetings Janm
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