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Absorption Spectra from Stars

  1. Oct 1, 2015 #1
    Hi pf. As I understand we know what a star is made of by looking at the absorption spectrum from a star. Since a star is a black-body it gives out a continuous spectrum of light but as this light passes through the Sun's atmosphere, certain wavelengths are absorbed depending on what the star is made of.

    My question is, how do we know what the rest of the star is made of? Surely from this method you only know what elements are in the atmosphere of the star?
  2. jcsd
  3. Oct 1, 2015 #2


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    hi Jimmy

    here's a comment that should answer you question ....

    from this site ....

    Note that the chromosphere is what we see as the visible surface of the sun that is at ~ 6000K
    so as such, you are looking at the spectra of the sun ( other star)

    EDIT .... I err'ed there, the photosphere is the visible surface of the sun sorry
    the chromosphere is directly above the photosphere

    Last edited: Oct 1, 2015
  4. Oct 1, 2015 #3
    Great thanks. But how do we know what the core is made of? You always here scientists say "the Sun is made up of a core of hydrogen". How do they know this because the absorption spectra only tells you about the composition of the chromosphere. Or do you just say that the chromosphere is a representation of what the whole star is made of?
  5. Oct 1, 2015 #4


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    Well, if we assume that a star is initially composed of mostly hydrogen, we can then determine what should happen to the core using known physical laws. These laws tell us that the core should be millions of degrees kelvin and under immense pressure. These laws also tell us that at the temperature and density that we think a star's core is at, hydrogen should fuse together to form helium in a specific chain of events that release a large neutrino flux from the core. The neutrino flux from the Sun has been measured and it indeed matches the amount predicted by theory.

    Prior to measuring the neutrino flux, we had to simply trust that our models were correct since they appeared to make good predictions about stellar temperatures vs mass and other features. In the end there really isn't one thing, or even a few things, that supports the idea that a main sequence star's core is composed primarily of fusing hydrogen. A great many different things all add together to form a complete model, and if a star's core were not made up of hydrogen then we'd have a very serious mismatch between different parts of the model. For one thing, we can see vast clouds of hydrogen gas collapsing under gravity. If a star's core isn't made up of hydrogen, then what else could it be made of? There isn't enough non-hydrogen material in these collapsing clouds to form the core of a star. If hydrogen isn't being fused in the core, then where is the energy coming from and why does the age of the solar system, dated through radioactive dating, match so well with the prediction that the Sun is halfway through its fuel supply of hydrogen? (no other fuel source gives enough energy for the Sun to last 4.5 billion years or so) Why does our model match many different observations so well if it's wrong?
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