Balmer absorption lines from stars - A level

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Offline survivalist13

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Balmer absorption lines from stars - A level
« on: 28/03/2012 18:23:47 »
We are doing astrophysics and have been doing about balmer series and how it can be used to work out the temperature of stars, well really just that the intensity of balmer lines varies with the temperature of a star. So this is the explanation we have been given:
Balmer series is the frequencies of photons that are absorbed/emitted when the electron in a hydrogen atom move up from the 2nd level or down to the 2nd level. These frequencies happen to be in the visible part of the spectrum hence why they are useful. When looking at the light from a star there will be dark lines in the spectrum, these are the absorption lines from the balmer series. How these dark lines are produced is where the issue lies.

As far as I can gather the inner part of the star produces light of all frequencies (some with a higher intensity than others) the outer part of the star is made of relatively cool hydrogen gas, the light from the inside of the  star passes through this cooler hydrogen which still depends on the temperature of the star. In a cold star this hydrogen will be mostly in the low energy state so won't absorb much light of the frequencies corresponding to the balmer series, in a hot star most of the hydrogen will be in energy states above the 2nd so it will also not absorb many photons of the correct wavelength. In medium temp stars the absorption will be greatest. Ok so this makes sense however those electrons need to get rid of that energy again, how do they do that? In chemistry we are told they re-emit it in smaller steps which I'm not really convinced by, surely this process would be symmetrical and balance out? In physics the answer is that the photons and re-emitted in random directions which sort of makes more sense and I would assume be easy to test, if we had a gas and passed a white light through it we should see dark lines looking straight ahead and the absorbed colours when looking off to the sides, I suspect there maybe some sort of scattering effect that might in validate this test (speaking of which how's scattering work on an atomic level?). Anyway back to the star, if the star has an even layer of "cold" hydrogen surrounding it surely the hydrogen that re-emits the light in random directions will all cancel out and no net effect will be observed (or a very small one) the slight problem I have with this is if it emits it back into the centre of the star, would this account for it?   

So can anyone help me out, or know anyone I could email who would know? Thanks in advance.   


Offline Soul Surfer

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Re: Balmer absorption lines from stars - A level
« Reply #1 on: 29/03/2012 13:53:10 »
You seem to have given quite a good description of the process however it is not easy to simulate fully in the laboratory because of the temperature pressure and depth needed to replicate a star's atmosphere.  Various other measurements can be made that allow the process of creating absobtion lines to be created.
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