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Old Mar 25th 2015, 07:04 AM   #1
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Light and signatures in it

How can we tell what’s in a star from what’s in the interstellar (and intergalactic) medium?

In both cases, isn’t light what we’re seeing? So how could anyone say, for example, that H and He signatures in the sunlight are from the sun, not from the interstellar medium?

Please give me a clear and detailed answer, since it’s a long time since this haunts me.

Thank you so much.

Last edited by neil; Mar 25th 2015 at 07:05 AM. Reason: clarification
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Old Mar 25th 2015, 09:23 AM   #2
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Light from stars contain emmision lines and absorption lines in its spectrum. Emission lines are symptomatic of the elements in the body of the star, caused by excitation of the electrons in the atom to higher energy levels. Apsorption lines are caused by absorbtion of certian wave lengths by elements in the atmosphere of the star. So we can determine the composition of the star by analyzing these emission and absorption lines in the spectrum of light from the star.

The composition of intervening gas and dust can also be analyzed by how they absorb certain wavelengths of light. The diffeence with absorption lines from the star is that the temperature of instellar gas and dust is much cooler than a star's atmosphere, so the absorption lines are different.
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Old Mar 26th 2015, 12:51 PM   #3
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Isn’t the very same light (sunlight, or starlight) we’re talking about? So on one hand you say it tells us about hot (of the star) and on the other hand it tells us about the cold (of the space)? How can that be?

And could you give me a link to a picture, or illustration? And in fact a link with a very detailed explanation?

Thanks.
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Old Mar 27th 2015, 09:51 AM   #4
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Take a look at this site and especially the section on "Stars and their properties," which covers emission and absorption lines, and also the section on interstellar medium:

http://en.m.wikipedia.org/wiki/Stellar_spectra

Post back if you still have questions on how this works.
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Old Apr 8th 2015, 11:33 AM   #5
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Thank you so much for your kindness, Chip.

I'll look into all that, and come back if necessary.
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