Composition of the Sun

  • #1

Main Question or Discussion Point

The question “how do we know the composition of the sun” falls shorts everywhere I look.
1. There is the spectral absorption lines. Explanations given how that works, indicate how we know the elements, but fails to tell us how we know about the preponderance of Hydrogen and Helium.
https://www.physicsforums.com/threads/how-do-we-know-what-the-sun-is-made-of.64077/

2. The Cecilia Payne story tells us she used ionisation theory developed by and Indian physicist, which somehow effects the absorbsion lines dependant on temperature, to be the first to declare that Sun was mostly Hydrogen in her 1925 PHD thesis, although she described her result as “spurious” because...

3. Henry Russell he told her it was crap, but he changed his mind after having derived the same result by different means and publishing it in 1929, but he did give her some credit, nice of him!

MY QUESTIONS:
1. What is the basic story of how with ionisation theory and spectral absorbsion lines, we can determine the proportion of the elements we observe in the spectral lines?
2. What are the “different means” Henry Russell used to come to the same conclusion and how do they work?
 

Answers and Replies

  • #2
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Spectral lines don't just tell you which elements are present - their intensity also tells you how much of it is present, and even in which state (neutral hydrogen atoms, molecules, ionized atoms, ...). Measuring the intensity of spectral lines is a bit more difficult than just seeing they are there but it can be done.
 
  • #3
How is there an intensity of black absorbsion lines?
I got to 2nd year uni physics, I can give a kindagarten version of how/why we get emission and absorption lines from a hydrogen atom.

When someone asks me “how do we know composition of the sun” I wanna be able to give a complete kindagarten version. There was an entire physicsforum devoted to this question, and no one asked about proportions. How do we know, how did we first know? It took some searching to find Cecilia.

This is not trivial, knowledge of the preponderance of Hydrogen & Helium was huge, supporting Big Bang theory etc. Earliest uses of spectral analysis revealed stars, hence likely all the universe, had a make up similar to our earth “40 to 45 of the elements here on earth are found in the sun, so....” was their thinking. So what changed, knowledge of how quantum mechanics explained the sprectral lines is not sufficient.

Also the idea learned from blackbody radiation was, that if we heated earth to the temperature of the sun, the thermal spectrum and spectral analysis would like the same - that was the thinking - HOW did Cecilia discover otherwise? How did Russell independently verify otherwise.

Sorry, I’m NOT asking for an complete answer, I’m wondering where to find it, I’ll double check my intro cosmology texts, etc. It is so basic and so fundamental to our view of the univesrse that I believe there SHOULD be a kindagarten version. I”m going to retire soon in tropical Cairns in Australia with lots of sun and plenty of time to write a kindagarten (should also include one for general relativity quantum field theory)
 
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  • #4
russ_watters
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Sorry, I’m asking for an complete answer, I’m wondering where to find it, I’ll double check my intro cosmology texts, etc. It is so basic and so fundamental to our view of the univesrse that I believe there SHOULD be a kindagarten version.
I was going to joke and say you should pick one or the other, but instead I'll say you are trivializing what is a highly developed science and you won't get very far if you don't correct that attitude.

However, for a kindergartner: Ever see a movie screen before a movie starts? What color is it?

In Lesson 2, we'll discuss Google.
 
  • #5
I was going to joke and say you should pick one or the other, but instead I'll say you are trivializing what is a highly developed science and you won't get very far if you don't correct that attitude.

However, for a kindergartner: Ever see a movie screen before a movie starts? What color is it?

In Lesson 2, we'll discuss Google.
Sorry, ment to say I’m NOT after a complete answer, relevant texts ok. But, dont you think it strange logic, there was an entire forum on physics forum about “how do we know the composition of the sun” and no one asked or explained the proportions? I mean, you cannot know the composition if you don’t know the proportions.
 
  • #6
russ_watters
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Sorry, ment to say I’m NOT after a complete answer, relevant texts ok. But, dont you think it strange logic, there was an entire forum on physics forum about “how do we know the composition of the sun” and no one asked or explained the proportions? I mean, you cannot know the composition if you don’t know the proportions.
[sigh] 0/2

Would you like to re-take the quiz?
 
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  • #7
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How is there an intensity of black absorbsion lines?
They are not completely black, just darker than the spectrum around them.
Here is an example, the yellow spectrum is from the Sun. Note how it has many dips, but none of them goes to exactly zero.
 
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  • #8
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You have to understand that the spectral analysis might be very complex task. The spectral absorption lines are not discrete lines with infinitesimal width at particular wavelength as it is often simplified, but rather are continuous (usually rapid) changes in the intensity as the wavelength changes. This rapid changes in the flux are superimposed on the curve of blackbody radiation, which can be seen as "background" of the continuous spectrum. Consequently, the lines have different intensity, width and profiles. The particular shape depends on the conditions prevailing in the stellar atmosphere and beside others it depends on the chemical composition and relative abundance of the elements. By building the models of the stellar atmospheres and comparing with observed spectra, all the details can be determined.
 
  • #9
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How is there an intensity of black absorbsion lines?
Also the idea learned from blackbody radiation was, that if we heated earth to the temperature of the sun, the thermal spectrum and spectral analysis would like the same
Surely not! The blackbody curve ("the background" of the spectrum) would be let's say the same, but due to different composition of the upper layers of the atmospheres, the absorption/emission lines would be different. Also theoretically, the blackbody radiation is applicable for "balck body", that means the body is absorbing all incident radiation - no wavelength are being reflected. In reality, such body probably doesn't exist, but stars are very close to it.

(... not mentioning that such situation is purely hypothetical: at such temperatures it would be very difficult to talk about any Earth's atmosphere. And I think that there is no mechanism which would heat the Earth's surface to such temperatures, unless it gains a LOT of mass and ignite a nuclear fusion in its core... )
 
  • #10
Bandersnatch
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  • #11
stefan r
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Why not take various mixes of gasses in a lab and heat them up? Compare the spectra.
 

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