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Comment by ars | Hacker News Reader

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ars·

It's more than just light elements, those elements are the main sequence elements made by Stellar nucleosynthesis.

If you look here, you'll see just how abundant they are compared to everything else:

https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elem...

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Been a while since chem & physics class for me as well.

How are these estimations calculated, and to what certainty? Surely, these are calculations only from the observable universe?

I'm not sure how they know, but see here: https://en.wikipedia.org/wiki/Stellar_nucleosynthesis for the reason for the quantities.

I'm not a scientist, but I imagine that they would use the emission spectrum[0] of the various elements. Perhaps they could sample the spectrum of all of the stars and that might give a reasonable approximation.

[0] http://en.wikipedia.org/wiki/Emission_spectrum

They do it using spectral analysis. Different elements have different absorption bands of light. You can look at a graph of light from a star, with intensity of light on the y-axis and frequency/wavelength (color) on the x-axis and that tells you quite a bit about the composition of the star.

That only tells you the composition of the surface of the star. The heavier elements will be deeper.

I suspect there is mathematical modeling.

Interstellar dust might provide the spectrum instead of stars, although that really only tells us the composition of the milky way.

Well, in order to find the composition of interstellar dust, they will use spectral analysis as well...

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