Data supporting abundance of H and He in universe I am a science educator, writing a lesson sequence for students on understanding evidence that supports the big bang theory.  I am trying to figure out whether there is accessible galaxy spectral DATA, that high school students could understand supporting the claim that H and He comprise the majority of mass in the universe. In an effort to build an inquiry, or constructivist approach to learning science, starting with actual phenomenon is important to helping students make sense of things.  But I can't find specific data sets that seem accessible. Thanks!
 A: Stellar spectra are not particularly simple to interpret. If you look at the sun's absorption spectra you will see some hydrogen lines, but you will also see some lines from things like sodium that have intensities way out of proportion to the concentration of sodium in the sun. It requires quite a bit of sophistication to determine the relative abundances of elements from stellar spectra. You can certainly show them that stellar spectra do all contain hydrogen lines, e.g.:

(Source: https://en.wikipedia.org/wiki/File:Dwarf_star_spectra_(luminosity_class_V)_from_Pickles_1998.png .)
An additional problem is that the luminous mass of a galaxy is disproportionately in the form of low-mass stars, but such stars are very dim, so they make little contribution to the spectrum of a galaxy as a whole. And most of the mass in a galaxy is not luminous matter -- quite a bit is not even baryonic matter.
You can get inexpensive spectrometers that students can use to observe emission and absorption spectra by holding them up to their eyes. I do this and have students first look at fluorescent lights in the classroom, then go outside and look at the sun's spectrum. We verify that hydrogen lines are present, and also that other lines are present. I explain that the intensities are not simply proportional to the abundances, and I point out that it's cool to be able to see what the sun is made out of without going there.

I am trying to figure out whether there is accessible galaxy spectral DATA, that high school students could understand supporting the claim that H and He comprise the majority of mass in the universe. 

Of course this is not really true -- they are the majority of the baryonic mass, but only a small fraction of the total mass-energy density.
A: This question reads like an interface between chemistry, physics, and astronomy.  While not yet in the consecution for teaching the whole story, there are some elements that are worth to consider:


*

*What is the element's abundance in the universe?  For this, there are lists and compilations, like the one on wikipedia (here).  Nicely, it does not stop while stating the corresponding percentages, but has a periodic system of the elements showing the origin of the elements, too: 



(loc. cit.)


*

*Similar to "typical chemical reactions", where energetic changes are caused by changes on the level of electrons, there are energetic changes in the generation of elements by fusion, or fission. A view on the nuclear binding energy may be helpful, especially around the peak of iron / Fe:



(source)


*

*The spectroscopic investigation comes with own complex aspects, even if restraining only to visible light.  Some elements are spot, because the light passing across a (relatively cold) cloud towards the observer contains them, and their characteristic wavelengths are hence absorbed (Fraunhofer lines).  In other instances, stars are so hot that these elements are excited and their emission is recorded, and deciphered (like in a flame test).  The two phenomena are complicated because the universe still follows expansion, leading to redshift of the spectra.


Example of absorption spectrum:

(source)
Example of emission spectrum:

(source)
with the "fingerprints" of the elements superimposed:

(enter link description here, Hubble).
The Sloan Digital Sky Survey III maintains a server with plenty of imaging, optical spectroscopy and infrared spectroscopic data like here to delve into detail; even if it takes some investment to learn how the database works.

