Chemical Abundances in the Universe

I have stated (early and often) that the Universe is primarily hydrogen and helium. What is this statement based upon? Fortunately, the statement is based on data. The Universe is roughly 90 % hydrogen and 10 % helium based on observation.

For the following, let me change a definition. I will give abundances in terms of mass fractions instead of number fractions. That is, if I have 10 grams of material, I will let X denote the fraction of the stuff which is hydrogen and I will let Y denote the fraction of the stuff which is helium. So, if 90 % of the stuff is hydrogen by number, then X = 0.9 m(hydrogen) / [ 0.9 m(hydrogen) + 0.1 m(helium) ] ~ 0.9 / 1.3 = 0.69 and Y ~ 0.31.

• Sun, Young stars, planetary nebulae, extra-galactic H II regions have been used. However, since stars produce helium, simply not enough to account for 10 % by number, exercise is a little difficult because we want to deduce how much helium was produced by the Big Bang. A clever thing to do would be to observe objects which are very old (and therefore relatively uncontaminted by stars). Some workers (as advocated by many people) have shown that extra-galactic H II regions may be useful. An interesting result is that the helium abundance is correlated with the abundances of elements such as carbon, nitrogen, and oxygen. The correlation is in the sense that the abundance of helium increases for regions which have high abundances of the CNO elements. This is very suggestive because the CNO elements are produced by stars. So, using this data one can infer what the primordial abundance of helium.

  • Y ~ 0.23 with an uncertainty of ~ 0.005

In the early Universe, recall that nucleosynthesis yields .... . A reasonable range of values based on the Big Bang picture is

• 0.23 < Y < 0.28 ===> Big Bang is fully consistent with the data.