What is the ratio of hydrogen to helium in the Sun?

[shirin]

Hi everyone,
There is problem that asks how many hydrogen atoms are there in sun for each helium atom.
I don't know which solution is correct:
1) to count all the hydrogens and heliums participating in one solar pp chain,
or,
2) as X=0.71 and Y=0.26, there is (0.71/0.26)*2 H for each He.
Thanks for help.

 

[qraal]

Hi everyone,
There is problem that asks how many hydrogen atoms are there in sun for each helium atom.
I don't know which solution is correct:
1) to count all the hydrogens and heliums participating in one solar pp chain,
or,
2) as X=0.71 and Y=0.26, there is (0.71/0.26)*2 H for each He.
Thanks for help.

The figures X & Y are mass fractions. In terms of numbers of atoms of each we need to divide by their relative masses, roughly 1:4, giving us 0.71 and 0.052, then divide those by the sum I.e. 0.71/0.762 & 0.052/0.762 which gives their atomic proportions. Divide the hydrogen figure by the helium figure and you have how many hydrogens for every helium.

 

[shirin]

thank you.

 

[phyzguy]

The 26% He by weight is more representative of the composition of the sun at the time of formation. The sun has been burning H to He for 4.5 billion years, so according to the standard solar model, the interior of the sun is now over 60% He. To calculate the atomic ratio, you would need to integrate over the whole volume - I don't know the answer, but I'm sure it is more than 26% He by weight when averaged over the whole sun. Here are a couple of references with Figure 2 in the first reference showing the mass fraction as a function of radius, and Table 1 in the second reference giving the He mass fraction in the core at >60%.

http://www.pha.jhu.edu/~france/PAPERS/solmodel.pdf

http://arxiv.org/abs/astro-ph/0204331