http://heasarc.gsfc.nasa.gov/docs/software/lheasoft/xanadu/xspec/manual/XSabund.html
## Allen's Astrophysical Quantities, ed. A. N. Cox (New York: Springer-Verlag), 27 Fraction of H (X) = 0.707 +/- 2.5 % Fraction of He (Y) = 0.274 +/- 6 % Fraction of Others (Z) = 0.0189 +/- 8.5 %