Abundance

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 %

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