mass fraction

Mass fraction indicates the quantity of a particular constituent (or grouping of constituents) of a substance, as a fraction of the total mass. The term is used in chemistry as well as astronomy.

Mass fraction has a particular use in astronomy to specify overall abundances within stars, clouds, entire galaxies, etc., or the entire universe, with these common symbols for various fractions:

• X - relative mass of hydrogen to the whole.
• Y - relative mass of helium to the whole.
• Z - relative mass of other elements (i.e., metals) to the whole, thus a measure of metallicity by mass.

The mass fractions present after Big Bang nucleosynthesis are often listed with a P (for "primordial") subscript: Y_p, etc. The values are a topic of research but X_p and Y_p are presumed both by theory and by extrapolation of observations to be roughly 0.75 and 0.25, with Z_p being very small. One means of determining that part of Y due to additions since the Big Bang is to survey for the ratio between changes to Y and Z over time (ΔY/ΔZ) and to use that ratio to estimate how much current helium was synthesized post-Big-Bang.