Metallicity
Mass fraction

Stellar composition is often simply defined by the parameters X, Y, and Z. Here X represents the mass fraction of hydrogen, Y is the mass fraction of helium, and Z is the mass fraction of all the remaining chemical elements. Thus

X + Y + Z = 1 {\displaystyle X+Y+Z=1}

In most stars, nebulae, H regions, and other astronomical sources, hydrogen and helium are the two dominant elements. The hydrogen mass fraction is generally expressed as X ≡ m H M , {\displaystyle \ X\equiv {\tfrac {m_{\mathsf {H}}}{M}}\,} where M is the total mass of the system, and m H {\displaystyle \ m_{\mathsf {H}}\ } is the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as Y ≡ m H e M. {\displaystyle \ Y\equiv {\tfrac {m_{\mathsf {He}}}{M}}~.} The remainder of the elements are collectively referred to as "metals", and the metallicity – the mass fraction of elements heavier than helium – is calculated as

Z = ∑ e > H e m e M = 1 − X − Y. {\displaystyle Z=\sum _{e>{\mathsf {He}}}{\tfrac {m_{e}}{M}}=1-X-Y~.}

For the surface of the Sun (symbol ⊙ {\displaystyle \odot }), these parameters are measured to have the following values:

Due to the effects of stellar evolution, neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition.
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