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 , where M is the total mass of the system, and m H is the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as Y ≡ ≡ m H e 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