Mean molecular weight per electron
= Mean molecular weight per electron
{title2=$\mu_e=\rho/(m_un_e)$}
The mean molecular weight per electron is the mass carried per free electron in units of atomic mass $m_u$. Fully ionized hydrogen has $\mu_e=1$, while helium, carbon, and oxygen have approximately $\mu_e=2$. It converts <electron number density> to bulk <mass density> in the <equation of state of a cold electron gas>.