Hydrogen-helium free-free composition factor
= Hydrogen-helium free-free composition factor
{title2=$n_e\sum_iZ_i^2n_i=2n_e^2/(1+x_H)$}
For fully ionized <hydrogen> and <helium> with hydrogen <mass fraction> $x_H$, $n_e=(1+x_H)\rho/(2m_p)$ and $\sum_i Z_i^2n_i=\rho/m_p$. With the same unit <Gaunt factor> for both species, <thermal bremsstrahlung> at fixed <electron> <number density> is multiplied by $2/(1+x_H)$ relative to pure ionized <hydrogen>. The factor is one for pure <hydrogen> and two for pure <helium>.