Stellar gas constant ($\mathcal{R}=k_B/m_u$)

In stellar ideal gas equations, the stellar gas constant $\mathcal{R}=k_B/m_u$ converts a dimensionless mean molecular weight $\mu$ into the specific thermal relation $P=\mathcal{R}\rho T/\mu$. Here $m_u$ is the reference atomic mass unit, commonly approximated by the proton mass in simplified hydrogen-helium models. It has units of energy per mass per temperature. It is the molar gas constant divided by the molar mass corresponding to $m_u$, rather than the molar gas constant used with a dimensional molar molecular weight.