= Stellar gas constant
{title2=$\mathcal R=k_B/m_u$}
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.
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