Specific gas constant ($\mathcal{R}$)

The gas constant per unit mass for a specified composition. An ideal gas obeys $p=\rho \mathcal{R}T$. With dimensionless mean molecular weight $\mu$ relative to an atomic mass unit, $\mathcal{R}=k_B/(\mu m_u)$; the common astrophysical notation $R^{\ast }/\mu$ uses a modified mass-based constant rather than the molar gas constant without a mass conversion.