 Cluster hydrostatic mass estimator

For spherical thermal hydrostatic equilibrium, $dP/dr=-{\rho }_{g}GM(<r)/r^2$. Inserting the ideal gas law $P={\rho }_g{k}_{B}T/(\mu m_p)$ with constant mean molecular weight gives the displayed estimator. The density ${\rho }_g$ is gas density, while $M$ includes every gravitating component. A varying $\mu$ adds $-d\log \mu /d\log r$ inside the logarithmic bracket. Nonthermal pressure requires differentiating the total supporting pressure rather than thermal pressure alone.