Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2018 / S2 / 1 / b / Solution

Integrating $dP/dr=-G{M}_r\rho /r^2$ in order of magnitude, with $\rho \sim M_{\ast }/R_{\ast }^3$, givesThe ideal gas law independently gives $P_c\sim M_{\ast }{k}_B{T}_c/(\mu m_H{R}_{\ast }^3)$. Equating the two estimates yieldsFor similar composition and an approximately fixed hydrogen-burning central temperature, $R_{\ast }\propto M_{\ast }$. Calibrating this rough relation on the Sun and using part a gives a nominal mass range of about $0.05$ to $5M_{\odot }$.

The endpoint luminosity ratio is $10^8$, whereas the inferred mass ratio is about $10^2$, giving $L\propto M^4$ across that range. The exponent is therefore consistent at this schematic level. The solar normalization and individual endpoint masses are much less reliable: using the luminosity relation itself gives approximately $0.1$ to $10M_{\odot }$. Central temperatures are not truly constant, low-mass stellar structure differs, and the uncorrected $V$ luminosities in the diagram are not bolometric. These limitations prevent treating the rough radius-based masses as a contradictory precision measurement.