Past exam of the short options paper of the physics Prelims of the University of Oxford / 2021 / S02 / 1 / b / Solution

TODO: numerical endpoints require the unreadable source diagram. Let ${\ell }_{V,\max }$ and ${\ell }_{V,\min }$ be the maximum and minimum main sequence visual luminosities, in units of the Sun's visual luminosity. At a distance $d$, the inverse-square law and the magnitude difference relation giveThus, using $m_{V,\odot }=-26.77$,At the naked-eye detection threshold $m_V=6$, the greatest distances for the intrinsically brightest and faintest main sequence stars would respectively beThe historical $m_V=1$ endpoint is a brightness reference, not the visibility threshold: if that endpoint were intended instead, the corresponding coefficient would be $1.736$ rather than $17.36$.

The source Hertzsprung-Russell diagram has lost the stellar loci and luminosity scale except for a surviving top tick; neither endpoint can be read reliably. If its scale was bolometric luminosity rather than visual luminosity, bolometric corrections would also be needed, especially for very hot and very cool stars. Equating the two luminosity ratios would then be an explicitly approximate assumption. These missing data are not replaced by invented endpoint values. Interstellar extinction is neglected in the displayed distance relation.