Hot bright stars lie toward the upper left: smaller $B-V$ means bluer/hotter, and smaller numerical $V$ means brighter. The vertical axis is inverted in the original diagram. The plot shows <Apparent magnitude>[apparent magnitudes], but cluster members at a common distance permit relative <luminosity> comparisons.

Solution

Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S26 / 1 / 3 / a / Solution

 Solution (source code)