This quantity can be calculated using the R-23 method. A galaxy with an extremely low value for this quantity nicknamed “Peekaboo” was discovered in November 2022. Closed, leaky, and accreting box models are used to derive the evolution of this quantity in galaxies. High values of this quantity cause line blanketing that reddens stellar spectra, particularly in galactic bulges. This quantity is essentially zero for Population III stars, (10[2])which formed soon after the Big Bang and thus were not chemically enriched by supernovae. Stellar composition is defined by this quantity and two other quantities (10[1])symbolized X and Y, which measure the two most abundant elements in the universe. For 10 points, name this astronomical quantity that measures the ratio of elements heavier than helium. ■END■ (0[2])