Schramm and Turner highlighted the discrepancy between this element’s observed abundance in stars and the theoretical amount predicted by the Big Bang, which is known as this element’s cosmological“problem.” This element and helium-4 are produced in the PP II [p-p two] branch of the proton-proton chain. T Tauri stars exhibit strong absorption lines of this element. The presence of deuterium and this element is often used to distinguish brown dwarves from (*) low-mass (10[1])stars. An isotope of this element is the heaviest stable product of Big (-5[2])Bang nucleosynthesis. This is the lightest (-5[1])element whose abundance contributes to a quantity that distinguishes Population I and II stars and is denoted Z. In stars, this element is depleted by fusing with a proton (-5[1])to form unstable beryllium-8. For 10 points, name this lightest metal. ■END■ (10[4]0[2])