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The origin of calcium production in the first stars (Pop III stars), whichformed out of the primordial matter of the Big Bang, and their fates, remainmost fascinating mysteries in astrophysics. Advanced nuclear burning andsupernovae were thought to be the dominant source of the Ca production seen inall stars. Here we report on a qualitatively different path to Ca productionthrough break-out from the "warm" carbon-nitrogen-oxygen (CNO) cycle. We extenddirect measurement of the $^{19}$F($p$, $\gamma$)$^{20}$Ne break-out reactiondown to an unprecedentedly low energy point of 186 keV and discover a keyresonance at 225 keV. In the domain of astrophysical interest, at around 0.1giga kelvin, this thermonuclear $^{19}$F($p$,$\gamma$)$^{20}$Ne rate is up to afactor of 7.4 larger than the previous recommended rate. Our stellar modelsshow a stronger break-out during stellar hydrogen burning than thought before,and may reveal the nature of Ca production in Pop III stars imprinted on theoldest known ultra-iron poor star, SMSS0313-6708. This result from the ChinaJinping Underground Laboratory, the deepest laboratory in the world, offeringan environment with extremely low cosmic-ray induced background, hasfar-reaching implications on our understanding of how the first stars evolveand die. Our rate showcases the impact that faint Pop III star supernovae canhave on the nucleosynthesis observed in the oldest known stars and firstgalaxies, key mission targets of the James Webb Space Telescope.

Measurement of $^{19}$F($p$,$\gamma$)$^{20}$Ne reaction suggests CNO break-out in first stars