In stellar cores, this element is rapidly destroyed by reacting with a proton, creating a beryllium atom that then degenerates into two alpha particles.
Plotting the baryon density on the x-axis against the ratio of one isotope of this element to hydrogen shows a minimum between 10 to the negative 10th and 10 to the negative 9th. The Spite plateau helps constrain the amount of this element in the early universe, which is inferred by measuring its abundance in old halo stars. Absorption lines of this element are characteristic in T Tauri stars. Neutrino production in the second branch of the proton-proton chain is accompanied by the production of this element.  Standard theories of the big bang nucleosynthesis predict that this element appears as the universe cools below 100 KeV, though BBN significantly (*) overestimates the amount of this element. In stellar cores, this element is rapidly destroyed by reacting with a proton, creating a beryllium atom that then degenerates into two alpha particles. The spectral lines of this element are used to distinguish brown dwarves from low-mass stars. For 10 points, name this element, which is produced after hydrogen and helium and is the lightest element that counts for metallicity measurements of stars.
Plotting the baryon density on the x-axis against the ratio of one isotope of this element to hydrogen shows a minimum between 10 to the negative 10th and 10 to the negative 9th.
lithium