A stellar source of lithium Free

Of all the elements of the periodic table, lithium has perhaps the most complicated and mysterious origins. It’s the only element that can be produced in large quantities in three ways: in Big Bang nucleosynthesis during the first three minutes of the universe’s existence; in nuclear reactions initiated by cosmic rays in the interstellar medium; and in stars, when helium-3 and helium-4 fuse into beryllium-7, which undergoes beta decay within tens of days to form ⁷Li. The stellar synthesis mechanism seems almost contradictory—the He fusion step requires high temperatures, but those same high temperatures destroy the nascent Li. Yet fully half of present-day Li is hard to account for without it. Now Akito Tajitsu (at the National Astronomical Observatory of Japan’s Subaru Telescope in Hawaii) and his colleagues report the first direct observational evidence of Li nucleosynthesis in a stellar system—specifically, a classical nova: a white dwarf whose outer layer is blown away in a thermonuclear explosion. Using the Subaru Telescope’s High Dispersion Spectrograph, the researchers monitored the absorption spectrum of the ejected gas during the weeks after the explosion. They found a group of spectral lines corresponding to ⁷Be. Because of that isotope’s short half-life, the ⁷Be must have formed in the nova explosion, and because it was ejected from the hot star at high speed, the ⁷Li it produced could remain safely stable. (A. Tajitsu et al., Nature 518, 381, 2015.)