When I had finally given up the hope, this year astronomers have reached twice a goal which I have dreamed to see for about 25 years. They caught in the act lithium formation in stars for the first time, observing it in the spectra of the matter ejected in two different novae, exploded in 2013.
V1369 Cen (Nova Centauri 2013) from the NTT ESO telescope, image taken in July 2015, one year and half after the explosion. It is the brightest star in the centre of the picture. Credit: ESO
Lithium is formed during the primordial Big Bang Nucleosynthesis (BBN). When the fireball begins expanding, its temperature progressively lowers, reaching thresholds which modify its main components. The timing for building up deuterium (D), tritium (3H), the isotopes 3 and 4 of helium (3He and 4He) and the main lithium isotope (7Li) is confined between about 2 minutes and 15 minutes of age of the newborn Universe, when the temperature passes through the range from a billion to about 300 million degrees.
Notice that all the other elements are synthesized much later, most of them by nuclear reactions in the interior of stars, while those heavier than iron are formed in some stars by two other processes, “rapid” and/or “slow” neutron capture onto iron seed nuclei.
The most abundant final outcome of BBN is the very stable nucleus of helium, one 4He every 10 protons (hydrogen nuclei). An appealing result of BBN is that this abundance of helium can be derived from a back of the envelope computation, and depends on the basic proton and neutron properties, not on the details of the model. On the contrary, the precise abundances of D, 3He and 7Li depend on the barion density with which our Universe was born (one of those interesting —> Read More