Introduction

Abstract

Models of galactic chemical evolution study how the chemical elements have formed and dispersed in the Universe. During the Big Bang, only light elements, such as hydrogen (H), deuterium (D), helium (He), and a very tiny fraction of lithium (⁷Li) were formed, while all the other elements from carbon to uranium and beyond were formed inside the stars. The chemical elements and their isotopes are characterized by their mass number (A), namely, the sum of the protons and neutrons composing their nuclei. So, when we write ⁷Li, it means that A= 7 for Li. Elements with A = 5 and A = 8 do not exist because they would not be stable, elements with A = 9, 10, 11 are the isotopes of berillium (Be) and boron (B), which are formed, together with ⁶Li and some ⁷Li during spallation processes, which derive from the interaction between cosmic rays and interstellar atoms of carbon (C), nitrogen (N), and oxygen (O). All the elements with A ≥ 12 starting from ¹²C have been synthesized inside the stars and the sum of all of them in Astronomy is called metallicity and indicated with capital Z.