Several processes were responsible for producing the current inventory of elements in the cosmos. Hydrogen, helium, and some lithium were created in the Big Bang, a massive explosion that is thought to have produced the universe. Elements heavier than hydrogen and helium, known as metals in astronomy, were produced in stars by processes collectively called stellar nucleosynthesis. The chemical elements other than hydrogen and helium in our solar system are the result of nucleosynthesis that occurred in stars that lived and died before the solar system formed. These processes involve fusion of light elements into heavy elements, sometimes at modest rates as stars evolve and sometimes at furious rates in stellar explosions. Significant amounts of a few rare elements, such as lithium, beryllium, and boron, were created via spallation reactions, in which collisions between highly energetic cosmic rays (typically protons or helium ions) and atoms break up the heavier nuclides into lighter fragments. And, of course, some nuclides have been produced by decay of radioactive nuclides. In this chapter, we will review these processes and discuss the evolution of the elemental abundances with time in the universe and the galaxy.
In the beginning
The cosmological model that best explains the origin of the universe is the Big Bang. According to this model, the universe began at a finite time in the past and at a discrete point in space, expanded from a hot dense initial state of very small size, and continues to expand to this day.