Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons). The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis, which formed the hydrogen and helium content of the first stars, and is responsible for the general hydrogen/helium ratio of the universe today. In space, cosmic ray spallation is also a significant source of specific nuclei (9Be and 10,11B) that are not created by stellar nucleosynthesis.
With the formation of stars, heavier nuclei were created from hydrogen and helium by stellar nucleosynthesis, a process that continues today. Some of these elements, particularly those lighter than iron, are thought to be delivered to the interstellar medium in the last stages of evolution of dying low mass stars, in the non-explosive ejection of the outer envelope gases of plantetary nebulae before these stars continue to form white dwarfs.
Supernova nucleosynthesis, nuclear reactions within exploding stars, is responsible for the abundant elements between magnesium (A=24) and nickel (A=60). Supernova nucleosynthesis is also thought to be responsible for the creation of elements heavier than iron and nickel (Z > 26-28), in the last few seconds of the explosion of a supernova, such as a type II supernova event. These elements must absorb energy as they are created, and do so from energy available in the supernova explosion. Some of the elements are created from the absorption of multiple neutrons in a space of a very short time (a few seconds) during an explosion. The elements formed in supernovas include formation of the heaviest elements known in the solar system, such as the long-lived primordial element radionuclides uranium and thorium. The nuclear fission of such elements releases the energy they absorbed during their creation.
In addition to these major processes responsible for the (growing) natural abundances of elements in the galaxy, a few minor natural processes continue to produce very small numbers of new nuclides on Earth. These nuclides are unimportant for the natural abundances, but may account for the presence of specific new nuclei on Earth. These nuclides are naturally produced via the decay of long-lived primordial radionuclides such as uranium and thorium (radiogenesis), from natural nuclear reactions caused by cosmic ray bombardment of elements on Earth (]s), and from other natural nuclear reactions powered by particles from radioactive decay (producing nucleogenic nuclides).
Read more about Nucleosynthesis: Timeline, History of Nucleosynthesis Theory, Processes, Empirical Evidence, Minor Mechanisms and Processes