Nucleogenesis (also known as nucleosynthesis) as a general phenomenon is a process usually associated with production of nuclides in the Big Bang or in stars, by nuclear reactions there. Some of these neutron reactions (such as the r-process and s-process) involve absorption by atomic nuclei of high-temperature (high energy) neutrons from the star. These processes produce most of the chemical elements in the universe heavier than zirconium (element 40), because nuclear fusion processes become increasingly inefficient and unlikely for elements heavier than this. By convention, such heavier elements produced in normal elemental abundance, are not referred to as "nucleogenic". Instead, this term is reserved for nuclides (isotopes) made on Earth from natural nuclear reactions.