Webonce carbon is present, it acts as a catalyst for the fusion of hydrogen to helium through a series of reactions known as the cno cycle (for. Webthere are three major sources responsible for the synthesis of the elements: Comological nucleosynthesis in the big bang, nucleosynthesis during stellar evolution,. Webmain sequence stars accumulate helium in their cores as a result of hydrogen fusion, but the core does not become hot enough to initiate helium fusion. Helium fusion first begins. Webstars evolve as protostars by gravitational contraction of a molecular cloud until the increasing core temperature and density triggers the ignition of hydrogen. Webthe big bang theory is supported by a wealth of evidence: It explains the cosmic background radiation, the abundances of light elements and the hubble expansion. Webdespite their diminutive contribution to the mass of the universe, stars play a vital role in the evolution of the cosmos. They are the engines that produce all the chemical elements. Webfrom about one second to a few minutes cosmic time, when the temperature has fallen below 10 billion kelvin, the conditions are just right for protons and neutrons to combine. Webthe chronology of the universe describes the history and future of the universe according to big bang cosmology. Research published in 2015 estimates the earliest stages of the. In perhaps the simplest nucleosynthesis.
