It is now known that the elements observed in the Universe were created in either of two ways. Light elements namely deuterium, helium, and lithium were produced in the first few minutes of the Big Bang, while elements heavier than helium are thought to have their origins in the interiors of stars which formed much later in the history of the Universe.
CNO-I cycle The helium nucleus is released at the top-left step.
Hydrogen fusion nuclear fusion of four protons to form a helium-4 nucleus  is the dominant process that generates energy in the cores of main-sequence stars. It is also called "hydrogen burning", which should not be confused with the chemical combustion of hydrogen in an oxidizing atmosphere.
There are two predominant processes by which stellar hydrogen fusion occurs: Ninety percent of all stars, with the exception of white dwarfsare fusing hydrogen by these two processes. In the cores of lower-mass main-sequence stars such as the Sunthe dominant energy production process is the proton—proton chain reaction.
This creates a helium-4 nucleus through a sequence of chain reactions that begin with the fusion of two protons to Nucleosynthesis hydrogen helium a deuterium nucleus one proton plus one neutron along with an ejected positron and neutrino.
In higher-mass stars, the dominant energy production process is the CNO cyclewhich is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in the end produces a helium nucleus as with the proton-proton chain.
The difference in energy production of this cycle, compared to the proton—proton chain reaction, is accounted for by the energy lost through neutrino emission.
As a result, the core region becomes a convection zonewhich stirs the hydrogen fusion region and keeps it well mixed with the surrounding proton-rich region.
The type of hydrogen fusion process that dominates in a star is determined by the temperature dependency differences between the two reactions.
This temperature is achieved in the cores of main sequence stars with at least 1. As a main sequence star ages, the core temperature will rise, resulting in a steadily increasing contribution from its CNO cycle. Triple-alpha process and Alpha process Main 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 when a star leaves the red giant branch after accumulating sufficient helium in its core to ignite it.
In stars around the mass of the sun, this begins at the tip of the red giant branch with a helium flash from a degenerate helium core and the star moves to the horizontal branch where it burns helium in its core. More massive stars ignite helium in their cores without a flash and execute a blue loop before reaching the asymptotic giant branch.
Despite the name, stars on a blue loop from the red giant branch are typically not blue in color, but are rather yellow giants, possibly Cepheid variables. They fuse helium until the core is largely carbon and oxygen.
The most massive stars become supergiants when they leave the main sequence and quickly start helium fusion as they become red supergiants. After helium is exhausted in the core of a star, it will continue in a shell around the carbon-oxygen core. This can then form oxygen, neon, and heavier elements via the alpha process.
In this way, the alpha process preferentially produces elements with even numbers of protons by the capture of helium nuclei. Elements with odd numbers of protons are formed by other fusion pathways.
Reaction rate[ edit ] The reaction rate per volume between species A and B, having number densities nA,B is given by:Primordial nucleosynthesis is believed by most cosmologists to have taken place in the interval from roughly 10 seconds to 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as the isotope helium-4 (4 He), along with small amounts of the hydrogen isotope deuterium (2 H or D.
We are all made of stardust. It sounds like a line from a poem, but there is some solid science behind this statement too: almost every element on Earth was formed at the heart of a star.
Nucleosynthesis. The Big Bang model predicts that nucleosynthesis, the process by which the elements formed, began approximately seconds after the Big ashio-midori.com by the immense temperature and pressure, nuclear fusion reactions converted hydrogen into helium.
Which shows the correct order of events during the process of nucleosynthesis ashio-midori.comen nucleus formed,isotope of hydrogen tritum formed,helium nucleus formed/5(9).
The process of forming the hydrogen and helium and other trace constituents is often called "big bang nucleosynthesis".
Schramm's figures for relative abundances indicate that helium is about 25% by mass and hydrogen about 73% with all other elements constituting less than 2%. Carroll & Ostlie give 23 to 24% helium.
Processes. There are a number of astrophysical processes which are believed to be responsible for nucleosynthesis. The majority of these occur in within stars, and the chain of those nuclear fusion processes are known as hydrogen burning (via the proton-proton chain or the CNO cycle), helium burning, carbon burning, neon burning, oxygen burning and silicon burning.