Editing Nucleosynthesis (section)

===Stellar nucleosynthesis===
{{Main|Stellar nucleosynthesis|Proton–proton chain|Triple-alpha process|CNO cycle|s-process|p-process|photodisintegration}}

Stellar nucleosynthesis is the nuclear process by which new nuclei are produced. It occurs in stars during [[stellar evolution]]. It is responsible for the galactic abundances of elements from carbon to iron. Stars are thermonuclear furnaces in which H and He are fused into heavier nuclei by increasingly high temperatures as the composition of the core evolves.<ref>{{cite book |last1=Clayton |first1=D. D. |year=1983 |title=Principles of Stellar Evolution and Nucleosynthesis |url=https://archive.org/details/principlesofstel0000clay/page/ |url-access=registration |at=[https://archive.org/details/principlesofstel0000clay/page/ Chapter 5] |edition=Reprint |publisher=[[University of Chicago Press]] |location=Chicago, IL|isbn=978-0-226-10952-7 }}</ref> Of particular importance is carbon because its formation from He is a bottleneck in the entire process. Carbon is produced by the [[triple-alpha process]] in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the s-process, in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel.<ref>{{cite journal |last1=Clayton |first1=D. D. |last2=Fowler |first2=W. A. |last3=Hull |first3=T. E. |last4=Zimmerman |first4=B. A. |title=Neutron Capture Chains in Heavy Element Synthesis |journal=[[Annals of Physics]] |date=1961 |volume=12 |issue=3 |pages=331–408 |doi=10.1016/0003-4916(61)90067-7|bibcode=1961AnPhy..12..331C }}</ref><ref name=ClaytonIsotopes7>{{cite book |last1=Clayton |first1=D. D. |year=1983 |title=Principles of Stellar Evolution and Nucleosynthesis |url=https://archive.org/details/principlesofstel0000clay/page/ |url-access=registration |at=[https://archive.org/details/principlesofstel0000clay/page/ Chapter 7] |edition=Reprint |publisher=[[University of Chicago Press]] |location=Chicago, IL|isbn=978-0-226-10952-7 }}</ref>

The products of stellar nucleosynthesis are generally dispersed into the interstellar gas through mass loss episodes and the stellar winds of low mass stars. The mass loss events can be witnessed today in the [[planetary nebula]]e phase of low-mass star evolution, and the explosive ending of stars, called [[supernova]]e, of those with more than eight times the mass of the Sun.

The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellar gas has become enriched with heavy elements as time passed. As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier. The detection of [[technetium]] in the atmosphere of a [[red giant]] star in 1952,<ref>{{cite journal |last1=Merrill |first1=S. P. W. |date=1952 |title=Spectroscopic Observations of Stars of Class |journal=[[The Astrophysical Journal]] |volume=116 |pages=21 |bibcode=1952ApJ...116...21M |doi=10.1086/145589}}</ref> by spectroscopy, provided the first evidence of nuclear activity within stars. Because technetium is radioactive, with a half-life much less than the age of the star, its abundance must reflect its recent creation within that star. Equally convincing evidence of the stellar origin of heavy elements is the large overabundances of specific stable elements found in stellar atmospheres of [[asymptotic giant branch]] stars. Observation of barium abundances some 20–50 times greater than found in unevolved stars is evidence of the operation of the s-process within such stars. Many modern proofs of stellar nucleosynthesis are provided by the [[isotopes|isotopic]] compositions of [[Cosmic dust#Stardust|stardust]], solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component of [[cosmic dust]] and is frequently called [[presolar grains]]. The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star's late-life mass-loss episodes.<ref name=Clayton2004>{{cite journal |last1=Clayton |first1=D. D. |last2=Nittler |first2=L. R. |title=Astrophysics with Presolar Stardust |journal=[[Annual Review of Astronomy and Astrophysics]] |date=2004 |volume=42 |issue=1 |pages=39–78 |bibcode=2004ARA&A..42...39C |doi=10.1146/annurev.astro.42.053102.134022}}</ref>