World Library  
Flag as Inappropriate
Email this Article

Alpha process

Article Id: WHEBN0000217607
Reproduction Date:

Title: Alpha process  
Author: World Heritage Encyclopedia
Language: English
Subject: Nucleosynthesis, Silicon-burning process, Oxygen-burning process, Triple-alpha process, Carbon-burning process
Collection: Nuclear Fusion, Nucleosynthesis, Stellar Physics
Publisher: World Heritage Encyclopedia

Alpha process

The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process.[1] While the triple-alpha process only requires helium once some carbon is present, these other reactions that consume helium are possible:

\mathrm{_6^{12}C} + \mathrm{_2^4He} \rightarrow \mathrm{_{8}^{16}O} + \gamma , Q = 7.16 МeV
\mathrm{_8^{16}O} + \mathrm{_2^4He} \rightarrow \mathrm{_{10}^{20}Ne} + \gamma , Q = 4.73 МeV
\mathrm{_{10}^{20}Ne} + \mathrm{_2^4He} \rightarrow \mathrm{_{12}^{24}Mg} + \gamma , Q = 9.31 МeV
\mathrm{_{12}^{24}Mg} + \mathrm{_2^4He} \rightarrow \mathrm{_{14}^{28}Si} + \gamma , Q = 9.98 МeV
\mathrm{_{14}^{28}Si} + \mathrm{_2^4He} \rightarrow \mathrm{_{16}^{32}S} + \gamma , Q = 6.95 МeV
\mathrm{_{16}^{32}S} + \mathrm{_2^4He} \rightarrow \mathrm{_{18}^{36}Ar} + \gamma
\mathrm{_{18}^{36}Ar} + \mathrm{_2^4He} \rightarrow \mathrm{_{20}^{40}Ca} + \gamma
\mathrm{_{20}^{40}Ca} + \mathrm{_2^4He} \rightarrow \mathrm{_{22}^{44}Ti} + \gamma
\mathrm{_{22}^{44}Ti} + \mathrm{_2^4He} \rightarrow \mathrm{_{24}^{48}Cr} + \gamma
\mathrm{_{24}^{48}Cr} + \mathrm{_2^4He} \rightarrow \mathrm{_{26}^{52}Fe} + \gamma
\mathrm{_{26}^{52}Fe} + \mathrm{_2^4He} \rightarrow \mathrm{_{28}^{56}Ni} + \gamma
\mathrm{_{28}^{56}Ni} + \mathrm{_2^4He} \rightarrow \mathrm{_{30}^{60}Zn} + \gamma

Q  is energy of the gamma ray.

Energy is produced in the isolated fusion reaction of nickel-56 with helium-4, but less than consumed to product of helium-4 by photodisintegration of heavier nuclei, causing alpha buildup of nickel-56 to be shut off due to the essential fact that nickel-56 has nucleon binding energy less zinc-60

All these reactions have a very low rate at the temperatures and densities in stars and therefore do not contribute significantly to a star's energy production; with elements heavier than neon (atomic number > 10), they occur even less easily due to the increasing Coulomb barrier.

Alpha process elements (or alpha elements) are so-called since their most abundant isotopes are integer multiples of four, the mass of the helium nucleus (the alpha particle). Stable alpha elements are: C, O, Ne, Mg, Si, S, Ar, Ca. They are synthesized by alpha capture prior to the silicon fusing process, a precursor to Type II supernovae. Silicon and calcium are purely alpha process elements. Magnesium can be burned by proton capture reactions. As for oxygen, some authors consider it an alpha element, while others do not. Oxygen is surely an alpha element in low-metallicity population II stars. It is produced in Type II supernovae and its enhancement is well correlated with an enhancement of other alpha process elements. Sometimes carbon and nitrogen are considered alpha process elements, since they are synthesized in nuclear alpha-capture reactions.

The abundance of alpha elements in stars is usually expressed in a logarithmic manner:

[\alpha/Fe] = \log_{10}{\left(\frac{N_{\alpha}}{N_{Fe}}\right)_{Star}} - \log_{10}{\left(\frac{N_{\alpha}}{N_{Fe}}\right)_{Sun}} ,

Here N_{\alpha} and N_{Fe} are the number of alpha elements and iron nuclei per unit volume. Theoretical galactic evolution models predict that early in the universe there were more alpha elements relative to iron. Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca and Ti) while Type Ia supernovae mainly produce elements of the iron peak (Ti, V, Cr, Mn, Fe, Co and Ni) but also alpha-elements.


  1. ^ Narlikar, Jayant V (1995). From Black Clouds to Black Holes. World Scientific.  

External links

  • The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.