Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Alpha process
open-in-new
<h2 id="alpha-process-elements">Alpha process elements</h2> <p>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 <a href="/facts/Alpha_particle/dzNMs9WP">alpha particle</a>). These isotopes are called <i><a href="/facts/Alpha_nuclide/LGETggTY">alpha nuclides</a></i>. </p> <ul><li>The stable alpha elements are: <a href="/facts/Carbon/ukrgQexo">C</a>, <a href="/facts/Oxygen/acyn6o8r">O</a>, <a href="/facts/Neon/5bF1FwGL">Ne</a>, <a href="/facts/Magnesium/XyIhd2FG">Mg</a>, <a href="/facts/Silicon/cgWx0hdr">Si</a>, and <a href="/facts/Sulfur/IK0amTfM">S</a>.</li> <li>The elements <a href="/facts/Argon/QX6NRH6d">Ar</a> and <a href="/facts/Calcium/hf252CC0">Ca</a> are <i>"<a href="/facts/Observationally_stable/vC0VVvqN">observationally stable</a>"</i>. They are synthesized by alpha capture prior to the <a href="/facts/Silicon_burning_process/qpYtHo8c">silicon fusing</a> stage, that leads to <a href="/facts/Type_II_supernova/623d9YYT">Type II supernovae</a>.</li> <li><a href="/facts/Silicon/cgWx0hdr">Si</a> and <a href="/facts/Calcium/hf252CC0">Ca</a> are purely alpha process elements.</li> <li><a href="/facts/Magnesium/XyIhd2FG">Mg</a> can be separately consumed by <a href="/facts/Proton_capture/eLQDChDd">proton capture</a> reactions.</li></ul> <p>The status of oxygen (<a href="/facts/Oxygen/acyn6o8r">O</a>) is contested – some authors<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> consider it an alpha element, while others do not. <a href="/facts/Oxygen/acyn6o8r">O</a> is surely an alpha element in low-<a href="/facts/Metallicity/nhhPqbYF">metallicity</a> <a href="/facts/Stellar_population/8ur1qDRK">Population II stars</a>: It is produced in <a href="/facts/Type_II_supernova/623d9YYT">Type II supernovae</a>, and its enhancement is well correlated with an enhancement of other alpha process elements. </p><p>Sometimes <a href="/facts/Carbon/ukrgQexo">C</a> and <a href="/facts/Nitrogen/Nf1QpGDz">N</a> are considered alpha process elements since, like <a href="/facts/Oxygen/acyn6o8r">O</a>, they are synthesized in nuclear alpha-capture reactions, but their status is ambiguous: Each of the three elements is produced (and consumed) by the <a href="/facts/CNO_cycle/ClM5eAGc">CNO cycle</a>, which can proceed at temperatures far lower than those where the alpha-ladder processes start producing significant amounts of alpha elements (including <a href="/facts/Carbon/ukrgQexo">C</a>, <a href="/facts/Nitrogen/Nf1QpGDz">N</a>, & <a href="/facts/Oxygen/acyn6o8r">O</a>). So just the presence of <a href="/facts/Carbon/ukrgQexo">C</a>, <a href="/facts/Nitrogen/Nf1QpGDz">N</a>, or <a href="/facts/Oxygen/acyn6o8r">O</a> in a star does not a clearly indicate that the alpha process is actually underway – hence reluctance of some astronomers to (unconditionally) call these three "alpha elements". </p> <h2 id="production-in-stars">Production in stars</h2> <p>The alpha process generally occurs in large quantities only if the star is sufficiently massive – more massive than about 10 <a href="/facts/Solar_mass/j6Auhku8">solar masses</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> These stars contract as they age, increasing core temperature and density to high enough levels to enable the alpha process. Requirements increase with atomic mass, especially in later stages – sometimes referred to as <a href="/facts/Silicon-burning_process/qpYtHo8c">silicon burning</a> – and thus most commonly occur in <a href="/facts/Supernova_nucleosynthesis/rbzAja5z">supernovae</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Type II supernovae mainly synthesize oxygen and the alpha-elements (<a href="/facts/Neon/5bF1FwGL">Ne</a>, <a href="/facts/Magnesium/XyIhd2FG">Mg</a>, <a href="/facts/Silicon/cgWx0hdr">Si</a>, <a href="/facts/Sulfur/IK0amTfM">S</a>, <a href="/facts/Argon/QX6NRH6d">Ar</a>, <a href="/facts/Calcium/hf252CC0">Ca</a>, and <a href="/facts/Titanium/53ArnDoy">Ti</a>) while <a href="/facts/Type_Ia_supernova/W4rEFuDK">Type Ia supernovae</a> mainly produce elements of the <a href="/facts/Iron_peak/GYAe3weY">iron peak</a> (<a href="/facts/Titanium/53ArnDoy">Ti</a>, <a href="/facts/Vanadium/UvKkKTSm">V</a>, <a href="/facts/Chromium/6sVk6Uu7">Cr</a>, <a href="/facts/Manganese/j9ELd1TL">Mn</a>, <a href="/facts/Iron/QK1JYy8E">Fe</a>, <a href="/facts/Cobalt/gqhScEw4">Co</a>, and <a href="/facts/Nickel/KwMCHYRP">Ni</a>).<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Sufficiently massive stars can synthesize elements up to and including the iron peak solely from the hydrogen and helium that initially comprises the star.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>Typically, the first stage of the alpha process (or alpha-capture) follows from the <a href="/facts/Triple-alpha_process/wV6lacSF">helium-burning</a> stage of the star once helium becomes depleted; at this point, free 6 12 C {\displaystyle {}_{6}^{12}{\textrm {C}}} capture helium to produce 8 16 O {\displaystyle {}_{8}^{16}{\textrm {O}}} .<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> This process continues after the core finishes the helium burning phase as a shell around the core will continue burning helium and <a href="/facts/Convection/E8vsCPhu">convecting</a> into the core.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The second stage (<a href="/facts/Neon-burning_process/gFaHhLDK">neon burning</a>) starts as helium is freed by the photodisintegration of one 10 20 Ne {\displaystyle {}_{10}^{20}{\textrm {Ne}}} atom, allowing another to continue up the alpha ladder. Silicon burning is then later initiated through the photodisintegration of 14 28 Si {\displaystyle {}_{14}^{28}{\textrm {Si}}} in a similar fashion; after this point, the 28 56 N i {\displaystyle \,{}_{28}^{56}\mathrm {Ni} \,} peak discussed previously is reached. The <a href="/facts/Supernova_remnant/tYboe2n4">supernova shock wave</a> produced by stellar collapse provides ideal conditions for these processes to briefly occur. </p><p>During this terminal heating involving photodisintegration and rearrangement, nuclear particles are converted to their most stable forms during the supernova and subsequent ejection through, in part, alpha processes. Starting at 22 44 Ti {\displaystyle {}_{22}^{44}{\textrm {Ti}}} and above, all the product elements are radioactive and will therefore decay into a more stable isotope; for instance, 28 56 N i {\displaystyle \,{}_{28}^{56}\mathrm {Ni} \,} is formed and decays into 26 56 Fe {\displaystyle {}_{26}^{56}{\textrm {Fe}}} .<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="special-notation-for-relative-abundance">Special notation for relative abundance</h2> <p>The abundance of total alpha elements in stars is usually expressed in terms of <a href="/facts/Logarithm/QeXaV97q">logarithms</a>, with astronomers customarily using a square bracket notation: </p> [ α Fe ] ≡ log 10 ( N E α N Fe ) S t a r − log 10 ( N E α N Fe ) S u n , {\displaystyle \left[{\frac {\alpha }{\,{\ce {Fe}}\,}}\right]~\equiv ~\log _{10}{\left(\,{\frac {N_{\mathrm {E} \alpha }}{\,N_{{\ce {Fe}}}\,}}\,\right)_{\mathsf {Star}}}-\log _{10}{\left({\frac {N_{\mathrm {E} \alpha }}{\,N_{{\ce {Fe}}}\,}}\,\right)_{\mathsf {Sun}}}~,} <p>where N E α {\displaystyle \,N_{\mathrm {E} \alpha }\,} is the number of alpha elements per unit volume, and N Fe {\displaystyle \,N_{{\ce {Fe}}}\,} is the number of iron nuclei per unit volume. It is for the purpose of calculating the number N E α {\displaystyle \,N_{\mathrm {E} \alpha }\,} that which elements are to be considered "alpha elements" becomes contentious. Theoretical <a href="/facts/Galaxy_formation_and_evolution/uAPtXxr4">galactic evolution</a> models predict that early in the universe there were more alpha elements relative to iron. </p> <h2 id="further-reading">Further reading</h2> <ul><li>Mendel, J. Trevor; Proctor, Robert N.; Forbes, Duncan A. (21 August 2007) [31 May 2007]. <a href="https://doi.org/10.1111%2Fj.1365-2966.2007.12041.x">"The age, metallicity and α-element abundance of galactic globular clusters, from single stellar population models"</a>. <i>Monthly Notices of the Royal Astronomical Society</i>. 379 (4) (published 26 July 2007): 1618–1636. <a href="/facts/ArXiv_(identifier)/H6EtgnBe">arXiv</a>:<a href="https://arxiv.org/abs/0705.4511v2">0705.4511v2</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1111%2Fj.1365-2966.2007.12041.x">10.1111/j.1365-2966.2007.12041.x</a>.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Narlikar, Jayant V. (1995). From Black Clouds to Black Holes. World Scientific. p. 94. ISBN 978-9810220334. <a href="978-9810220334" target="_blank">978-9810220334</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Fewell, M.P. (1995-07-01). "The atomic nuclide with the highest mean binding energy". American Journal of Physics. 63 (7): 653–658. Bibcode:1995AmJPh..63..653F. doi:10.1119/1.17828. ISSN 0002-9505. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Nave, Carl R. (c. 2017) [c. 2001]. "The most tightly bound nuclei". Physics and Astronomy. hyperphysics.phy-astr.gsu.edu. HyperPhysics pages. Georgia State University. Retrieved 2019-02-21. <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin2.html#c1" target="_blank">http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin2.html#c1</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Fewell, M.P. (1995-07-01). "The atomic nuclide with the highest mean binding energy". American Journal of Physics. 63 (7): 653–658. Bibcode:1995AmJPh..63..653F. doi:10.1119/1.17828. ISSN 0002-9505. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Burbidge, E. Margaret; Burbidge, G.R.; Fowler, William A.; Hoyle, F. (1957-10-01). "Synthesis of the elements in stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="/wiki/Margaret_Burbidge" target="_blank">/wiki/Margaret_Burbidge</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Mo, Houjun (2010). Galaxy formation and evolution. Frank Van den Bosch, S. White. Cambridge: Cambridge University Press. p. 460. ISBN 978-0-521-85793-2. OCLC 460059772. <a href="978-0-521-85793-2" target="_blank">978-0-521-85793-2</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Truran, J.W.; Heger, A. (2003), "Origin of the Elements", Treatise on Geochemistry, Elsevier, pp. 1–15, doi:10.1016/b0-08-043751-6/01059-8, ISBN 978-0-08-043751-4, retrieved 2023-02-17 <a href="978-0-08-043751-4" target="_blank">978-0-08-043751-4</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Truran, J. W.; Cowan, J. J.; Cameron, A. G. W. (1978-06-01). "The helium-driven r-process in supernovae". The Astrophysical Journal. 222: L63 – L67. Bibcode:1978ApJ...222L..63T. doi:10.1086/182693. ISSN 0004-637X. <a href="https://doi.org/10.1086%2F182693" target="_blank">https://doi.org/10.1086%2F182693</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Truran, J.W.; Heger, A. (2003), "Origin of the Elements", Treatise on Geochemistry, Elsevier, pp. 1–15, doi:10.1016/b0-08-043751-6/01059-8, ISBN 978-0-08-043751-4, retrieved 2023-02-17 <a href="978-0-08-043751-4" target="_blank">978-0-08-043751-4</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Mo, Houjun (2010). Galaxy formation and evolution. Frank Van den Bosch, S. White. Cambridge: Cambridge University Press. p. 460. ISBN 978-0-521-85793-2. OCLC 460059772. <a href="978-0-521-85793-2" target="_blank">978-0-521-85793-2</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Clayton, Donald D. (1983). Principles of stellar evolution and nucleosynthesis : with a new preface. Chicago: University of Chicago Press. pp. 430–435. ISBN 0-226-10953-4. OCLC 9646641. <a href="0-226-10953-4" target="_blank">0-226-10953-4</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Truran, J.W.; Heger, A. (2003), "Origin of the Elements", Treatise on Geochemistry, Elsevier, pp. 1–15, doi:10.1016/b0-08-043751-6/01059-8, ISBN 978-0-08-043751-4, retrieved 2023-02-17 <a href="978-0-08-043751-4" target="_blank">978-0-08-043751-4</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Clayton, Donald D. (1983). Principles of stellar evolution and nucleosynthesis : with a new preface. Chicago: University of Chicago Press. pp. 430–435. ISBN 0-226-10953-4. OCLC 9646641. <a href="0-226-10953-4" target="_blank">0-226-10953-4</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> </ol>