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Stellar Nucleosynthesis by Tarun P. Roshan,

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Stellar Nucleosynthesis by Tarun P. Roshan,

  1. 1. Nucleosynthesis :An important nuclear Astrophysics phenomenon: Stellar Nucleosynthesis By Tarun P. Roshan IISER MOHALI, India
  2. 2. Cosmic Abundances: The relative proportion of the elements in the Universe is “COSMIC ABUNDANCE” Almost all natural elements are found on Earth and on other bodies of The Solar system. Scientists recognize 93 natural elements. Same 93 elements are also found everywhere in the Universe.
  3. 3. Hydrogen and Helium in the known Universe Hydrogen is the most abundant element in the known Universe; helium is second Heavier elements constitute less than 1% of the total matter in the known Universe. Cosmological observations suggest that only 4.6% of the universe comprises the visible baryonic matter which constitutes stars, planets and living beings. The rest is made up of dark energy (72%) and dark matter (23%)
  4. 4. All the Elements except, Hydrogen and Helium, have been synthesized in the Stars during their evolution. The NUCLEOSYNTHESIS process is related with the evolution of Stars. Elements are formed inside the Stars since birth and death of the stars is a continues process.
  5. 5. Universe started with aUniverse started with a BIG BANG.BIG BANG. After this time, the Universe was in an extremely hotAfter this time, the Universe was in an extremely hot and dense state and began expanding rapidly.and dense state and began expanding rapidly.
  6. 6. The first seconds after the BIG BANG  it was very hot   then the Universe started  to cool down.
  7. 7. Right after the BIG BANG, the p+ , the n0 and the e- were flying around without control.      When the Universe started to cool down the quarks       started making primitive elements.
  8. 8. In 1938, Hans Albrecht Bethe and Weizsacker analyzed two process in stars, p-p chain and CNO cycle and believed to be the source of energy in Stars. They showed the possibility to converting hydrogen into helium through nuclear reaction. These reactions take place at high temperature and high densities, such conditions are readily available in the interior of Stars.
  9. 9. The process of creation new nuclear species by fusion reaction in stars is Nucleosynthesis. Major Processes by which elements are synthesized: 1.) Hydrogen burningHydrogen burning -a first stage of NUCLEOSYNTHESIS.NUCLEOSYNTHESIS. The p-p chain reaction, which causes fusion of four hydrogenThe p-p chain reaction, which causes fusion of four hydrogen nuclei to form helium.nuclei to form helium. Sun and other similar stars generate their energy by this reaction.Sun and other similar stars generate their energy by this reaction.
  10. 10. Hydrogen Burning: The first stage of Nucleosynthesis is the fusion of Hydrogen nuclei and consequent formation of Helium, called the p-p chain. Sun and other similar stars generate their energy by this process.
  11. 11. CNO cycle Helium as its end product. It starts with carbon and acts as a catalyst for the reaction. It produces energy in main sequence stars. The main part of the cycle involves C and N, while the ON cycle usually contributes little energy.
  12. 12. Helium Burning: the triple-alpha reaction. Simplest reaction in a helium gas should be the fusion of two helium nuclei. There is no stable configuration with A=8. For example the beryllium isotope 8 Be has a lifetime of only 2.6×10-16 s But a third helium nucleus can be added to 8 Be before decay, forming 12 C by the “triple-alpha” reaction
  13. 13. Thus helium burning proceeds in a two-stage reaction, and energy released is Q3α = [ 3ΔM( 4 He) − ΔM(12 C)]c 2 = 7.275MeV In terms of energy generated per unit mass 5.8 ×1013 JKg-1≡ (I.e. 1/10 of energy generated by H-burning).
  14. 14. Carbon and oxygen burning: Fusion of two Carbon nuclei requires temperature above 5×108 K , and Oxygen nuclei requires temperature above 109 K. . Once carbon is formed by triple-α reaction in the core, formation of heavier nuclei becomes possible. .
  15. 15. Carbon and Oxygen burning are very similar, in both cases a compound is produced at an excited energy level. These reactions produce p, n, α-particles, which are immediately captured by heavy nuclei, thus many isotopes created by secondary reactions. 12 C +12 C→24 Mg + γ , 16 O+16 O → 32 Si + γ →23 Mg + n → 31 S + n →23 Na + p →31 P + p →20 Ne + α → 28 Si + α →16 O + 2α →24 Mg + 2α
  16. 16. Neon Burning: At temperature of about 3 x 108 K , We obtain nuclei of Neon 16 8 O + 4 2 He → 20 10 Ne + gamma ( before the onset of carbon burning) This reaction can be reversed in presence of gamma ray photon which split up the nuclei, this process is called Photo-disintegration. 20 10 Ne + gamma →16 8 o + α-particles, At temperature 1.5 x 109 , these alpha particles can react with nuclei of Neon ( that have not undergone photo-disintegration) to form Magnesium. This process is called as Neon Burning. At temperature of about 2 x 109 K the Oxygen nuclei start to react to form silicon by Oxygen burning.
  17. 17. Silicon Burning : Photo-disintegration-rearrangement After oxygen burning, the core contracts yet and temperature rises about 3 x 109 K, the photo-disintegration of silicon starts: 28 Si + gamma → 24 Mg + α-particles, these alpha particles rapidly undergo fusion reactions with silicon and with subsequent products of fusion, 28 Si + α → 32 S + gamma, 32 S + α → 36 Ar + gamma, 36 Ar + α → 40 Ca + gamma, This reaction will proceeds as far as producing elements with atomic masses up to A-56, such as Iron , chromium, cobalt and nickel.
  18. 18. Every time a particular type of nuclear fuel is used up completely,Every time a particular type of nuclear fuel is used up completely, Core contraction due to gravitation takes place. As aCore contraction due to gravitation takes place. As a consequence, the temperature is raised of the core, thusconsequence, the temperature is raised of the core, thus another reaction becomes possible.another reaction becomes possible. This cycle continues till all the nuclei in the core have become iron nuclei. This means that with increasing mass number, the nuclei are more tightly bound and are more stable. Iron cannot combine with other nuclei to to produce heavier nuclei. Then, How did the elements heavier than iron form  ?
  19. 19. Every time a particular type of nuclear fuel is used up completely,Every time a particular type of nuclear fuel is used up completely, Core contraction due to gravitation takes place. As aCore contraction due to gravitation takes place. As a consequence, the temperature is raised of the core, thusconsequence, the temperature is raised of the core, thus another reaction becomes possible.another reaction becomes possible. This cycle continues till all the nuclei in the core have become iron nuclei. This means that with increasing mass number, the nuclei are more tightly bound and are more stable. Iron cannot combine with other nuclei to to produce heavier nuclei. Then, How did the elements heavier than iron form  ?
  20. 20. s- and r-processes The elements heavier than iron probably synthesized by the absorption of one neutron at a time. S-process : The process of absorption of one neutron at a time is slow process. In this way, all the elements up to Bi209 are formed. This process stops at Bi209. The elements heavier than Bi209 are unstable and emit beta- particles before they can absorb a neutron. R-process: If neutrons become available in large number, the nuclei can absorb neutrons rapidly. Elements right up to uranium are synthesized in stars by these processes.
  21. 21. What happens after all the nuclear reactions have stopped and the stellar core consist of iron only ? Iron has highest binding energy per nucleon. Thus, the core is forced to contract. The gravitational energy heats the core resulting in the disintegration of iron into nuclei of helium, even the helium cannot remain intact as nuclei, they break up into protons and neutrons.
  22. 22. Energy transfers from the core to the envelope, very high temperature is produced. Finally, the envelope explodes. The explosion is called Supernova. The elements built inside the star are thrown into the interstellar medium. The new stars born from this enriched medium contain a small proportion of heavy elements, these stars are called second generation stars. Well, the envelope explodes but what happens to the core ?
  23. 23. The core keeps collapsing...................... If the initial mass of the star is 12MS to 3MS , the matter in such stars is mostly neutrons. At the extremely high density in the core, the neutrons become degenerate. The pressure exerted by the degenerate neutrons is sufficiently high to halt the collapse . The core stabilizes in the form of a Neutron star If the initial mass of the exploding star is close to 15MS or more, the core is left behind a mass more than 3MS . The core of this mass cannot attain equilibrium. It keeps contracting, its gravitational field becomes so strong that even light cannot escape it. It becomes a Black Hole
  24. 24.                  thank you !

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