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The most important reactions in stellar nucleosynthesis:
When the core temperature exceeds 100MK, helium nuclei begin to fuse to form carbon and oxygen. At this point non-relativistic degeneracy sets in, and the active life of the star has reached an end. The outer envelope has been shed out, and what remains is a hot, inert stellar core, called a white dwarf. It is very small and dense (the mass must be lower than 1.4 times the solar mass), and the size is very similar to the size of the Earth. All the results of stellar evolution become forever trapped within the white dwarf that begins to cool down to eventually become a dark cinder in the sky. For higher mass stars, however, the evolution does not end up there, but it continues to subsequent stages. There, at yet deeper levels, heavier elements are synthesized by the fusion of helium nuclei up to iron-56. Elements having mass numbers less than 56 and that are not multiples of 4 are produced in side reactions with neutrons. Moving in toward the core of the star helium is converted into carbon by the triple alpha process at 10 8 K. At a larger depth, the temperature increases to the point where carbon atoms will undergo fusion to produce neon at temperatures in the range of 10 9 K. As the depth (and temperature) of the star continues to increases neon will go on to form oxygen. Oxygen will fuse to form silicon, and silicon in turn will go on to form nickel. At this point the star is classified as a red giant and has been undergoing stellar evolution. Silicon will begin to burn at 4 x 10 9 K forming iron which cannot undergo any further stellar nucleosynthesis because of its high binding energies. A few elements having masses larger than 5 6Fe are formed through the equilibrium process. Eventually the fuel will be exhausted at which point energy production ceases, gravity causes the core to collapse, and the star undergoes a massive explosion, or type II supernova. The elements synthesized just prior to a supernova explosion would include: hydrogen, helium, carbon, oxygen, neon, magnesium, silicon, sulfur, chlorine, argon, potassium, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, and nickel. (14, 15, 16, 17, 18, 19).
Utilizing the Cornell Method of note taking and cooperative learning strategies, such as brainstorming, numbered heads together, reciprocal teaching, and team teaching, the first few days of the curriculum unit will focus on the students researching the Big Bang theory, stellar evolution, Big Bang, stellar, and supernova nucleosynthesis. As an assessment project, each group of students will be given a different list of astrophysicists and mathematicians from whom they could choose to write a report. Additionally, students will be able to create timelines involving the Big Bang and the expansion of the Universe, as well as creating posters illustrating different concepts of Big Bang, stellar, or supernova nucleosynthesis.
Stellar nucleosynthesis is the process by which the ..
His life held many contributions to physics including work on the first atomic bomb, work on developing the hydrogen bomb, the crystal field theory, a Nobel Prize in Physics in 1967 (for his work on stellar nucleosynthesis), and work on nuclear reactions.
The main activity for this unit will be for each cooperative learning group to prepare and present a fifteen minute power point presentation on a different aspect of the origin of the elements. After briefly presenting the basic foundations of the Big Bang theory and nucleosynthesis, each group will be assigned one of the following topics for their presentation: history of the Big Bang theory, stellar evolution, Big Bang nucleosynthesis, stellar nucleosynthesis, supernova nucleosynthesis, nuclear fusion reactions, and neutron capture reactions. The students will be escorted to the high school library where they will research their topics for two days using both written and electronic resources. The following two days each group will prepare their power point presentation. Presentation and discussion of their work will take place over the last few days of this unit.
Principles of Stellar Evolution and Nucleosynthesis.
This unit has been designed in such a way that the students should be able to: (1) explain the three different processes by which the elements are synthesized, (2) list the elements synthesized during Big Bang nucleosynthesis, (3) explain why elements with mass numbers greater than iron-56 do not undergo nuclear fusion in stars, (4) explain the differences between the s-process and r-process, and (5) explain how spectroscopy is used to identify elements. These behavioral objectives are in alignment with the Pennsylvania Academic Standards for Science and Technology as well as the School District of Philadelphia's core curriculum for chemistry.
For this activity, the behavioral objectives are: (1) the students will be able to understand the primary type of nucleosynthetic process that takes place in stars, and (2) the students will be able to write balanced equations for nuclear fusion reactions up to oxygen. This is a card game in which students are dealt seven cards from a deck containing the symbols, in relative proportions, of various isotopes, electrons, neutrons, positrons, and energy. The object is for the student to use the cards in their hand and/or chosen from the deck to form a nuclear fusion reaction. Scores are based on the mass number of each element that is formed. One class period will be allotted for completion and discussion of this activity.
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Stellar nucleosynthesis is the process by which the natural ..
Stellar nucleosynthesis occurs in layers or shells depending on the temperature within the star. At the surface, the temperature is never high enough to undergo any nuclear processing. Since the temperature increases with depth, a region will be reached where it is about 10 7 K, where hydrogen undergoes fusion to form helium. At this point, there is a fundamental difference between low mass stars (stars with masses lower than about 2-3 times the mass of the Sun), and higher mass stars. All stars begin evolving towards the Giant branch when hydrogen is exhausted in the stellar core. As the star evolves in this stage, there is some mass loss from the surface that is expanding, as the core is shrinking.
Principles of stellar Evolution and Nucleosynthesis.
Stellar nucleosynthesis of the elements up to iron has involved exothermic nuclear fusion reactions. During supernova nucleosynthesis, the heavier elements are created through endothermic reactions involving primarily neutron capture and to a lesser degree proton capture. When a star undergoes a supernova explosion, a high concentration of neutrons, called the neutron flux, will be emitted. The entire process of neutron capture during supernova nucleosynthesis is called the r-process and occurs at extremely high temperatures in a matter of seconds. The elements synthesized during this process are formed by a rapid absorption of neutrons producing the neutron rich isotopes of the heavier elements up to mass number 254. Highly unstable isotopes are produced during this process since the rate of neutron capture is greater than that of b-decay. These unstable radioactive nuclides will eventually decay into stable isotopes. The proton capture p-process, is believed to be responsible for the synthesis of approximately thirty to thirty-six proton rich elements which are heavier than iron. These elements cannot be synthesized by either the s-process or r-process. Because of the problems associated with overcoming very high Coulomb barrier forces, the p-process is believed to be a relatively minor process that requires extremely high temperatures (20, 21, 22, 23).
Principles of stellar Evolution and Nucleosynthesis
Hans Bethe in 1939 considered two methods by which energy was produced in stars. These two methods were the proton-proton (PP) chain and the carbon-nitrogen-oxygen (CNO) cycle. The proton-proton chain actually involves three different sets of nuclear reactions and requires temperatures in excess of 10 MK. The PPI process takes place at temperatures between 10 MK to 14 MK. The temperature requirements for PPII are between 14 MK and 23 MK. For temperatures above 23 MK, PPIII reactions occur. The elements produced by way of the PP chain include 3He, 4He, 7Be, 7Li, and 8B. Fred Hoyle later proposed a mechanism by which nuclear fusion reactions would be able to synthesize the elements from carbon to iron in stars. It was the 1957 review article by Burbridge, Burbridge, Fowler, and Hoyle that laid the framework for stellar nucleosynthesis. In that paper they outlined eight processes by which stellar nucleosynthesis could take place from hydrogen. Without going into the details, they included: converting hydrogen to helium, burning helium to carbon, oxygen, and neon, the capture of alpha particles, the equilibrium e-process, the slow s-process of neutron capture, the rapid r-process of neutron capture, the proton capture p-process, and an unknown x-process. Since then, the most important stellar nucleosynthetic processes appear to be: 1. hydrogen burning which involves the proton-proton chain and the CNO cycle, 2. helium burning involving both the alpha and triple alpha processes, and 3. the burning of heavier elements including carbon, neon, oxygen, and silicon.
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