The idea that stars fuse together the atoms of light elements was first proposed in the 1920's, by Einstein's strong supporter Arthur Eddington.
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).
A. Apply concepts about the structure and properties of matter.
Effective classroom management is due, in large part, to the structuring of the lesson. Structuring not only involves the arrangement of students within groups, but it also includes the manner in which individual lessons are designed and presented. These structures, designs, or activities are meant to improve such areas as team building, information sharing, thinking skills, communication skills, and content mastery. A brief list of classroom structures and lesson designs include: brainstorming, jigsaw, numbered heads together, rally table, round robin, roundtable, student teams achievement division (STAD), team projects and think pair share. A detailed review of each activity is given in (26).
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.
History of nucleosynthesis theory.
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.