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1By Mr. Kriengkrai Wongsaroj Nuclear Fusion &Stellar EvolutionBy Mr. Kriengkrai Wongsaroj
2Stage 1. An Interstellar Cloud Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies.The first stage in the star-formation process is a dense interstellar cloud.Typical temperatures are about 10 K throughout .The clouds contain thousands of time the mass of the sun,mainly in the form of cold atomic and molecular gas .
3Stage 2 and 3 - A Contracting Cloud Fragment The cloud breaks up into smaller, denser areas which may again break intostill smaller areas - the outcome being a cluster of protostars. This certainly agrees with the observation that star clusters are common, so stars are often found in groups known as clusters which appear to have formed at around the same time .
4Stage 4. ProtostarA Protostar is the name given to a stage in the development of a star and it is a period after clouds of hydrogen, helium and dust begin to contract and before the star reaches the main sequence.As the protostar evolves, it shrinks, its density increases, and its temperature rises,where its center seethes at about 1,000,000 Kbut the temperature is still short of the 107 K needed to ignite the proton–proton nuclear reactions that fuse hydrogen into helium.
6Stage 5. Protostellar Evolution By stage 5, the protostar has shrunk to about 10 times the size of the Sun, its surface temperature is about 4000 K, and its luminosity has fallen to about 10 times the solar value. The central temperature has risen to about 5,000,000 K, but the protons still do not have enough thermal energy for nuclear fusion to begin.
7Stage 6 and 7 - A Newborn Star Some 10 million years after its first appearance, the protostar finally becomes a true star. By stage 6, when our roughly one-solar-mass object has shrunk to a radius of about 1,000,000 km, the contraction has raised the central temperature to 10,000,000 K—enough to ignite nuclear burning. Protons begin fusing into helium nuclei in the core, and a star is born. The star finally reaches the main sequence just about where our Sun now resides.
8Nuclear FusionIf light nuclei are forced together, they will fuse with a yield of energy because the mass of the combination will be less than the sum of the masses of the individual nuclei and that decrease in mass comes off in the form of energy according to the Einstein relationshipThe famous Einstein relationship for energyE : Energym : mass of particleC : The velocity of light
10Nuclear Fusion Step 1. Deuterium formation The first step involves the fusion of two hydrogen nuclei 1H (protons) into deuterium 2H, releasing a positron as one proton changes into a neutron, and a neutrino.This step carrying energies up to 0.42 MeV
11Nuclear Fusion Step 2. Deuterium-proton fusion After this the deuterium produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3HeThis step carrying energies up to 5.49 MeV
12Nuclear Fusion Step 3. Helium-3 fusion helium-4 comes from fusing two of the helium-3 nuclei producedThis step carrying energies up to MeV
13Nuclear FusionIn the core of the Sun, at temperatures of million Kelvin The most promising of the hydrogen fusion reactions which make up the deuterium cycle is the fusion of deuterium and tritium. The reaction yields 17.6 MeV of energy to keep the Sun burning - and to sustain life on Earth.
15Stage 8 and 9 - Sub Giant and Red Giant Branches Hydrogen( yellow ) and helium ( orange ) abundance are shown.Only a few percent of the mass has been converted from hydrogen to helium
16A star’s core convertsMore and more of itshydrogen to heliumThe change speeds up as the nuclear burning rate increases with time
17Red Giant BranchesThe shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying layers, causing the hydrogen there to fuse even more rapidly than before. This hydrogen-shell-burning stage, in which hydrogen is burning at a furious rate in a relatively thin layer surrounding the nonburning inner core of helium. The pressure exerted by this enhanced hydrogen burning causes the star’s nonburning outer layers to increase in radius. Even while the core is shrinking and heating up, the overlying layers are expanding and cooling. The star, aged and unbalanced, is on its way to becoming a red giant.
18Stage 10. Helium Fusion A star’s temperature rise sharply called the “helium flash”, the helium burns ferociously like an uncontrolled bomb for a few hours.Eventually ,the star structure catches up . The core expand , its density drops , being to burn helium into carbon at temperature above 108 K
19Helium Fusion He4 + He4 → Be8 Be8 + He4 C12 + γ The conversion of helium-4 into carbon-12 is therefore accomplished through the following two reactions:He4 + He4→Be8Be8 + He4C12 + γ
20Stage 11. Back to the Giant Branch Nuclear reactions in stars proceed at rates that increase very rapidly with temperature . As helium fuses to carbon, a new carbon-rich inner core begins to form and phenomena similar to the earlier buildup of helium occur.The nonburning carbon core shrinks in size—even as its mass increases due to helium fusion—and heats up as gravity pulls it inward, causing the hydrogen- and helium-burning rates in the overlying layers to increase. The star now contains a contracting carbon-ash inner core surrounded by a helium-burning shell, which is in turn surrounded by a hydrogen-burning shell.
21Giant BranchThe burning rates in the shells surrounding the inner core are much fiercer during the star’s second trip into the red giant region, and the radius and luminosity increase .Its carbon core continues to shrink, driving the hydrogen- and helium-burning shells to higher and higher temperatures and luminosities.
22The Death of a Low-Mass Star Stage 12 A planetary NebulaFor solar-mass stars, The temperature never reaches the 600 million K needed for new nuclear reactions to occur. Its carbon core is, for all practical purposes, dead, while the outer hydrogen and helium burning shells consume fuel at a furious and increasing rate.The former red giant now consists of two distinct pieces: the exposed core, very hot and still very luminous, surrounded by a cloud of dust and cool gas—the escaping envelope—expanding away at a typical speed of a few tens of kilometers per second.
23Stage 13 A White DwarfFormerly concealed by the atmosphere of the red giant star, the carbon core becomes visible as a white dwarf as the envelope recedes. The core is very small, about the size of Earth, with a mass about half that of the Sun. Shining only by stored heat, not by nuclear reactions, this small star has a white-hot surface when it first becomes visible, although it appears dim because of its small size.
24Table2 Summarizes the key stages through which our solar-mass star evolves
25The Death of a High-Mass Star Heavy Element FusionThe 8-solar-mass star, however, can fuse not just hydrogen, helium, and carbon but also oxygen, neon, magnesium, and even heavier elements as its inner core continues to contract and its central temperature continues to rise.Evolution proceeds so rapidly in the 8-solar-mass star that it doesn’t even reach the red giant region before helium fusion begins. The star achieves a central temperature of while still quite close to the main sequence. As each element is burned to depletion at the center, the core contracts, heats up, and fusion starts again. A new inner core forms, contracts again, heats again, and so on.
26Secondary Helium Fusion Processes Heavy Element FusionFor temperatures that enable the triple-alpha process to proceed, other nuclear reactions are possible involving helium that create elements with atomic masses that are multiples of 4. These processes are as follows:C12 + He4→O16 + γO16 + He4Ne20 + γNe16 + He4Mg24 + γSecondary Helium Fusion ProcessesThe CNO hydrogen fusion process converts carbon-12 and the oxygen-16 into four other isotopes as hydrogen is converted into helium-4. These isotopes are carbon-13, nitrogen-14, nitrogen-15, and oxygen-15.
28C13 + He4 → O16 + n N14 + He4 → F18 + γ F18 O18 + e+ + νe O18 + He4 → The destruction of carbon-13 proceeds through the following reaction with helium-4:C13 + He4→O16 + nThe destruction of nitrogen-14 through the absorption of helium-4 creates the unstable nucleus fluorine-18, which decays to oxygen-18.N14 + He4→F18 + γF18O18 + e+ + νeThe oxygen-18 created from nitrogen-14 can be destroyed by absorbing a helium-4 nucleus. This interaction has two branches, one that creates neon-21, and a second that creates neon-22. The first of these reactions is as follows:O18 + He4→Ne21 + nO18 + He4→Ne22 + γThe net result of these opposing forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron
29The Death of a High-Mass Star Once the inner core begins to change to iron, our high-mass star is in trouble. Nuclear fusion involving iron does not produce energy, because iron nuclei are so compact that energy cannot be extracted by combining them into heavier elements. In effect, iron plays the role of a fire extinguisher, damping the inferno in the stellar core. With the appearance of substantial quantities of iron, the central fires cease for the last time, and the star’s internal support begins to dwindle. The star’s foundation is destroyed, and its equilibrium is gone forever. Even though the temperature in the iron core has reached several billion kelvins by this stage, the enormous inward gravitational pull of matter ensures catastrophe in the very near future. Gravity overwhelms the pressure of the hot gas, and the star implodes, falling in on itself.Then the core consists entirely of electrons, protons, neutrons, and photons at enormously high densities and it is still shrinking. As the density continues to rise, the protons and electrons are crushed together, combining to form more neutrons and releasing neutrinos.
30The Death of a High-Mass Star SupernovaDriven by the rebounding core, an enormously energetic shock wave then sweeps outward through the star at high speed, blasting all the overlying layers—including the heavy elements outside the iron inner core—into space. Although the details of how the shock reaches the surface and destroys the star are still uncertain, the end result is not: The star explodes in one of the most energetic events known in the universeThis spectacular death rattle of a high-mass star is known as a supernova.
31Supernovae & Formation of the Heavy Element We have already seen something of how heavy elements are created from light ones by nuclear fusion. Hydrogen fuses to helium, then helium to carbon and oxygen. Subsequently, in high-mass stars, carbon and oxygen can fuse to form still heavier elements. Neon, magnesium, sulfur, silicon—in fact, elements up to and including iron—are created in turn by fusion reactions in the cores of the most massive stars. However, fusion stops at iron.
32How then were even heavier elements, such as copper, lead, gold, and uranium, formed? Some of these elements were formed during the late red-giant stages of low-mass stars via reactions involving neutrons. Similar reactions occur in supernovae too, where neutrons and protons, produced when some nuclei are ripped apart by the almost unimaginable violence of the blast, are crammed into other nuclei, creating heavy elements that cannot have formed by any other means. Many of the heaviest elements were formed after their parent stars had already died, even as the debris from the explosion signaling the stars’ deaths was hurled into interstellar space.
33References www.wikipedia.org/protostar www.wikipedia.org/nuclearfusion Eric,Chaissan & Steve,McMillan : Astronomy,A Beginner’s Guide to the Universe,4th ed. ( 2004 ),Pearson Education,Inc.Michael, A.Seeds , Foudations of Astronomy,8th ed.( 2005 ),Thomson Learning Academic Resource CenterJohn,D.Fix,Astronomy:Journey to the cosmic frontier,4th ed.(2006),The McGraw Hill Companies.formationevolution