feb 08 bov great lecture - as.utexas.edu
TRANSCRIPT
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Heaven’sKitchens
Origins of theChemical Elements
David L. LambertThe W.J. McDonaldObservatory
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I mean by elements, as those chemists thatspeak plainest do by their Principles, certainprimitive and simple, or perfectlyunmingled bodies, which not being made ofany other bodies, or of one another, are theingredients of which all perfectly mixtbodies are immediately compounded.
Robert BoyleThe Sceptical Chemist, 1661
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• An atom consists of a ‘large’ electron cloudaround a ‘tiny’ nucleus
• Electrons have negative charge• Nucleus has
– protons of positive charge– neutrons of no charge.
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• Elements are distinguished by Atomic Number,Z = number of protons in the nucleus
• Eighty-one natural elements run from Hydrogen(Z=1) to Bismuth (Z = 83).
• Technetium (Z = 43) and Promethium (Z = 61) areradioactive and have decayed.
• There are a few naturally occurring radioactiveactive elements: Uranium (Z = 92) and Thorium(Z = 90) and their decay products.
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How, when, and where were thechemical elements synthesized?
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Evidence provided by
• Spectroscopy of stars, …• Cosmic rays• Stardust in meteorites
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Nous concevons la possibilité de déterminerleurs formes, leurs distances, leurs grandeurset leurs mouvements; tandis que nous nesaurions jamais étudier par aucun moyen leurcomposition chimique, ou leur structureminéralogique, et, à plus forte raison, lanature des corps organisés qui vivent à leursurface, etc.
Auguste Comte(1798 – 1857)
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Primordial Soup
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• Primordial soup from the Big Bang via the3-minute recipe– Thermonuclear fusion in rapidly cooling
expanding mixture of protons andneutrons
– Main constituents of the soup are1H and 4He with 4He/1H º 10%
– Minor constituents are 2H, 3He, and 7Li– Synthesis limited by rapid expansion and
lack of stable A = 5 and 8 nuclei
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Stellar Entrées
Braised BetelgeuseFresh TechnetiumChandra Burgers
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I think that the suspicion has beengenerally entertained that the stars arethe crucibles in which the lighter atomswhich abound in the nebulae arecompounded into more complexelements.
A.S. Eddington
Stellar Entrées
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We do not argue with the critic who urgesthat the stars are not hot enough for thisprocess; we tell him to go and findA HOTTER PLACE.
A.S. Eddington
Stellar Entrées
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Introduction to Stellar Structure
• Gravity balanced by pressure (gas,radiation, degenerate electron) gradient
• Temperature gradient → Luminosity• Energy flow out by radiation and
convection• Energy replaced by nuclear energy and/or
gravitational collapse
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Braised Betelgeuse
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Evolution of High Mass Stars
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Hydrogen Burning
• 4H → He + Energy• E = mc2
• 0.7% of H to Energy• Sunlight from H-burning
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Supernova Explosion!
• Inner layers of ‘onion’ reprocessed• Neutron flood makes heavier elements• Products ejected into the Galaxy
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Need to scan the supernova
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Fresh Technetium
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Evolution of Low Mass Stars
• Experience H and then He-burning• ⇒ core of C and O• Core supported by degenerate electrons: pressure
~ T independent• Mass shed by red giant leaves C-O core as white
dwarf with MWD ≤ 1.4 MØ
• White dwarf cools and fades
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Red Giants
• Outer layers ejected at low velocity• Key contributions to
– carbon– heavy elements (technetium) by
neutron capture
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Chandra Burgers
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Supernovae Type Ia
Thermonuclear explosion of white dwarfdrives to exceed Chandrasekhar mass
Explosive nucleosynthesis → “Fe”
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Interstellar Desserts
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Interstellar Desserts
• Spallation and fusion reactionsbetween cosmic rays and ambientnuclei e.g.,
p + O → Li, Be, Bα + α → 6Li, 7Li
Li, Be, B
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Fun at the telescope
• The magnesium isotopes: 24Mg, 25Mg, 26Mg– Models of supernovae predict yield of
25Mg and 26Mg to decline as the initialcomposition of the star tends to pure Hand He
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Evolution ofthe
MagnesiumIsotopes
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Evolution of the Magnesium Isotopes
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Evolution of theMagnesium Isotopes
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• Hydrogen, Z = 1– Water, H2O– Big Bang survivor
• Helium, Z = 2– Airships, coolant– Earth: Product of U and Th decay– Stars: 90% Big Bang, 10% Stars
Where do common elements come from?
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• Lithium, Z = 3– Greases, medicine, nuclear bombs– 10% Big Bang, 10% Spallation, 80% Red
giants, Supernovae II• Beryllium, Z = 4
– Alloys, beryl (emeralds)– 100% Spallation
• Boron, Z = 5– Pyrex, bleaches, fireproofing– 30% Spallation, 70% Supernovae II (?)
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• Carbon, Z = 6 to Nickel, Z = 28– Lighter elements – Carbon to Silicon –
primarily from massive stars and theirsupernovae (Type II)
– Heavier elements – Silicon to Nickel –primarily from exploding white dwarfs(Supernovae of Type Ia)
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Beyond nickel, elements are made byneutron captures on iron
• Barium, Z = 56– X-ray medicine, spark plugs– Red Giants
• Europium, Z = 63– Red phosphor in TVs– Supernovae Type II
• Gold, Z = 79– Precious metal– Red giants and Supernovae Type II
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In My Craft OrSullen ArtBy Dylan Thomas
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In my craft or sullen artExercised in the still nightWhen only the moon ragesAnd the lovers lie abedWith all their griefs in their armsI labour by singing lightNot for ambition or breadOr the strut and trade of charmsOn the ivory stagesBut for the common wagesOf their most secret heart.
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Not for the proud man apartFrom the raging moon I writeOn these spindrift pagesNor for the towering deadWith their nightingales and psalmsBut for the lovers, their armsRound the griefs of the ages,Who pay no praise or wagesNor heed my craft or art.