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ASTR 1101-001 Spring 2008. Joel E. Tohline, Alumni Professor 247 Nicholson Hall [Slides from Lecture25]. Chapter 8: Principal Topics. How old is the Solar System?

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## ASTR 1101-001 Spring 2008

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**ASTR 1101-001Spring 2008**Joel E. Tohline, Alumni Professor 247 Nicholson Hall [Slides from Lecture25]**Chapter 8: Principal Topics**• How old is the Solar System? • Nebular Hypothesis + Planetesimals + Core Accretion: A model that explains how the solar system acquired its key structural properties. • Directions and orientations of planetary orbits • Relative locations of terrestrial and Jovian planets • Size and compositions of planets • Observational evidence for extrasolar planets**How old is the Solar System?**• Radioactive dating • First, let’s discuss the idea of radioactive isotopes of atomic elements**How old is the Solar System?**• Radioactive dating • First, let’s discuss the idea of (radioactive & non-radioactive) isotopes of atomic elements**Chemical Elements & Their Isotopes**Courtesy of:http://atom.kaeri.re.kr/**Chemical Elements & Their Isotopes**Hydrogen**Chemical Elements & Their Isotopes**Hydrogen**Chart of Nuclides**14 C**Chart of Nuclides**14 C 6 + 8 = 14**How old is the Solar System?**• Radioactive dating • First, let’s discuss the idea of (radioactive & non-radioactive) isotopes of atomic elements • Now let’s discuss radioactivity and the concept of “half-life” • “The half-life of an isotope is the time interval in which one-half of the nuclei decay.” [See Box 8-1 in the textbook.] • http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html**Radioactive Decay**t1/2 = half-life Courtesy of: www.splung.com/content/sid/5/page/radioactivity**Decay of 14C to 14N**14C t1/2 = 5730 years 14N**Decay of 87Rb to 87Sr**t1/2 = 47 billion years**Decay of 238U to 206Pb**238U t1/2 = 4.5 billion years 206Pb**Radioactive Decay**• As the abundance of the radioactive isotope(for example, 14C) decreases steadily over time, the abundance of the final stable isotope(for example, 14N) steadily increases.**14C Dating**• Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14N/14C = 3. How old is the weapon if we assume that, originally, the wood contained no 14N ?**14C Dating**14N 14C**14C Dating**At what time does the abundance ratio 14N/14C = 3 ? 14N 14C**14C Dating**At what time does the abundance ratio 14N/14C = 3 ? 14N 14C**14C Dating**At what time does the abundance ratio 14N/14C = 3 ? 14N 14C Age of wood = 2 t1/2 = 2 x (5730 yrs) = 11,460 yrs**14C Dating**• Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14N/14C = 3. How old is the weapon if we assume that, originally, the wood contained no 14N ? • ANSWER: 11,460 years • In practice, the ‘dating’ technique is messier than this, but this should give you a general idea of how the radioactive dating technique works.**14C Dating**• Suppose an archeologist digs up a primitive weapon made partly of wood and determines that the wood contains an isotopic abundance ratio 14N/14C = 3. How old is the weapon if we assume that, originally, the wood contained no 14N ? • ANSWER: 11,460 years • In practice, the ‘dating’ technique is messier than this, but this should give you a general idea of how the radioactive dating technique works.**238U Dating**• Suppose an astronomer discovers a meteorite and determines that the meteorite contains an isotopic abundance ratio 206Pb/238U = 1. How old is the meteorite if we assume that, when it originally formed, the meteorite contained no 206Pb ?**Atomic Bombs**• Rely on spontaneous fission (radioactive decay) of heavy nuclei, such as Uranium and Plutonium • A ‘critical mass’ of fissionable material will explode because the decay products from spontaneous fission strike nearby nuclei and induce those nuclei to fission runaway chain reaction • First atomic bombs were constructed during World War II in the so-called ‘Manhattan Project’ centered at Los Alamos, NM http://www.lanl.gov/history/

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