ASTR 1101-001 Spring 2008
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Transcript of ASTR 1101-001 Spring 2008
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
Chemical Elements & Their Isotopes
Helium
Chemical Elements & Their Isotopes
Helium
Chemical Elements & Their Isotopes
Carbon
Chemical Elements & Their Isotopes
Carbon
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
Chart of Nuclides
C14
Chart of Nuclides
C14
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
Color Indicates Approximate “Half-Life”
Some more precise “Half-Life” values
Some more precise “Half-Life” values
Some more precise “Half-Life” values
Decay of 14C to 14N
14C
14N
t1/2 = 5730 years
Some more precise “Half-Life” values
Decay of 87Rb to 87Sr
t1/2 = 47 billion years
Some more precise “Half-Life” values
Decay of 238U to 206Pb
t1/2 = 4.5 billion years
238U
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 Dating14N
14C
14C Dating14N
14C
At what time does the abundance ratio 14N/14C = 3 ?
14C Dating14N
14C
At what time does the abundance ratio 14N/14C = 3 ?
14C Dating14N
14C
At what time does the abundance ratio 14N/14C = 3 ?
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/