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/