Download Slides from Lecture09

ASTR 1102-002
2008 Fall Semester
Joel E. Tohline, Alumni Professor
Office: 247 Nicholson Hall
[Slides from Lecture09]
Chapter 16: Our Star, the Sun
A Problem with Time Scales!
• Kelvin-Helmholtz contraction explains how the
Sun’s interior could stay hot (even as it radiates
copious amounts of heat from its surface) for
hundreds of thousands of years.
• But geological and fossil records show that the
Earth is far older than this (the Earth-Moon
system is about 4.6 billion years old!).
A Problem with Time Scales!
• Perhaps the Sun’s interior remains hot because
the Sun is burning fuel in a manner similar to the
way we humans burn fuel (for example, wood or
coal) to generate heat/energy.
– This type of “burning” involves building or breaking
chemical bonds.
• This won’t work because, in practice, relatively
little energy is released through chemical
burning processes.
– To generate the Sun’s luminosity via chemical
burning, the entire Sun would be consumed in about
10,000 years!
Is there a Solution?
• Is there some other source of energy that can be
called upon to explain how the Sun’s structure
can remain virtually unchanged for billions of
years?
• The answer is…the Sun “burns” its fuel
(hydrogen) via nuclear reactions, rather than via
chemical reactions.
• Energy is released when the nuclei of hydrogen
atoms are fused together to produce helium.
Nuclear Reactions
• Two basic types of nuclear reactions:
– Fusion = the nuclei of two or more elements “fuse” together to
create the nucleus of a heavier element
– Fission = the nucleus of one element breaks apart to form nuclei
of two or more lighter elements
• Some reactions generate heat/energy (exothermic);
other reactions absorb heat/energy (endothermic)
• Rule of thumb: The dividing line falls within the Fe-Ni
(iron-nickel) group of elements
– Energy/heat generated via fusion when elements lighter than the
Fe-Ni group are involved in the reaction
– Energy/heat generated via fission when elements heavier than
the Fe-Ni group are involved in the reaction
Chemical Elements & Their Isotopes
Courtesy of: http://atom.kaeri.re.kr/
Chemical Elements & Their Isotopes
Hydrogen
Chemical Elements & Their Isotopes
Helium
Chemical Elements & Their Isotopes
Carbon
Chart of Nuclides
Chart of Nuclides
14
6 + 8 = 14
C
How is Energy Generated?
(and how much energy?)
• Mass is converted into energy!
• The total mass of the nuclei (or nucleus)
produced by a reaction, Moutput, is less than the
total mass of the nuclei (or nucleus) that start(s)
the reaction, Minput.
• E = (DM)c2
– Here, the quantity, DM = (Minput – Moutput)
• Example: When 4 hydrogen nuclei fuse to form
1 helium nucleus, 0.7% of Minput is converted into
energy/heat
Is there a Solution?
• Via nuclear fusion reactions (so-called, nuclear
burning), the Sun can “live” for approximately 10
billion years and only use up approximately 10%
of its total fuel supply!
• We can express this mathematically:
– tage = fMc2/L
– For M = Msun, L = Lsun, and f = 0.7% x 10%, tage = 10
billion years
Figure 16-4
Sun’s Internal Structure
Chapter 19: Stellar Evolution:
On & after the Main Sequence
Apply the “Age” Concept
to Other Stars
• How long can other stars live?
– tage = fMc2/L
– (tage /1010 years) = (M/Msun)/(L/Lsun)
Apply the “Age” Concept
to Other Stars
Checking Accuracy of Solar Model
• Solar Oscillation measurements
• Solar Neutrino measurements
Solar Oscillation Measurements
Solar Neutrino Measurements