Announcements
description
Transcript of Announcements
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Announcements• Wednesday night star parties begin this
week, 8:45 pm, weather permitting. Attend one for 4 points extra credit! (Staff signature required.)
• 10:00 office hour cut short today• Majors Fest at Union Building Gallery
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Nuclear Reactions in Stars (part 2)
1 November 2006
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Today:
• What makes the stars shine?• How long do they last?• Then what happens?
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The Sun’s Interior
Thermonuclear energy zone
Radiative zone
Convective zone
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The Sun’s Interior
Nuclear reactions only take place in the innermost 30% of the sun’s radius.
The central density is 150 times that of water; the central temperature is 15 million kelvin.
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In summary…
The sun is a mass of incandescent gas,
A giant nuclear furnace,
Where hydrogen is built into helium
At a temperature of millions of degrees.
-- Zim and Baker, A Golden Guide to Stars, 1951; borrowed by They Might Be Giants, 1993
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Can we test any of this theory?Yes! Look for the neutrinos…
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Fusion of Hydrogen into Helium
4 1H (protons) 4He
This reaction powers all main-sequence stars.
The more massive the star, the more pressure at its center and therefore the faster the reaction occurs.
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Masses of Stars
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Sizes of Main-Sequence Stars
Should be white, not green!
Hottest stars are actually somewhat larger
Reds are greatly exaggerated!
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Main Sequence Lifetimes(predicted)
Mass (suns)
Surface temp (K)
Luminosity (suns)
Lifetime (years)
25 35,000 80,000 3 million15 30,000 10,000 15 million3 11,000 60 500 million
1.5 7,000 5 3 billion1.0 6,000 1 10 billion0.75 5,000 0.5 15 billion0.50 4,000 0.03 200 billion
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What happens when the core of a star runs out of hydrogen?
• With no energy source, the core of the star resumes its collapse…
• As it collapses, gravitational energy is again converted to thermal energy…
• This heat allows fusion to occur in a shell of material surrounding the core…
• Due to the higher central temperature, the star’s luminosity is greater than before…
• This increased energy production causes the outer part of the star to expand and cool (counterintuitive!)…
• We now have a very large, cool, luminous star: a “red giant”!