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Nuclear Reactions in Stars
(part 2)
1 November 2006
Today:
• What makes the stars shine?
• How long do they last?
• Then what happens?
The Sun’s Interior
Thermonuclear
energy zone
Radiative
zone
Convective
zone
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.
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
Can we test any of this theory?
Yes! Look for the neutrinos…
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.
Masses of Stars
Sizes of Main-Sequence Stars
Hottest stars are
actually somewhat
larger
Should be white,
not green!
Reds are greatly
exaggerated!
Main Sequence Lifetimes
(predicted)
Mass
(suns)
25
15
3
1.5
1.0
0.75
0.50
Surface temp
(K)
35,000
30,000
11,000
7,000
6,000
5,000
4,000
Luminosity
(suns)
80,000
10,000
60
5
1
0.5
0.03
Lifetime
(years)
3 million
15 million
500 million
3 billion
10 billion
15 billion
200 billion
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”!