PHYS
205
Powerhouse
PHYS
205
Possible sources
Chemical Energy:
Sun has hydrogen and if it has oxygen, than we can make water.
will last 18,000 years.
there is not enough oxygen.
Gravitational Energy:
Gravity compresses the Sun, the gases will be heated
will last 30,000,000 years.
Falling meteorites will heat the Sun
you need Earth’s mass every 100 years
will change Earth’s orbit 4 minutes each century.
Nuclear Energy:
Only option left. So how??
PHYS
205
Nuclear Reactions
Nuclear Fission - produces energy by breaking up massive nuclei
Nuclear Fusion - produces energy by fusing together light nuclei
Stars are made up of light nuclei:
The energy source of the stars is nuclear fusion.
PHYS
205
Nuclear fusion
Fusion requires extremely high temperatures:
To fuse hydrogen we need a few million degrees (5-8 million K)
To fuse elements with a greater positive charge we need higher speeds
(higher temperatures). Helium fuses at 100 million K.
Heavier nuclei fuse at even higher temperatures.
PHYS
205
Einstein??
E = mc2
This formula means that mass can be converted into energy.
10 gr chalk -> 9.1014 Joules.
Take water at 20 oC. You can boil
3.5x108 tons of water with this
energy.
Cube of water with side 70m.
In the Sun, Hydrogen (one proton)
is converted into Helium (two
protons and two neutrons).
PHYS
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Sun’s fusion
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Reactions
1H
+ 1H  2H + e- +  + 
1.4 x 1010 years
2H
+ 1H  3H + 
6s
3H
+ 3H  4He + 21H
106 years
1H
: 1.007825u
4 1H :
4.03130u
4He
4.00268u
:
--------------------0.02862u
1 kg H  0.0071 kg = 7.1 g mass defect  6.4 x 1014 Joules
Sun’s luminosity : 4 x 1026 joules/sec  6 x 1011 kg/sec  Sun lasts 1010 years
PHYS
205
Equilibrium
How is this all possible??
1.
2.
3.
Sun is made up of gas

Heated gases expand

They have a pressure increasing T increases p
Sun is stable

Gravitation should collapse it, but does not

So T in the center should be 15,000,000 K  hydrostatic equilibrium
Temperature does not change

4.
Heat generated = heat lost  thermal equilibrium
Heat is transferred: conduction, convection, radiation
PHYS
205
Radiation
During the fusion reactions two kinds
of radiation are generated.
1. Photons (gamma rays): It takes
about 1,000,000 years to reach the
surface.
2. Neutrinos (massless): It takes
about 2 seconds to reach the
surface.