# Astronomy 1021 Chapter Notes - Chapter 11: Radiation, Ideal Gas Law, Nuclear Fission

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16 Aug 2016
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Chapter 11: The Sun
The Sun
Its total power output, called luminosity, is about 3.8 x 1026 watts
o1 second of the sun's luminosity can meet our energy demands for 500,000 years
Gravitational Contraction- the collapse of gas and dust to form stars, planets and other objects, under
gravitational attraction. The process leads to the release of potential energy as the mass becomes more
closely packed
Source of the Sun's Energy
E= mc2
Energy has a lot of mass, and the formula explains that the Sun's mass is enough to product
large amounts of energy for billions of years
Nuclear fusion is how the sun is powered
o4 hydrogen atoms joining together for form 1 helium atom through Quantum Mechanics
*note: nuclear power plants use nuclear fission, not fusion
Strong force- binds neutrons and protons together in an atomic nucleus, but only act on very
short distances
oFor forces at larger distances, a dominant force is electrostatic repulsion
oAt low speeds, electromagnetic repulsion prevents the collision of nuclei
oAt high speeds, nuclei come close enough for the strong force to bind them together,
and this is how protons are glued together (since they usually repel each other)
oThis is the force that allows for hydrogen fusion in the sun which provides its energy
o4 protons --> 1 He + 2 gamma rays + 2 positrons + 2 neutrinos
The gamma ray is a result of the energy loss because 1 helium nucleus is 0.7%
lighter than the mass of 4 combined hydrogen nuclei, thus the energy lost is emitted
in the form of gamma ray, and so it becomes sunlight
oWe not only need high speeds to have fusion reactions, we also need high density to
have enough protons enough around to collide
oFusion requires a temperature of about 15 millions degrees
Q: If we can fuse Hydrogen into Helium, what would we need fused bigger nuclei?
A: More heat, a big atom gun, and higher pressure
How Did the Sun Get Hot Enough for Fusion?
A collapsing cloud of interstellar gas go through gravitational contraction
Density and pressure increase in the cloud center, thus temperature increase
Eventually, density and temperature in the core is high enough to sustain hydrogen fusion, and
the Sun starts shining
The energy produced by fusion heats the gas and builds up enough pressure to balance gravity
Therefore, the Sun is shining because it is fighting gravity, which is its own weight
oThe natural balance between gravity pulling inward and pressure pushing outward is
called gravitational equilibrium or hydrostatic equilibrium
The sun was born with enough hydrogen to shine for 10 billion years, we are about half way
through
Q: Could the Sun survive as just a core?
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A: no
The Solar Thermostat
Nuclear reactions rates are very sensitive to temperature
oA small increase in temperature would lead to a large increase in fusion rate and thus a
large increase in energy production
The solar thermostat- a natural feedback mechanism (the ideal gas law) prohibits this from
having severe consequences:
oFusion rates would increase a lot if the temperature increased just a little bit
oEven though this would increase temperature further, the core would expand
oIf it expands, it will cool down
Q: How would the temp of Doc Ock's mini-star change when it pulled in material around it
A: go up because there is more fuel available
Why the Sun is Brightening
Since the sun is converting 4 particles into 1, it is taking less space as time goes on
This decreases the pressure
the core contracts slightly
Fusion rate increases
Energy production goes up
Therefore the total energy production slowly increases, and the sun gets brighter
How does the energy get out from its core?
2 problems:
1. How does the light get out
2. Hydrogen Fusion makes the same photon (piece of light) every time (gamma ray)
Extreme high density causes photons to bounce off some particles within less than 1 mm
These interactions change the direction in which the photon is moving randomly
oRandom walk process, aka radiative diffusion
oReview energy levels of an electron in an atom from previous chapters
oIt takes up to 1 million years for photons to get out of the Sun
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