Why does the Sun shine?
The Sun shines because it is hot.
The Sun is hot because in its core, nuclear fusion is
The nuclear fusion that is happening in the Sun's core is
of hydrogen into helium.
From models of stars and the Sun, we estimate that this
process will keep the Sun shining as it does today for another 4-5
The Sun is hot.
The surface of the Sun is 5800 degrees Kelvin. The
temperature inside the Sun is much hotter.
Its continuous spectrum is a blackbody with peak at a
wavelength 5000 Angstroms (yellow)
The total luminosity of the Sun is 390 trillion trillion
The density of the Sun is about 40% greater than water --
but the Sun is a gas.
The Sun is in HYDROSTATIC EQUILIBRIUM:
The collapse of the Sun be gravity is countered by the
pressure of the gas.
The heat generated by nuclear fusion in the core of the Sun
keeps the pressure high enough for the Sun to not collapse.
FUSION: 4 hydrogen nuclei combine to yield one helium
The helium nucleus weighs less than the 4 hydrogen nuclei, and the
difference in mass is converted into energy, described by the famous
Note: A very small amount of m makes a lot of E
The Sun is fusing 600 million tons of hydrogen per
596 million tons of helium.
Fission vs. Fusion:
Fusion only can occur if the gas temperature (ie the speed of
the atoms) is very,very high -- greater than 10 million degrees Kelvin.
In the Sun, the following chain of events makes the energy
the Sun hot.
ASIDE -- A-bombs vs. H-bombs; Nuclear Power.
The atom bomb, e.g. the bombs developed by the
Manhattan Project and dropped on Nagasake and Hiroshima during World
FISSION of Plutonium or uranium-235. The split
the atom into smaller fragments with a neutron. U-235 means that
the number of 92 protons + 143 neutrons = atomic weight of 235
in the nucleus.
Naturally occuring uranium is 99.3% U-238 and 0.7%
The trick is to keep the fuel in separate, subcritical
masses, and then put them together quickly so that enough neutrons are
in a small enough volume to start a "chain reaction" and maintain it.
The H-bomb, or thermonuclear bombs.
FUSION of deuterium or tritium to make helium.
Deuterium is "heavy hydrogen" or n+p. Tritium is
Tested by the US in the South Pacific in 1952.
Trick is to produce and store tritium and deuterium,
which are both gases.
Tritium spontaneously decays into deuterium and normal
hydrogen very rapidly.
And then you have to compress the deuterium at high
enough temperature to start fusion: usually done with an A-bomb.
Nuclear Power plants provide 17% of the world's
electricity; as much as 75% of the electricity in France.
Work by Fission: Split Uranium-235 with a
One pound of uranium (size of a baseball) can yield
energy as a million gallons of gasoline (50ftx50ftx5 story building).
For nuclear reactors to work, uranium must be
that it contains 2-3% U-235.
Weapons-grade uranium is 90% U-235.
Control rods are inserted into the slightly
supercritical uranium so to slow the neutrons and slow the reaction.
If the cooling stops, you can have a
meltdown/explosion, dispersing radioactive material for miles:
Spent nuclear fuel is toxic for centuries,
be stored safely.
Transporting fuel to the nuclear reactor, or on
submarines/rockets/ships is dangerous.
Dirty Bombs, or Neutron Bombs
Conventional bomb (or small thermonuclear bomb) used to
disperse radioactive material, anthrax, etc., producing "minimal" blast
Currently, only USA, Israel and China have neutron bombs
Countries with Nuclear Weapons: USA, Russia, United
Kingdom, France, China, India, Pakistan, Israel
Countries with active Nucelar Weapons Development Programs:
Iran, Iraq (turned out it wasn't as active as we thought), North