Lots of fast moving nuclei means high temperature.
The core of the Sun has a temperature of 15 million degrees Kelvin. (ouch!)
Fusion is the source of energy for hydrogen bombs.
We are trying to harness fusion to generate electricity:
tokamak - magnetic confinement machine as envisioned for ITER shown to the right
inertial confinement fusion - Lawrence Livermore National Lab
Fusion Powers the Sun
Temperatures in the cores of stars are above the approximately 8 million K needed to fuse hydrogen nuclei together.
Calculations: observed power output of the Sun consistent with fusion of hydrogen nuclei.
Observation: neutrinos from Sun produced by fusion reactions.
Hypothesis: all stars produce energy by nuclear fusion.
fuse two hydrogen, H (1 proton) to make deuterium, 2H (1 proton, 1 neutron), neutrino and positron
fuse one deuterium and one hydrogen to make helium-3 3He (1 proton, 2 neutrons), gamma ray (energetic photon)
fuse two helium-3 to make helium 4He plus two hydrogen
Fusion would be simpler if four protons would collide simultaneously to make one helium nucleus
Simpler, but less likely
rare for four objects to collide simultaneously with high enough energy
chance of this happening are very, very small
rate too slow to power the Sun
proton-proton chain: each step involves collision of two particles
chance of two particles colliding and fusing is much higher, so nature slowly builds up the helium nucleus.
fusion only in Sun's core
only place its hot enough
heat from fusion determines Sun's structure
Fusion and Solar Structure
Heat from Core determines Sun's Size
Hydrostatic equilibrium: balance between
thermal pressure from the hot core pushing outwards
gravity squeezes the star collapse to the very center
Nuclear fusion rate is very sensitive to temperature.
A slight increase/decrease in T causes fusion rate to increase/decrease by a large amount.
Newton's second law: F = ma
static equilibrium: no acceleration if forces on object balance