The Sun A Nuclear Powerhouse

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Lots of fast moving nuclei means high temperature.

The core of the Sun has a temperature of 15 million degrees Kelvin. (ouch!)

Slide 16

Fusion Power on Earth

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

ITER reactor

Slide 17

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.

Slide 18

proton-proton chain

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

Slide 19

Why a complicated chain?

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.

Slide 20

fusion only in Sun's core

only place its hot enough

heat from fusion determines Sun's structure

Fusion and Solar Structure

Slide 21

Heat from Core determines Sun's Size

Force equilibrium

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.

Slide 22

Gravity and Pressure

force equilibrium

Newton's second law: F = ma

static equilibrium: no acceleration if forces on object balance