The Sun A Nuclear Powerhouse

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gravity tries to pull 1/4 pounder to center of the Earth

pressure from table opposes gravity

hydrostatic equilibrium on Sun

"cloud of gas" (like 1/4 pounder)

gravity pulls cloud to the center

pressure from gas below opposes gravity

heat from fusion in the hot core increases pressure

energy output controls size of sun!

pressure from table

weight from gravity

cloud

pressure from hot gas

weight from gravity

Slide 23

Temperature and Pressure

temperature: random motion of atoms in a gas

pressure: amount of force per unit area on piston from gas

generally pressure increases with increasing temperature

Slide 24

Balancing Fusion, Gravity and Pressure

If the rate of fusion increases, then:

thermal pressure increases causing the star to expand.

star expands to a new point where gravity would balance the thermal pressure.

the expansion would reduce compression of the core

the temperature in the core would drop

the nuclear fusion rate would subsequently slow down

the thermal pressure would then drop

the star would shrink

the temperature would rise again and the nuclear fusion rate would increase

Stability would be re-established between the nuclear reaction rates and the gravity compression

Slide 25

Hydrostatic Equilibrium

balance between pressure, heat from fusion and gravity determines Sun's size

big stars have cooler cores, small stars have hotter cores – more compressed

Slide 26

Other Particles

Helium is not the only product in the fusion of hydrogen.

Two other particles produced

Positron

Neutrino

Slide 27

Gamma Ray Propagation in the Sun

Positrons quickly annihilate with electrons.

Photons produced in core of the Sun take about a million years to move to the surface.

Slow migration because they scatter off the dense gas particles

move about only a centimeter between collisions.

In each collision, they transfer some of their energy to the gas particles.

As they reach the photosphere, gamma rays have become visible photons.

Slide 28

Neutrinos

Nearly massless particles with no charge.

Rarely interact with ordinary matter.

Neutrinos travel extremely fast

Almost at the speed of light if small mass.

Neutrinos pass from the core of the Sun to the surface in only two seconds.