The Sun

Discussion

What does it mean to say the Sun is in hydrostatic equilibrium?

Discussion

What does it mean to say the Sun is in thermal equilibrium?

Discussion

How does the Sun maintain hydrostatic and thermal equilibrium?

Discussion

What happens if all fusion in the Sun suddenly ceases?

Discussion

If the Sun contracts, it heats up. Why doesn’t the increased pressure stop the contraction?

Discussion

If the Sun is in thermal equilibrium and it is generating heat in its core via thermonuclear fusion, what must happen to that energy?

Heat Transport in the Sun

• Conduction – particles transfer energy via collisions

• Convection – energy transferred by movement of material from hotter to cooler regions

• Radiative Diffusion – energy transferred via photons

Discussion

Which would you rather do, put your hand in an oven at 450 degrees F or put you hand on a 450 degree F stove top? Why is there a difference?

Radiative Diffusion

Radiative zone – inner 71 percent of the Sun’s Interior where all atoms are ionized.

Takes a photon 170,000 years to reach the convective zone. Each time a photon is absorbed it loses energy.

Convection

Convective Zone – outer 29 percent of Sun’s interior. Bottom of convective zone is cool enough for heavy atoms to regain electrons and absorb light.

Discussion

What happens to the bottom layer of the convection zone as it absorbs light from the radiative zone.

Solar Granulation

Differential Rotation causes Sun’s magnetic field

Sun is made of gas. Thus it does not all have to rotate at the same rate!

The equator of the Sun rotates faster than the poles. Sidereal rotation period at equator is about 25 days, while at the poles the sidereal rotation rate is about 35 days.

The Radiative Zone rotates as a solid body!

However, the radiative zone rotates with a sidereal period of 27 days at all latitudes.

Only the convection zone rotates differentially.

Discussion

How do you think we know the rotation rate of the radiative zone when we can only observe the photosphere?

Discussion

Differential rotation creates a tremendous amount of shear as one layer slides past the other. A similar thing happens in Earth’s atmosphere when a cold air mass meets a warm air mass.

What is the result in Earth’s atmosphere?

Prominence

Flare

Coronal mass ejections

The Sun-Earth connection

Coronal mass ejections and solar flares can be directed at Earth. Luckily for us, Earth has a magnetic field and an atmosphere to protect us.

Aurora

When high speed particles from the Sun collide with atoms in Earth’s upper atmosphere. The electrons are knocked into higher energy orbitals and emit light when returning to the ground state.

Discussion

Are stars without a convection zone likely to have starspots, prominences and flares? Explain.

Discussion

As the Sun converts 4 protons, into 1 He nucleus, what happens to the Sun’s core?

Discussion

What will happen to the Earth as the Sun’s luminosity increases?

Discussion

As the Sun becomes more luminous, what happens to the habitable zone?

Discussion

What happens to the core when the Sun fuses all of its Hydrogen in the core into helium?

Discussion

What happens to the hydrogen that was just outside the core but never hot enough to fuse into helium?

Discussion

The fusion of H into He around the Sun’s core produces a lot of heat and pressure. Can this stop the contraction of the core? What effect does this extra pressure have on the outer layers of the Sun?

Discussion

Do you think stellar winds will be stronger, less than, or about the same in red giant stars as in main sequence stars? Explain.

Triple alpha process

As long as more He is dumped on the core it continues to contract, to heat up and the star’s outer layers continue to expand.

Eventually, the core reaches a temperature of 100 million K and 3 He atoms can fuse into a carbon atom.

Discussion

Why do you think a higher temperature 100 million K is required to fuse helium into carbon than the 10 million K required for the proton-proton chain?

Discussion

What happens when the Sun runs out of He in its core?

Ring nebula

Abell 39

Owl nebula

Saturn nebula

Cat’s Eye nebula

Spirograph nebula

NGC 7027

Stingray nebula

NGC 6543

MyCn 18

Discussion

A solar mass star leaves behind a white dwarf that contains nearly pure carbon. While matter in the interior is degenerate, there is a layer of normal matter on the surface. What does this layer become once the surface cools to room temperature?