The Sun

1. The Sun

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

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

4. Discussion How does the Sun maintain hydrostatic and thermal equilibrium?

5. Discussion What happens if all fusion in the Sun suddenly ceases?

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

7. 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?

8. 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

9. 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?

10. 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.

12. 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.

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

14. Solar Granulation

18. 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.

19. 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.

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

22. 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?

23. Prominence

26. Flare

27. Coronal mass ejections

30. 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.

33. 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.

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

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

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

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

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

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

44. 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?

47. 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.

50. 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.