1. The source of light, heat and (nearly all) energy on the earth.
THE SUN       
The source of light, heat and (nearly all) energy on the earth.

2. Diameter: 1,390,000 km (4.6 light seconds)
Mass: x 1030 kg (333,000 times Earth’s mass)
Temperature: 5800 K (“surface”) 15,600,000 K (core)
> 99.8% of the Solar System’s mass (Jupiter has most of the rest)
Hydrogen 92.1%, Helium 7.8%, other elements: 0.1%     2

3. One million Earth’s would fit into a hollow Sun

4. The Sun has existed for 4.6 billion years.
Where does all the Sun’s energy come from?
A long history of speculation has preceded our understanding of how the Sun generates energy.

5. We can constrain the types of energy sources that might fuel the Sun by comparing its age with the length of time various sources of energy are able maintain the Sun’s energy output, i.e., its luminosity.

6. CHEMICAL ENERGY (Is it on fire)?
NO. Coal or more efficient fuels would last the Sun less than 106 years. However, geological evidence that the earth (therefore, presumably the Sun) is over 108 years old has been convincing since the late 1800's.

7. GRAVITATIONAL ENERGY (from shrinking)?
NO. An object such as a star, planet, or cloud of gas converts gravitational potential energy into other forms of energy as it shrinks. This process provides most of the excess heat from Jupiter and Saturn, but would last fewer than 108 years for the Sun.

8. NUCLEAR ENERGY (from nuclear fusion)?
YES! The Sun and all stars generate energy through nuclear fusion. If the mass of an atomic nucleus is LESS THAN the SUM of the masses of two nuclei jammed together to produce that atom, then the difference between initial masses and the final mass is converted to lots of ENERGY, via E = mc2

9. Fusion of hydrogen into helium in the core of the Sun provides sufficient energy to power the Sun for 10 billion years.
Since the Sun is approximately 4.5 billion years old, it is half-way through its life.

10. There are two types of nuclear reactions
fission
fusion
FISSION: Big nucleus splits into smaller pieces. This happens with radioactive isotopes and is the basis of power from nuclear power plants.
FUSION: Small nuclei stick together to make a bigger one. This is the energy source of stars.

11. This difference in mass, m, is converted to energy: E = mc2.
fission
fusion
In both fission and fusion, the mass of the product(s) produced by the reaction is less than the mass of the particle(s) before the reaction.
This difference in mass, m, is converted to energy: E = mc2.

12. How does nuclear fusion occur in the Sun?

13. The Proton–proton chain is how hydrogen fuses into helium in Sun
Sun releases energy by fusing four hydrogen nuclei (protons) into one helium nucleus.
The Proton–proton chain
is how hydrogen fuses into helium in Sun

15. IN
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower.

16. Fusion Requires
Very high temperatures (> 15 million Kelvin in the core of the Sun) to provide high enough velocities to overcome electrical repulsion
Very high densities to make collisions frequent
At low speeds, electromagnetic repulsion prevents the collision of nuclei
At high speeds, nuclei come close enough for the strong force to bind them together
Start here on 9/13.

17. The temperatures and densities required for the proton-proton chain only exist in the core of the Sun.
Start here on 9/13.

18. Why is the core of the Sun so hot and dense?
Because the Sun is in “hydrostatic equilibrium” (sometimes referred to as “gravitational equilibrium”)
Start here on 9/13.

19. Hydrostatic equilibrium: The condition where gravitational forces seeking to shrink an object are precisely balanced by pressure seeking to expand an object.
Start here on 9/13.

20. Weight of upper layers compresses lower layers
-> lower layers are more compressed, i.e. denser.

21. Hydrostatic (Gravitational) equilibrium:
Energy provided by fusion maintains the pressure.
Pressure comes from collisions between particles (gas pressure) and from photons interacting with matter (radiation pressure)

22. Gravitational contraction…
provided energy that heated the core as the Sun was forming.
Contraction stopped when fusion began replacing the energy radiated into space.
Gravitational potential energy transformed into thermal (kinetic) energy

23. ConceptTest
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
(red) The core would expand and heat up slightly.
(yellow) The core would expand and cool.
(blue) The Sun would blow up like a hydrogen bomb.

24. ConceptTest Solar thermostat keeps burning rate steady
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy?
(red) The core would expand and heat up slightly.
(yellow) The core would expand and cool.
(blue) The Sun would blow up like a hydrogen bomb.
Solar thermostat keeps burning rate steady

25. Feedback
If nuclear fusion rates varied, so would the Solar temperature
Need a feedback mechanism to keep this in check
Rate of nuclear fusion is very sensitive to temperature: (pp chain rate goes as T4.6 , thus 2x increase in T, 25% increase in fusion rate)
Suppose the core temp rose: fusion rate would increase, pressure would push the core apart, make it larger, cooling it down

26. Solar Thermostat
Decline in core temperature causes fusion rate to drop, so core contracts and heats up
Rise in core temperature causes fusion rate to rise, so core expands and cools down

27. What is the Sun’s structure?

28. Core:
The innermost 20% by radius. Essentially all of the sun's energy is produced by fusion reactions in the solar core, in the region where the temperature ranges from ~ 6 to 15 million K

29. Energy generated in the cores of stars is transported outwards toward the surface by two methods:
RADIATIVE TRANSPORT: microscopic process: photons carry energy from one location where they are emitted to another, where they are absorbed.
Works best in a vacuum, but also works in low density gases and some other fluids Examples: ordinary incandescent bulbs; warmth of sunlight (IR).
CONVECTIVE TRANSPORT: macroscopic transport by blobs of matter; this only works in liquids or gases or plasmas (i.e., in any fluid) Example: boiling water in a pot

30. Radiative zone:
No energy is produced here, but the huge power generated in the core is carried outwards by photons, whose average energy slowly decreases from X-ray into UV as the temperature and density slowly decline.

31. Convective zone:
While photons are still wending their ways outwards through this relatively low density region, the only way all of the luminosity can be carried out is if blobs of hot plasma flow outward and colder blobs sink inward.

32. Convection:
material heated from below expands, becomes buoyant, and rises into a cooler region.
It gives heat to material in this new region, cools, contracts, and sinks back to lower hotter layers.

33. Convection cells convective cells in clouds
Granules on Sun, tops of convective cells
convective cells in heated liquid

34. Photosphere:
“Surface” of Sun
~ 6,000 K

35. Why does the Sun, a ball of gas and plasma, have a “surface”?

36. Photosphere: ~400 km deep region
At a depth of 400 km, a photon can go about 200 km before it hits an atom.
At 200 km, about 1/2 the photons that are moving outward manage to make it all the way out.
At 0 km, nearly all of them manage to make it into outer space.

37. Chromosphere:
Middle layer of solar atmosphere
~ 104–105 K

38. Corona:
Outermost layer of solar atmosphere
~1 million K

39. Solar wind:
A flow of charged particles from the surface of the Sun