The Sun

1. The Sun

2. Discussion Why does the Sun shine?

3. Discussion How do you know the Sun is hot?

4. Infrared radiation The Sun feels warm because of the infrared radiation it emits. Anaxagoras (500 – 428 B.C.E.) believed the Sun was a very hot, glowing rock about the size of the Greek peninsula.

5. The setting Sun is red because • The Earth is rotating away from the setting Sun, so it is redshifted. • The setting Sun is cooler at sunset, so Wien’s law says the frequency of maximum emission shifts to lower frequencies, thus appears redder. • At sunset the light has to travel through more of the Earth’s atmosphere, which has lots of absorption lines in the blue portion of the spectrum. • None of the above

6. Solar Data Radius: 109 Earth radii Mass: 333,000 Earth masses Composition: 74% hydrogen 25% helium Mean density: 1.41 g/cm3 Luminosity: 3.86  1026 Watts

7. The Sun as a big cosmic light bulb Suppose every human being on Earth turned on 1000, 100-watt light bulbs. With about 6 billion people this would only be 6  1014 watts. We would need 670 billion more Earth’s doing the same thing to equal the energy output of the Sun.

8. Discussion What kind of spectrum does the Sun have?

11. Discussion Why is there less solar intensity at sea level than there is at the top of Earth’s atmosphere?

12. Discussion Where do you think that energy goes?

13. Discussion The Sun releases lots of energy each second, what if it were cooling down over time. How could we tell?

14. Thermal equilibrium The Sun is not measurably heating up or cooling down.

15. No cooling ember At the rate that the Sun is emitting energy, the Sun must have been much hotter just a few hundred years earlier, making life on Earth impossible. The Sun must have an energy source; a way of generating its own heat.

16. Discussion Given the composition of the Sun, why is it unlikely that it could be heated by the burning of wood or coal?

17. Kelvin-Helmoltz contraction As things contract gravitationally, they become hotter.

18. Discussion Why do you think gravitational contraction leads to a temperature increase?

19. Discussion If the Sun is getting its energy from Kelvin-Helmholtz contraction, what observation could you make to test this? Do you think this is an easy measurement to make? Explain.

20. Hydrostatic Equilibrium The Sun is not measurably expanding or contracting

21. The age of the Sun Sedimentary rocks on Earth which were deposited in liquid water are 3.8 billion years old. Rocks containing fossils are 3.5 billion years old. The Sun must have been shining for at least this long.

22. What energy source can keep the Sun hot for 3.8 billion years? Burning coal: Sun would last 10,000 years Kelvin-Helmholtz contraction: if the Sun’s heat were generated from contraction of the Sun’s mass, it would shine for only 25 million years.

23. E = m c2 Energy equals the mass times the speed of light squared. Matter is a form of frozen energy.

24. The Sun is huge! A little bit of matter can be turned into a large amount of energy. If the Sun’s mass could be converted to energy it could shine for hundreds of billions of years. The Sun needs to convert 4.3 million tons of matter to energy every second.

25. The Sun’s Mass is Converted to Energy 4 hydrogen atoms have a mass of 6.693  10-27 kg 1 helium atom has a mass of 6.645  10-27 kg Thus, 0.048  10-27 kg are converted to energy.

26. Thermonuclear Fusion The Sun fuses 4 hydrogen atoms together to produce 1 helium atom releasing energy. In the Sun about 600 million tons of hydrogen is converted to helium per second.

27. Discussion How can we change a positively charged proton from a hydrogen nucleus into a neutral neutron?

28. How does it work? We need a new form of matter called anti-matter. Antimatter is made up of anti-particles which have the same mass as ordinary particles but opposite charge. Matter and antimatter will annihilate each other if they come in contact producing energy.

29. Proton-Proton chain Helium nuclei can be built up one proton at a time in what we call the proton-proton chain. Normally, two protons will repel each other with the electrostatic force, but if they are smashed together with enough force they can stay together via the strong nuclear force.

31. Neutrinos Neutrinos () are particles that only interact with matter via the weak nuclear force (the force responsible for radioactive decay). To stop a typical neutrino emitted from the Sun would require 1 light-year (5 trillion miles) of lead.

32. Discussion Why must matter be so hot, 10 million K, for H to fuse into He?

33. Discussion If it takes on average 14 billion years to make a deuterium atom, how can the Sun fuse 600 million tons of hydrogen into helium each second?

34. Discussion How can atoms with more than one proton in the nucleus stay together? Why don’t they just fly apart?

37. Discussion Fusion keeps the Sun hot, but fusion requires the Sun to be hot. How did the Sun ever get hot enough to start fusion?

38. How do we know thermonuclear fusion is taking place in the Sun? “We do not argue with the critic who urges that stars are not hot enough for this process; we tell him to go and find a hotter place.” Eddington (1926)

39. We can test the theory that the Sun is powered by thermonuclear fusion by: • Modeling the solar interior • Direct observations of solar neutrinos

41. Discussion Which acrobat would you rather be and why?

43. Discussion What does this mean for the pressure on the gas as you descend into the interior of the Sun?

44. Pressure increases toward the center of the Sun To maintain equilibrium, the pressure below each layer of the Sun must be greater than the pressure above that layer.

46. Discussion What happens if you squash a gas?

47. Density increases toward center of the Sun The Sun is gaseous. If you apply pressure to a gas is compresses, i.e. it’s density goes up.

48. Temperature increases toward the center of the Sun As the pressure goes up toward the center of the Sun, the temperature also increases.

49. Fusion only takes place in the Sun’s core In the inner 1/4 of the Sun’s radius can fusion take place. Even at 15 million K, it takes on average 14 billion years at a rate of 100 million collisions per second to fuse two protons to produce a deuterium atom.

50. Thermal equilibrium The Sun is not heating up or cooling down. The temperature of each layer, although different, is constant with time. Heat generated in the core by thermonuclear fusion must be equal to the energy emitted from the Sun’s surface.