THE SUN

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

2. The Sun: Our Prototype Star • We know much more about it than any other star • We describe all other stars in terms of it • So you MUST memorize its key properties

3. Solar Properties • Mass: M = 1.989 x 1033 g = 2.0 x 1030 kg • Radius: R = 6.96 x 1010 cm = 7.0 x 105 km • Surface (photospheric) temperature, T = 5760 K = 9900 F • Luminosity: L = 3.9 x 1033 erg/s = 3.9 x 1026 W • Other important facts to memorize:Central temperature, Tc = 1.5 x 107 K Central density, c = 160 g cm-3 Mean density, = 3M/4R3 = 1.4 g cm-3

4. THE SOLAR INTERIOR • The innermost 20% by radius, or about 0.8% by volume, is the NUCLEAR ENERGY GENERATING ZONE. Essentially all of the sun's power is produced by fusion reactions in the solar core, in the region where the temperature exceeds 6 million K. • Energy is carried by high energy (-ray & X-ray) photons. They scatter or are absorbed every cm or so, but their descendants of much lower energy eventually emerge from the sun's photosphere. • These photons stream to us as the visible, IR and UV solar radiation that drives the earth's climate and provides the root source (via photosynthesis) of nearly all energy used on earth.

5. Pop Quiz # 2 • Put away all books, papers, computers, etc. • Take out a sheet of paper. • 1- Print your name (1). • 2- List the seven bands of the electromagnetic spectrum in order, starting from that with the longest wavelength (6). • 3- Write down the mass and radius of the Sun (4). Don’t forget units!

6. A Solar Slice

7. Energy Transport Zones • THE RADIATIVE ZONE extends from about 0.2 R out to 0.72 R. No more 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. • The outer quarter by radius, (but over half of the volume) of the sun is the 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.

8. Radiative & Convection Zones

9. ENERGY TRANSPORT CAN BE: • CONVECTIVE: macroscopic transport by blobs of matter; this only works in liquids or gases or plasmas (i.e., in any fluid). Examples: boiling oil in a pot; -- earth's outer mantle driving plate tectonics. • RADIATIVE: 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).

10. ENERGY TRANSPORT CAN ALSO BE • CONDUCTIVE: a microscopic process where individual atoms or molecules collide with their neighbors, thereby transferring some kinetic energy of motion, which is felt as heat flow. Works best in solids, but can function in liquids or gases too. Examples: --poker in a fireplace; --handle of a pot on the stove; --bare feet on a stone floor feel colder than bare feet on a carpet of the same temperature since the stone conducts heat away faster. (Conduction, while important in the planets, is not important for stars, so will be ignored in the rest of the course.)

11. How do photons get from the core of the Sun to the surface? • They bounce from atom to atom–absorbed, then emitted, losing energy as they go • They are conducted (heat conduction) • Gamma rays • X rays • None of the above

12. How do photons get from the core of the Sun to the surface? • They bounce from atom to atom–absorbed, then emitted, losing energy as they go • The are conducted (heat conduction) • Gamma rays • X rays • None of the above

13. STELLAR ENERGY PRODUCTION IS NOT: • CHEMICAL (coal or better 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. • due to self-gravity induced CONTRACTION (usually called Kelvin-Helmholtz contraction) which provides most of the excess heat from Jupiter and Saturn but would only last fewer than 108 years for the Sun.

14. STELLAR ENERGY PRODUCTION IS NOT: • from FISSION; while fission is far more efficient than either of the above mechanisms, fissile materials (like Uranium or Radium or Plutonium) are extremely rare and their abundances in the Sun are so low that they couldn't power it for very long either.

15. Stellar Energy Production IS • due to FUSION. If the mass of a nucleus is LESS THAN the SUM of the masses of two nuclei jammed together then the difference between the sum of the initial masses and the final mass is converted to lots of ENERGY, via • E = m c2 • The Sun and most other stars during the bulk of their lives use the PROTON-PROTON chain. The basic version is • p + p  deuteron (2H or d) + positron (e+) + neutrino () • d + p 3He + gamma-ray () • 3He + 3He 4He + p + p PPchain applet • THE NET REACTION IS: 4 protons (or nuclei of ordinary hydrogen-one) make one helium-four nucleus (or alpha-particle) and give off lots of energy (in the form of fast moving protons, photons, neutrinos and positrons).

16. Fusion Requires • Very high temperatures to overcome electrical repulsion • and to make collisions frequent • Coulomb repulsion applet • Also, very high densities to make the collisions frequent

17. Solar Fusion Reactions in Pictures

18. Why does the Sun shine? • The Sun is a mass of interstellar gas • It’s a giant nuclear furnace • It’s where hydrogen changes into helium • At a temperature of millions of degrees • All of the above

19. Why does the Sun shine? • The Sun is a mass of interstellar gas • It’s a giant nuclear furnace • It’s where hydrogen changes into helium • At a temperature of millions of degrees • All of the above* *These are lyrics from a song sung by “They Might Be Giants” and are really much older: I sang them in my 4th grade class play.

20. What is a hydrogen nucleus–the particle that fuses in the Sun? • A neutron • A proton • An electron • None of the above

21. What is a hydrogen nucleus–the particle that fuses in the Sun? • A neutron • A proton • An electron • None of the above

22. BATTLE: Gravity vs Pressure • All stars (planets too) must be in HYDROSTATIC EQUILIBRIUM • Inward self-gravity is balanced by outward change in pressure

23. Thought Question What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly B. The core would expand and cool C. The Sun would blow up like a hydrogen bomb

24. Thought Question What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly B. The core would expand and cool C. The Sun would blow up like a hydrogen bomb Solar thermostat keeps burning rate steady

25. 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 Thermostatic Equilibrium Applet

26. If the fusion in the Sun’s core sped up slightly, releasing more energy, what would happen? • Not much since the core is deep inside • The core would expand • The color of the Sun would change • None of the above

27. If the fusion in the Sun’s core sped up slightly, releasing more energy, what would happen? • Not much since the core is deep inside • The core would expand • The color of the Sun would change • None of the above

28. SOLAR MODEL Combines • Hydrostatic equilibrium • Fusion energy generation • Energy transport • To get density, temperature and composition at all locations in sun’s interior • Pressure inside Sun • Temperature inside Sun

29. SOLAR MODELS NOW AGREE WITH HELIOSEISMOLOGY & NEUTRINOS Patterns of vibration on surface tell us about what Sun is like inside

30. Data on solar vibrations agree very well with mathematical models of solar interior

31. NEUTRINO ASTRONOMY PROBES SOLAR CORE Neutrinos created during fusion fly directly through the Sun Observations of these solar neutrinos can tell us what’s happening in core

32. Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number. Only 1/3 of predicted amount was detected! Was the solar model wrong or was understanding of neutrinos inadequate?

33. Solar neutrino problem: More recent observations find the right number of neutrinos, but some have changed form (electron to mu or tau neutrinos) Here astrophysics revealed key truth about particle physics

34. What is the Solar Lifetime ()? • FUSION of 4 H nuclei into one 4He nucleus turns about 0.007 (0.7%) of their masses into energy. • Typical energy/nuclear reaction is 1 MeV or 1.6 x 10-6 erg • # of reactions needed/second = L /Energy per reaction Roughly, one proton is used up per reaction and therefore These numbers are approximate: correct value is 1.0 x 1010 years.

35. Solar Atmosphere

36. Atmospheric Layers • PHOTOSPHERE: visible, IR and UV continuum radiation streams out from here. • Thickness about 400-500 km • 4500 < T < 5800 K; usually say T = 5760 K • density between 10-5 and 10-8 g/cm3 • Granulation from underlying convection zone is visible • emerging spectrum is continuum formed in the denser layers with superposed absorption lines formed in the cooler, less dense, outer layers

37. Solar Granulation (orange peel look)

38. Solar (Fraunhofer) Spectrum

39. Spectral Line Formation

40. Chromosphere & Transition Layer • mostly UV emission line radiation, some visible • irregular thickness, averaging 5000 km in SPICULES • most chromospheric gas w/in about 1500 km layer • 4500 < T < 10,000 K(up to 50,000 K in transition zone) density averages ~ 10-10 g/cm3 • only visible when photosphere is obscured

41. Spicules and Corona

42. CORONA • mostly X-ray emission • Irregular thickness, typically out to 2-3 times Solar radius • Average T = 1 x 106 K extremely low density • only visible when photosphere is obscured or via X-ray telescopes • heated via magnetic energy originating in the convective zone of the sun: probably by both magnetohydrodynamic shocks and magnetic reconnection.

43. Since the Sun’s outer atmosphere or corona is millions of degrees but not very dense, • We can’t really see it • We see it–it emits orange light and big flames • We see X rays coming from it • We only see the lower layers of the Sun’s atmosphere, which are much more dense

44. Since the Sun’s outer atmosphere or corona is millions of degrees but not very dense, • We can’t really see it • We see it–it emits orange light and big flames • We see X rays coming from it • We only see the lower layers of the Sun’s atmosphere, which are much more dense

45. Finally, the SOLAR WIND • small amount of matter boiled off CORONA • Typical speed, 500 km/s (roughly the Sun's escape velocity) • Mostly protons, Helium nuclei and electrons • Continually hitting earth's magnetosphere; usually only a small fraction penetrates and reaches the Earth’s atmosphere

46. SOLAR ACTIVITY • Spectacular activity: PROMINENCES, FLARES and CORONAL MASS EJECTIONS • These can extend to 100,000 km or more into the corona. • Typically large amounts of matter following magnetic field lines. • Big flares yield lots of COSMIC RAYS (mostly protons) moving close to the speed of light. • Cosmic Rays can penetrate to the earth's atmosphere, yielding spectacular auroral displays, power grid failures and disrupted communications.

47. Solar Prominences UV image from SOHO Cooler (dark) and hotter (bright) emissions from TRACE. The big prominence is over 100,000 km long

48. Solar Flares • More powerful than prominences, flares are explosions that only take a few minutes to erupt; gas escapes from magnetic confinement • Spots (visible) +photosphere (UV) +magnetic loops (EUV)

49. Solar Flares & Multiwavelength Activity X-ray, UV and EUV Sun

50. Coronal Mass Ejections & Coronal HolesSOHO Yohkoh