16. Our Star, the Sun

1. 16. Our Star, the Sun • Sun data • Hydrogen fusion produces the Sun’s energy • Sun’s energy moves from core to surface • Sunquakes give information about the interior • The problem of the missing solar neutrinos • Photosphere: The 1st atmospheric layer • Chromosphere: The 2nd atmospheric layer • Corona: The 3rd atmospheric layer • Sunspots are relatively cool magnetic storms • Sunspots exhibit a 22-year cycle • Other magnetic effects on the Sun

2. Sun Data (Table 16-1)

3. The Sun Is An “Average” Star • The usual descriptions • The Sun’s diameter is about midway • The Sun’s mass is about midway • The Sun’s surface temperature is about midway • The Sun’s chemical composition is about midway • More accurate descriptions • Sun is in the middle of possible star masses • Least massive star can be ~ 0.08 x the Sun’s mass • Most massive star can be ~ 110 x the Sun’s mass • Sun is in the middle of possible star luminosities • Least luminous star can be ~ 10-4 x the Sun’s luminosity • Most luminous star can be ~ 10 6 x the Sun’s luminosity About 95% of all stars are less massive than the Sun

4. Some Important Concepts • Reflection vs. Emission • Planets, asteroids & comets shine byreflectinglight • Sunshines byemitting light • Luminosity • Total energy emitted per second • For the Sun, this is 3.9 . 1026 watts • For the Sun, this is 3.9 . 1026 joules . sec-1 • The Sun’s spectrum • This is very nearly perfect blackbody radiation

5. Hydrogen Fusion Makes the Sun’s Energy • Early speculation about Sun’s energy source • Chemical combustion • Each combusting atom releases ~ 10-19 joules • This would require ~ 3.9 . 1045 atoms . sec-1 • The Sun contains ~ 1057 atoms • Sun could produce chemical energy for ~ 3.0 . 1011 sec • Sun would burn itself out in only 10 thousand years • Modern understanding of Sun’s energy source • Einstein’s Special Theory or Relativity • e = m . c2 • c [speed of light] is a very large number, so c2 is huge • 4 hydrogen atoms fuse into 1 helium atom • Mass lost per helium atom = 4.8 . 10-26 g Only ~ 0.7% = 4.3 . 10-12 joules • Sun converts ~ 6.0 . 1011 kg . sec-2 of H2 into He • Sun will exhaust core hydrogen after ~ 10 billion years

6. Critical Terminology • Misleading astronomical terminology • Hydrogen burning • Misleading because it implies chemical combustion • Precise scientific terminology • Hydrogen fusion • Not misleading because it clearly indicates nuclear fusion Thus: Always use “hydrogen fusion” Never use “hydrogen burning”

7. Hydrogen Fusion: Deuterium Synthesis

8. Hydrogen Fusion: 3He Synthesis

9. Hydrogen Fusion: 4He Synthesis

10. Energy Moves from Core to Surface • Extremely high density ~ 1.6 . 105 kg . m-3 • ~ 14 times as dense as lead • Extremely high pressure ~ 3.4 . 1011atmos. • ~ 340 billion times as dense as Earth’s atmosphere • Extremely high temperature > 1.0 . 107 K • Hydrostatic equilibrium • Long-term pressure stability inside the Sun • Downward force = Upward force • Thermal equilibrium • Long-term temperature stability inside the Sun • Heat generation rate = Heat escape rate • Heat travels from hot areas to cool areas

11. Hydrostatic Equilibrium In Water

12. Hydrostatic Equilibrium In the Sun

13. Generic Heat Transport Mechanisms • Conduction • Energy transfer by contact between adjacent atoms • Atoms vibrate around an essentially fixed location • Relatively inefficient in the Sun because it is gaseous • Convection • Energy transfer by circulation of atoms • Atoms can move great distances • Relatively efficient where pressure is relatively low • Radiative diffusion • Energy transfer by photon absorption & re-emission • The Sun’s gases are dense enough to permit this • Relatively efficient where pressure is relatively high

14. Sun’s Heat Transport Mechanisms • Computer models of the Sun’s interior • Different models use same physics equations • Different models use different assumptions • All models produce nearly identical internal structures • Accepted model for the Sun’s heat transport • Deepest regions • Radiative diffusion is dominant heat transport mechanism • The core ~ 25% the Sun’s radius • Intermediate regions • Radiative diffusion is dominant heat transport mechanism • The radiative zone ~ 71% the Sun’s radius • Shallowest regions • Convection is dominant heat transport mechanism • The convective zone 100% the Sun’s radius • ~ 170,000 years for a photon to escape the Sun

15. Sun’s Interior Physical Properties

16. The Sun’s Internal Structure

17. Seismic Waves Probe Sun’s Interior • The Sun vibrates at many frequencies • Discovered by Robert Leighton of Cal Tech in 1960 • Extremely high-precision Doppler shift analyses • Many possibilities exist • Move ~ 10 meters every 5 minutes ~ 3.3 mm . sec-1 ~ 0.003 hertz [cycles . sec-1] ~ 13 octaves lower than humans can hear • Longer periods from 20 to 160 minutes • The science of helioseismology • Sunquakes comparable to earthquakes • Substantial evidence regarding the Sun’s interior • Set limits on the amount of He in Sun’s core & convective zone • Estimate layer thickness between radiative & convective zones • Convective zone is thicker than computer models predict

18. The Vibrating Sun

19. The Missing Neutrino Problem • Hydrogen fusion releases abundant neutrinos • Abundant energy ~ 1.0 . 1038 neutrinos . sec-1 produced by solar H fusion ~ 1.0 . 1014 neutrinos . sec-1penetrate every m2 of Earth • No electrical charge • Little or no mass < 1.0 . 10-4 times the mass of an electron Travel very slightly slower than light in vacuum • Extremely little interaction with matter • Neutrino detectors • 3 detector types search for different phenomena • Brookhaven National Lab ~ 35% of expected value • GALLEX & SAGE ~ 55% of expected value • Kamiokande ~ 45% of expected value

20. Interpretation of Neutrino Flux • Our models of the Sun’s interior are wrong • The Sun’s core is cooler than predicted by models • 10% temperature reduction would account for neutrinos • 10% temperature reduction would reduce Sun’s diameter • Our understanding of neutrinos is wrong • Neutrinos may behave in unpredicted ways • There are actually three kinds of neutrinos • Only one kind of neutrino is produced in the Sun • Detectors are designed to detect only this kind of neutrino • If neutrinos change types, an answer might be at hand • Super Kamiokande • Neutrino oscillation may take place • Super Kamiokande was severely damaged in 12 November 2001 • Super Kamiokande was back in full operation in June 2006

21. Photosphere: 1st Atmospheric Level • The Sun has no solid surface • Gas pressure decreases smoothly moving outward • A 400 km thick layer of the Sun is “visible” • This is only 0.057% the Sun’s radius • This seems like a very well defined surface • Limb darkening results • Density is ~ 10-4 times Earth’s atmospheric pressure • The photosphere approximates a blackbody • Produces a continuous spectrum Hot high-density • Temperature of ~ 5,800 K • Produces an absorption spectrum Cool low-density • Fraunhofer lines produced by coolest upper photosphere • Temperature of ~ 4,400 K ~ 24% drop • Not “cool” by Earth standards • Very “cool” by Sun standards

22. Fraunhofer Lines in the Solar Spectrum

23. Solar Photosphere: “Light” Sphere

24. The Quiet Sun: Granulation • Small-scale convection in the photosphere • The convection is broken up into small cells • Granules average ~ 1,000 km in diameter • The size of Texas + Oklahoma • Temp. drops ~ 300 K from center to edge of a granule • The center is ascending, the edge is descending • Granules last only a few minutes • The Sun has ~ 4 million granules at any one time

25. Granules & Photosphere Convection

26. The Quiet Sun: Supergranules • Medium-scale convection in the photosphere • Another scale of convection is superimposed • Supergranules are similar to granules • Supergranules average ~ 35,000 km in diameter • Supergranules last about one day

27. Solar Supergranule Convection

28. Chromosphere: 2nd Atmospheric Level • Chromosphere means “sphere of color” • Invisible under ordinary viewing conditions • Density is ~ 10-8x Earth’s atmospheric pressure • Chromosphere is dominated by emission lines • Characteristic of hot low-density gases • The Ha line at 656.3 nm is very strong • An electron falls from the n = 3 to the n = 2 energy level • Color is a very vibrant red • Color appears pink during a solar eclipse • Gas density is very low, so there are very few atoms emitting • Hafilter makes chromosphere visible without an eclipse • Chromosphere temperature increases with altitude • Lowest chromosphere level is ~ 4,400 K • Highest chromosphere level is ~ 25,000 K

29. Chromosphere: Color Emission

30. The Chromosphere’s Spicules • The chromosphere has many tall spikes • Rapidly rising gas jets • ~ 20 km . sec-1 ~ 45,000 mph • Typical spicules last ~ 15 minutes • ~ 300,000 spicules exist at any one time • They cover only ~ 1% of the Sun’s surface • Typical spicules form at supergranule edges

31. Corona: 3rd Atmospheric Level • Corona means “crown” • Invisible under ordinary viewing conditions • ~ 1.0 . 10-6 times as bright as the photosphere • This is the brightness of the full moon • Visible during a total solar eclipse • The corona is dominated by emission lines • Extremely unusual emission lines • Green line at 530.3 nm is from highly ionized iron • 13 of 26 electrons have been stripped away • Temperature > 2.0 . 106 K • Heated by magnetic energy from the photosphere

32. The Corona & the Solar Wind • The Sun’s atmosphere • Retained by the Sun’s extremely strong gravity • The Sun’s escaping matter • Temperatures are extremely high • Very high speeds of atoms & molecules • ~ 1 million km .hr-1 • Statistically, some will exceed escape velocity • This is the solar wind

33. The Solar Corona: The “Crown”

34. Solar Upper Atmosphere Temperatures

35. An X-Ray View of the Sun

36. Sunspots Basics • The active Sun • Several features that vary considerably over time • Sunspots • These are one type of active Sun feature • Irregular dark regions imbedded in the photosphere • Typically several thousand kilometers in diameter • Typically last a few hours to a few months • Typical sunspots have two parts • Umbra Cool central part Appears red • ~ 4,300 K ~ 30% as much energy as the photosphere • Penumbra Warm outer part Appears orange • ~ 5,000 K ~ 55% as much energy as the photosphere

37. Sunspots Imaged Close-Up A mature sunspot Overlapping sunspots

38. Sunspot Movement

39. The Sunspot Cycle: A First Look • Heinrich Schwabe1843 • Had observed many years of sunspot activity • Sunspots vary cyclically in number • Times of minimal sunspots • Recently 1976 1986 1996 2010 • Times of abundant sunspots • Recently 1978 1989 2000 2013? • Sunspots vary cyclically in latitude • Times of minimal sunspots • Sunspots first appear at ~ 30° latitude on the Sun • Times of abundant sunspots • Sunspots migrate to < 5° latitude on the Sun

40. Sunspots: Cool Magnetic Storms • George Ellery Hale 1908 • Discovered that sunspots are magnetic storms • Basic scientific principles • Zeeman effect Magnetic fields can split spectral lines Magnitude depends on field strength • Plasma At least some atoms are ionized Moving plasma generate magnetic fields Plasma can be deflected by the Sun • Basic observations • Large sunspots have strong magnetic fields • Opposite sunspot polarities in opposite hemispheres • Basic tool • Magnetograms show sunspot polarities • Leading sunspots in the N hemisphere have one polarity • Leading sunspots in the S hemisphere have opposite polarity

41. Sunspots: Strong Magnetic Fields

42. Mapping the Sun’s Magnetic Field

43. Sunspots Exhibit a 22-Year Cycle • The visual sunspot cycle • Repeats on an approximately 11 year cycle • The magnetic sunspot cycle • Repeats on an approximately 22 year cycle • Sun’s North Pole is magnetic north for 11 years • Sun’s North Pole is magnetic south for 11 years • A proposed cause • The magnetic-dynamo model • Sun’s differential axial rotation stretches magnetic field • They eventually stretch so far that they “snap” • Some anomalies • Extremely few sunspots from 1645 to 1715 • Europe’s “Little Ice Age” & western U.S. severe drought • Excessive sunspots during 11th & 12th centuries • Earth was warmer than it is today

44. The Sunspot Cycle

45. Babcock’s Magnetic Dynamo

46. Rotation Rates in the Solar Interior

47. Other Magnetic Effects on the Sun • Magnetic heating of the corona • Twisting & short-circuiting magnetic loops • Other magnetic phenomena • Plages • Chromosphere precursors of many sunspots • Filaments • Relatively cool dark streaks in the chromosphere • Prominences • Filaments seen against a dark background seem bright • Flares • Eruptive events associated with major sunspots • Coronal mass ejections (CME’s) • ~ 1.0 . 1012 kg of gas ejected into space at high speeds • Super-sized versions of solar flares • Cause aurorae in Earth’s atmosphere

48. Magnetic Heating of the Sun’s Corona

49. A Solar Prominence

50. A Coronal Mass Ejection (CME) Before the CME Early stages 16 minutes after (b)