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2. Slide 2 The Sun – Our Star Chapter 8

3. Slide 3 The preceding chapter described how we can get information from a spectrum. In this chapter, we apply these techniques to the sun, to learn about its complexities. This chapter gives us our first close look at how scientists work, how they use evidence and hypothesis to understand nature. Here we will follow carefully developed logical arguments to understand our sun. Most important, this chapter gives us our first detailed look at a star. The chapters that follow will discuss the many kinds of stars that fill the heavens, but this chapter shows us that each of them is both complex and beautiful; each is a sun. Guidepost

4. Slide 4 Summary of General Properties Average star Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 10 pc or 32.6 light years) Central temperature = 15 million 0 K 333,000 times Earth’s mass 109 times Earth’s diameter Consists entirely of gas (av. density = 1.4 g/cm 3 ) Only appears so bright because it is so close. Spectral type G2 Surface temperature = 5800 0 K

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6. Slide 6 The Sun: applying black-body radiation laws What we see: Radius R = 700,000 km Distance = 1 AU = 1.5x10 8 km Mass M = 2x10 30 kg Yellow light: ~ 520 nm (G2) Maximum of the black-body spectrum: (Wien’s law) Surface temperature T  5800 K (Class G2) Total radiated power (luminosity) L =  T 4 4  R 2  4  10 26 W The Stefan-Boltzmann law Solar flux at Earth: 1360 W/m 2

7. Slide 7 What we want to know: Internal structure and composition Source of energy Lifetime Origin of Sun’s activity and variability

8. Slide 8 The Composition of Stars From the relative strength of absorption lines one can infer the composition of stars.

9. Slide 9 Stars are gaseous spheres held together by gravity Mean density Higher than the density of water! Central density is 160 g/cm 3 : 15 times higher than the density of lead Still, stars are gaseous due to very high temperatures Central temperature T c  1.5  10 7 K Matter is in the state of plasma (ionized gas) Kinetic energy of particles is much larger than their potential energy

10. Slide 10 What defines an internal structure? Central temperature T c  1.5  10 7 K Surface temperature T c  5800 K Heat transfer from the center to the surface! Heat transfer determines the internal structure Energy generated in the sun’s center must be transported outward.

11. Slide 11 Heat transfer mechanisms Conduction Convection Radiation

12. Slide 12 Convection

13. Slide 13 Conduction, Convection, and Radiation (SLIDESHOW MODE ONLY)

14. Slide 14 Internal Structure Temperature, density and pressure decreasing Energy generation via nuclear fusion Energy transport via radiation Energy transport via convection Flow of energy Basically the same structure for all stars with approx. 1 solar mass or less. Sun

15. Slide 15 Energy Transport in the Sun-like stars Energy generated in the star’s center must be transported to the surface. Inner layers: Radiative energy transport Outer layers (including photosphere): Convection Bubbles of hot gas rising up Cool gas sinking down Gas particles of solar interior  -rays

16. Slide 16 Convection Bubbles of hot gas rising up Cool gas sinking down ≈ 1000 km Bubbles last for ≈ 10 – 20 min. Convection is the most efficient way to transport heat

17. Slide 17 Granulation … is the visible consequence of convection

18. Slide 18 It takes 10,000 years for a photon emitted in the core to reach the surface! The solar atmosphere

19. Slide 19 The Solar Atmosphere Heat Flow Solar interior Temp. incr. inward Only visible during solar eclipses Apparent surface of the sun

20. Slide 20 Apparent surface layer of the sun The Photosphere The solar corona Depth ≈ 500 km Temperature ≈ 5800 o K Absorbs and re-emits radiation produced in the solar interior

21. Slide 21 The Chromosphere Chromospheric structures visible in H  emission (filtergram) Region of sun’s atmosphere just above the photosphere. Visible, UV, and X-ray lines from highly ionized gases Temperature increases gradually from ≈ 4500 o K to ≈ 10,000 o K, then jumps to ≈ 1 million o K Transition region Filaments

22. Slide 22 The Chromosphere (2) Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere. Visible in H  emission. Each one lasting about 5 – 15 min.

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24. Slide 24 The Magnetic Carpet of the Corona Corona contains very low-density, very hot (1 million o K) gas Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”). Precise mechanism is unknown. Computer model of the magnetic carpet

25. Slide 25 Changing Face of the Sun Solar Activity, seen in soft X-rays

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27. Slide 27 Sun Spots Cooler regions of the photosphere (T ≈ 4200 K). Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky!

28. Slide 28 Sun Spots (2) Active Regions Visible Ultraviolet

29. Slide 29 Sun Spots (3) Magnetic field in sun spots is about 1000 times stronger than average. Magnetic North Poles Magnetic South Poles  Sun Spots are related to magnetic activity on the photosphere

30. Slide 30 Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic Field Lines

31. Slide 31 Magnetic Loops Magnetic field lines

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33. Slide 33 Zeeman effect

34. Slide 34 The Sun’s Magnetic Dynamo This differential rotation might be responsible for magnetic activity of the sun. The sun rotates faster at the equator than near the poles.

35. Slide 35 The Sun’s Magnetic Cycle After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged.  New magnetic field structure is similar to the original one, but reversed!  New 11-year cycle starts with reversed magnetic-field orientation

36. Slide 36 11-year period of solar activity

37. Slide 37 The Solar Cycle 11-year cycle Reversal of magnetic polarity After 11 years, North/South order of leading/trailing sun spots is reversed => Total solar cycle = 22 years

38. Slide 38 The Maunder Minimum Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum The sun spot number also fluctuates on much longer time scales:

39. Slide 39 The Little Ice Age: unusually cold winters in the XVII-XVIII cen.

40. Slide 40 Prominences Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops Relatively cool gas (60,000 – 80,000 o K) May be seen as dark filaments against the bright background of the photosphere

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43. Slide 43 Eruptive Prominences (Ultraviolet images) Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis).

44. Slide 44 Solar Flares Sound waves produced by a solar flare ~ 5 minutes

45. Slide 45 Coronal Holes X-ray images of the sun reveal coronal holes. These arise at the foot points of open field lines and are the origin of the solar wind.

46. Slide 46 Coronal holes

47. Slide 47 The Solar Wind Constant flow of particles from the sun. Velocity ≈ 300 – 800 km/s  Sun is constantly losing mass: 10 7 tons/year (≈ 10 -14 of its mass per year)

48. Slide 48 Solar wind creates a hot, rarefied plasma bubble in space

49. Slide 49 Solar wind hits the Earth’s magnetosphere

50. Slide 50 Coronal mass ejections

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53. Slide 53 Aurora

54. Slide 54 Summary – solar activity It is driven by the Sun’s magnetic field The magnetic field is generated by the differential rotation of the Sun Active regions are associated with sunspots that carry a strong magnetic field (~ 1000 G) Prominence is a solar plasma trapped in magnetic field arches above the active region Violent eruptive phenomena: solar flares (equivalent to billions of H-bombs!) and coronal mass ejections Ejected plasma shakes the Earth’s magnetic field and causes the magnetic storm