The Sun – Our Star

1. Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

2. The Sun – Our Star Chapter 8

3. Guidepost 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.

4. Outline I. The Solar Atmosphere A. Heat Flow in the Sun B. The Photosphere C. The Chromosphere D. The Solar Corona E. Helioseismology II. Solar Activity A. Sunspots and Active Regions B. The Sunspot Cycle C. The Sun's Magnetic Cycle D. Magnetic Cycles on Other Stars E. Chromospheric and Coronal Activity F. The Solar Constant

5. Outline (continued) III. Nuclear Fusion in the Sun A. Nuclear Binding Energy B. Hydrogen Fusion C. The Solar Neutrino Problem

6. General Properties • Average star • Spectral type G2 • Only appears so bright because it is so close. • Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) • 109 times Earth’s diameter • 333,000 times Earth’s mass • Consists entirely of gas (av. density = 1.4 g/cm3) • Central temperature = 15 million 0K • Surface temperature = 5800 0K

7. Very Important Warning: Never look directly at the sun through a telescope or binoculars!!! This can cause permanent eye damage – even blindness. Use a projection technique or a special sun viewing filter.

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

9. The Photosphere • Apparent surface layer of the sun • Depth ≈ 500 km • Temperature ≈ 5800 oK • Highly opaque (H- ions) • Absorbs and re-emits radiation produced in the solar interior The solar corona

10. Energy Transport in the Photosphere Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through Convection: Cool gas sinking down Bubbles of hot gas rising up ≈ 1000 km Bubbles last for ≈ 10 – 20 min.

11. Granulation … is the visible consequence of convection

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

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

14. The Layers of the Solar Atmosphere Ultraviolet Visible Sun Spot Regions Photosphere Corona Chromosphere Coronal activity, seen in visible light

15. The Magnetic Carpet of the Corona • Corona contains very low-density, very hot (1 million oK) gas • Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”) Computer model of the magnetic carpet

16. The Solar Wind Constant flow of particles from the sun. Velocity ≈ 300 – 800 km/s • Sun is constantly losing mass: 107 tons/year (≈ 10-14 of its mass per year)

17. Energy Production Energy generation in the sun (and all other stars): Binding energydue to strong force= on short range, strongest of the 4 known forces:electromagnetic, weak,strong,gravitational Nuclear Fusion = fusing together 2 or more lighter nuclei to produce heavier ones. Nuclear fusion can produce energy up to the production of iron; For elements heavier than iron, energy is gained by nuclear fission. proton proton animation

18. Energy Generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1H 4He + energy Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). T ≥ 1070K = 10 million 0K 4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He. • Energy gain = Dm*c2 = 0.43*10-11 J per reaction. Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

19. The Solar Neutrino Problem The solar interior can not be observed directly because it is highly opaque to radiation. But neutrinos can penetrate huge amounts of material without being absorbed. Early solar neutrino experiments detected a much lower flux of neutrinos than expected (the “solar neutrino problem”). Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem. Davis solar neutrino experiment

20. Helioseismology The solar interior is opaque (i.e. it absorbs light) out to the photosphere. • Only way to investigate solar interior is through Helioseismology = analysis of vibration patterns visible on the solar surface: Approx. 10 million wave patterns!

21. Sun Spots Cooler regions of the photosphere (T ≈ 4240 K). Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky!

22. Sun Spots (2) Active Regions Visible Ultraviolet

23. Face of the Sun Solar Activity, seen in soft X-rays

24. Magnetic Fields in Sun Spots Magnetic fields on the photosphere can be measured through the Zeeman effect Sun Spots are related to magnetic activity on the photosphere

25. Sun Spots (3) Magnetic field in sun spots is about 1000 times stronger than average. Magnetic North Poles Magnetic South Poles In sun spots, magnetic field lines emerge out of the photosphere.

26. Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic Field Lines

27. Star Spots? Image constructed from changing Doppler shift measurements Other stars might also have sun spot activity:

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

29. The Solar Cycle (2) Maunder Butterfly Diagram Sun spot cycle starts out with spots at higher latitudes on the sun Evolve to lower latitudes (towards the equator) throughout the cycle.

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

31. Magnetic Loops Magnetic field lines

32. 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

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

34. Magnetic Cycles on Other Stars H and K line emission of ionized Calcium indicate magnetic activity also on other stars.

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

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

37. Space Weather ~ 5 minutes Solar Aurora Sound waves produced by a solar flare Coronal mass ejections

38. 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.

39. New Terms sunspot granulation convection supergranule limb limb darkening transition region filtergram filament spicule coronagraph magnetic carpet solar wind helioseismology active region Zeeman effect Maunder butterfly diagram differential rotation dynamo effect Babcock model prominence flare reconnection aurora coronal hole coronal mass ejection (CME) solar constant Maunder minimum weak force strong force nuclear fission nuclear fusion Coulomb barrier proton–proton chain deuterium neutrino

40. Discussion Questions 1. What energy sources on Earth cannot be thought of as stored sunlight? 2. What would the spectrum of an auroral display look like? Why? 3. What observations would you make if you were ordered to set up a system that could warn astronauts in orbit of dangerous solar flares? Such a warning system exists.

41. Quiz Questions 1. What effect does the formation of negative hydrogen ions in the Sun's photosphere have on solar observations? a. We can view the Sun's interior through special filters set to the wavelength of the absorption lines created by such ions. b. Concentrations of such ions form sunspots that allow us to track solar rotation. c. It divides the Sun's atmosphere into three distinct, easily observable layers. d. The extra electron absorbs different wavelength photons, making the photosphere opaque. e. These ions produce the "diamond ring" effect that is seen during total solar eclipses.

42. Quiz Questions 2. What evidence do we have that the granulation seen on the Sun's surface is caused by convection? a. The bright centers of granules are cooler than their dark boundaries. b. The bright centers of granules are hotter than their dark boundaries. c. Doppler measurements indicate that the centers are rising and edges are sinking. d. Both a and c above. e. Both b and c above.

43. Quiz Questions 3. Which layer of the Sun's atmosphere contains the cooler low density gas responsible for absorption lines in the Sun's spectrum? a. The photosphere. b. The chromosphere. c. The corona. d. The solar wind. e. All of the above.

44. Quiz Questions 4. Which of the following is true about granules and supergranules? a. They are both about the same size. b. Granules and supergranules each fade in about 10 to 20 minutes. c. They are both due to convection cells in layers below. d. Both a and c above. e. Both b and c above.

45. Quiz Questions 5. What is revealed by observing the Sun at a very narrow range of wavelengths within the 656-nanometer hydrogen alpha line? a. The structure of the photosphere. b. The structure of the chromosphere. c. The structure of the corona. d. We can see the electrons make the transition from energy level 3 to level 2. e. Nothing is seen; all light is absorbed at this wavelength.

46. Quiz Questions 6. What are the general trends in temperature and density from the photosphere to the chromosphere to the corona? a. The temperature increases and density decreases. b. The temperature increases and density increases. c. The temperature decreases and density decreases. d. The temperature decreases and density increases. e. The temperature and density remain constant.

47. Quiz Questions 7. What physical property of the Sun is responsible for "limb darkening"? a. The chromosphere is hotter than the photosphere. b. The chromosphere is cooler than the photosphere. c. The lower photosphere is cooler than the upper photosphere. d. The lower photosphere is hotter than the upper photosphere. e. Both a and d above.

48. Quiz Questions 8. The spectrum of the corona has bright spectral lines of highly ionized elements. What does this reveal? a. The corona is a very hot, high density gas. b. The corona is a very hot, low density gas. c. The corona is very irregular in shape. d. The corona extends out to 20 solar radii. e. Both b and d above.

49. Quiz Questions 9. What heats the chromosphere and corona to high temperatures? a. Long-wavelength electromagnetic radiation emitted by layers below. b. Visible light emitted by layers below. c. Short-wavelength electromagnetic radiation emitted by layers below. d. Sungrazing comets, giving up their energy of motion as they vaporize in these two layers. e. Fluctuating magnetic fields from below that transport energy outward.

50. Quiz Questions 10. How are astronomers able to explore the layers of the Sun below the photosphere? a. Short-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun. b. Long-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun. c. Highly reflective space probes have plunged below the photosphere and sampled the Sun's interior. d. By measuring and modeling the modes of vibration of the Sun's surface. e. By observing solar X-rays and gamma rays with space telescopes. These shorter wavelengths are emitted from hotter regions below the photosphere.