Chapter 08

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Published on November 12, 2007

Author: Cinderella

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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).:  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). The Sun – Our Star:  The Sun – Our Star Chapter 8 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. Guidepost 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 Outline Outline (continued):  III. Nuclear Fusion in the Sun A. Nuclear Binding Energy B. Hydrogen Fusion C. The Solar Neutrino Problem Outline (continued) General Properties:  General Properties Average star Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years) Central temperature = 15 million 0K 333,000 times Earth’s mass 109 times Earth’s diameter Consists entirely of gas (av. density = 1.4 g/cm3) Only appears so bright because it is so close. Spectral type G2 Surface temperature = 5800 0K Very Important Warning::  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. The Solar Atmosphere:  The Solar Atmosphere Heat Flow Solar interior Temp. incr. inward The Photosphere:  Apparent surface layer of the sun The Photosphere The solar corona Depth ≈ 500 km Temperature ≈ 5800 oK Highly opaque (H- ions) Absorbs and re-emits radiation produced in the solar interior Energy Transport in the Photosphere:  Energy Transport in the Photosphere Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through Convection: Bubbles of hot gas rising up Cool gas sinking down ≈ 1000 km Bubbles last for ≈ 10 – 20 min. Granulation:  Granulation … is the visible consequence of convection The Chromosphere:  The Chromosphere Chromospheric structures visible in Ha 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 oK to ≈ 10,000 oK, then jumps to ≈ 1 million oK Transition region Filaments The Chromosphere (2):  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. The Layers of the Solar Atmosphere:  The Layers of the Solar Atmosphere Visible Photosphere Ultraviolet Chromosphere Coronal activity, seen in visible light Corona Sun Spot Regions The Magnetic Carpet of the Corona:  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 The Solar Wind:  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) Helioseismology:  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! Sun Spots:  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! Sun Spots (2):  Sun Spots (2) Active Regions Visible Ultraviolet Face of the Sun:  Face of the Sun Solar Activity, seen in soft X-rays Magnetic Fields in Sun Spots:  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 Sun Spots (3):  Sun Spots (3) Magnetic field in sun spots is about 1000 times stronger than average. In sun spots, magnetic field lines emerge out of the photosphere. Magnetic North Poles Magnetic South Poles Magnetic Field Lines:  Magnetic Field Lines Magnetic North Pole Magnetic South Pole Magnetic Field Lines Star Spots?:  Star Spots? Other stars might also have sun spot activity: Image constructed from changing Doppler shift measurements The Solar Cycle:  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 The Solar Cycle (2):  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. The Sun’s Magnetic Dynamo:  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. Magnetic Loops:  Magnetic Loops Magnetic field lines The Sun’s Magnetic Cycle:  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 The Maunder Minimum:  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: Magnetic Cycles on Other Stars:  Magnetic Cycles on Other Stars H and K line emission of ionized Calcium indicate magnetic activity also on other stars. Prominences:  Prominences Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops Relatively cool gas (60,000 – 80,000 oK) May be seen as dark filaments against the bright background of the photosphere Eruptive Prominences:  Eruptive Prominences (Ultraviolet images) Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis). Space Weather:  Space Weather Solar Aurora Sound waves produced by a solar flare ~ 5 minutes Coronal mass ejections Coronal Holes:  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. Energy Production:  Energy Production Energy generation in the sun (and all other stars): 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. Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational Energy Generation in the Sun: The Proton-Proton Chain:  Energy Generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1H  4He + energy 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. Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity. T ≥ 107 0K = 10 million 0K The Solar Neutrino Problem:  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. Davis solar neutrino experiment 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. 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 New Terms 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. Discussion Questions Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  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. Quiz Questions:  Quiz Questions 11. What is responsible for the Sun's surface and atmospheric activity? a. The Sun's magnetic field. b. Many comets impacting the Sun. c. Gravitational contraction of the Sun. d. The Sun sweeping up interstellar space debris. e. Gravitational interactions between the Sun and the planets. Quiz Questions:  Quiz Questions 12. What is the source of the Sun's changing magnetic field? a. The differential rotation of the Sun. b. Convection beneath the photosphere. c. The Sun's large iron core. d. Both a and b above. e. Both a and c above. Quiz Questions:  Quiz Questions 13. What evidence do we have that sunspots are magnetic? a. The spectral lines of sunspots are split by the Zeeman Effect. b. Observations show that the north pole and south pole sunspots attract one another and move closer together over time. c. Observations at far ultraviolet show material arched above the Sun's surface from one sunspot to another. d. Both a and b above. e. Both a and c above. Quiz Questions:  Quiz Questions 14. Which active feature in the Sun's atmosphere, seen from a different point of view, corresponds precisely to the dark filaments that are observed with an hydrogen alpha filter? a. A sunspot. b. A solar flare. c. A prominence. d. A spicule. e. A coronal hole. Quiz Questions:  Quiz Questions 15. What does a Maunder Butterfly Diagram show? a. During the 11-year sunspot cycle, the spots begin at high latitude and then form progressively closer to the equator. b. Between the years 1645 and 1715 the low activity on the Sun correlates with the Little Ice Age. c. The Sun's magnetic field is simple at the beginning of a sunspot cycle and grows progressively more complex due to differential rotation. d. Planetary nebulae do not all have spherical symmetry. e. When a butterfly flaps its wings in Brazil it affects the climate worldwide. Quiz Questions:  Quiz Questions 16. How constant is the solar constant; that is, by how much has the solar constant of 1360 joules per square meter per second been observed to vary over a few years? a. About 20%. b. About 10%. c. About 5%. d. About 1%. e. About 0.1%. Quiz Questions:  Quiz Questions 17. How does the Sun maintain its energy output? a. Gravitational contraction. b. Fusion of hydrogen nuclei. c. The impact of small meteoroids. d. Coal burning in pure oxygen. e. Fission of Uranium 235. Quiz Questions:  Quiz Questions 18. Why does nuclear fusion require high temperatures? a. Protons have positive charge, and like charges repel one another. b. Two protons must get close enough together to overcome the Coulomb barrier. c. Two protons must get close enough for the strong force to bind them together. d. Both a and b above. e. All of the above. Quiz Questions:  Quiz Questions 19. What happens to the neutrinos that are produced in the proton-proton chain? a. They collide immediately with other particles, thus adding to the gas pressure that supports the Sun against gravitational contraction. b. They combine with antineutrinos and form a pair of gamma rays. c. They head out of the Sun at nearly the speed of light. d. They are blocked by the Coulomb barrier and remain inside the Sun. e. They spiral out along magnetic field lines to become cosmic rays. Quiz Questions:  Quiz Questions 20. What solved the solar neutrino problem? a. The discovery that neutrinos oscillate between three different types. b. The standard model of energy production within the Sun was modified. c. It was discovered that electron neutrinos do not penetrate rock as easily as expected. d. Some of the radioactive argon gas was found leaking out of the neutrino detector undetected. e. The finding that chlorine does not interact with electron neutrinos as predicted. Answers:  Answers 1. d 2. e 3. a 4. c 5. b 6. a 7. d 8. b 9. e 10. d 11. a 12. d 13. e 14. c 15. a 16. e 17. b 18. e 19. c 20. a

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