1Our Star, the SunChapter EighteenGuiding Questions1. What is the source of the Sun’s energy?2. What is the internal structure of the Sun?3. How can astronomers measure the properties of the Sun’s interior?4. How can we be sure that thermonuclear reactions are happening in the Sun’s core?5. Does the Sun have a solid surface?6. Since the Sun is so bright, how is it possible to see its dim outer atmosphere?7. Where does the solar wind come from?8. What are sunspots? Why do they appear dark?9. What is the connection between sunspots and the Sun’s magnetic field?10. What causes eruptions in the Sun’s atmosphere?An Overview of the DetailsThe Sun’s energy is generated by thermonuclearreactions in its core• The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E = mc2• Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 107K• In the Sun, fusion occurs only in the dense, hot core The Sun’s energy is produced by hydrogen fusion, not in a single step, but in a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus2A theoretical model of the Sun shows how energygets from its center to its surface • Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius• The core is surrounded by a radiative zone extending to about 0.71 solar radius– In this zone, energy travels outward through radiativediffusion• The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure– In this zone, energy travels outward primarily through convectionSolar Model ResultsUnderstanding Hydrostatic Equilibrium Understanding Hydrostatic Equilibrium II3How do we know about the solar interior?• Helioseismology is the study of how the Sun vibrates• These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun By using the Sun’s own vibrationsMore Model ResultsA Subatomic InterludeA Subatomic Interlude II A Subatomic Interlude III4A Subatomic Interlude IIIIA Subatomic Interlude V• Neutrinos are produced in the “Weak Interaction”, for example– Neutrinos from the earth• natural radioactivity– “Man-made” neutrinos• accelerators, nuclear power plants.– Astrophysical neutrinos• Solar neutrinos• Atmospheric neutrinos• Relic neutrinos– left over from the big bang.Neutrino DetectionDetecting neutrinos requires a differentkind of a detector.Neutrino Factoids• The earth receives about 40 billion neutrinos per second per cm2from the sun.– About 100 times that amount are passing through us from the big bang.• This works out to about 330 neutrinos in every cm3of the universe!• By comparison there are about 0.0000005 protons per cm3in the universe.• Our body emits about 340 million neutrinos per day from 40K.• Neutrinos don’t do much when passing through matter.• Remember, it is very difficult to observe neutrinos.Neutrino Detection II• The neutrino is observed by detecting the product of its interaction with matter.νeElectronνμMuonNeutrinos reveal information about the Sun’score—and have surprises of their own• Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected• Recent neutrino experiments explain why this is so5The Photosphere -the lowest of three main layers in the Sun’s atmosphere• The Sun’s atmosphere has three main layers– the photosphere– the chromosphere– the corona• Everything below the solar atmosphere is called the solar interior• The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphereThe spectrum of the photosphere is similar to that of a blackbody at a temperature of 5800 KThe Sun is a sphere, although it appears as a disk.This leads to a phenomenon known as limb darkening.Convection in the photosphere produces granulesMoreConvectionThe Chromosphere -characterized by spikes of rising gas• Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere• Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranulesCrossSectionalView of theSolarAtmosphere6•The solar corona blends into the solar wind at great distances from the SunThe Corona –outermost layer of the solar atmosphere, made of very high-temperature gases at extremely low densityThe corona ejects mass into space to form the solar windActivity in the corona includes coronal mass ejections and coronal holesSunspots -low-temperature regions in the photosphere7Sunspot Cycle -Sunspots on the moveSunspots are produced by a 22-year cyclein the Sun’s magnetic field• The Sun’s surface features vary in an 11-year cycle – the sunspot cycle• The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next• This is related to a 22-year cycle (the solar cycle) in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity• Two sunspot cycles make up one 22-year solar cycleThe magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic fieldThe solar magnetic changes are caused by convection and the Sun’s differential rotation8Rotation of the Solar InteriorThe Sun’s magnetic field also produces otherforms of solar activity • A solar flare is a brief eruption of hot, ionized gases from a sunspot group• A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona9Key Words• 22-year solar cycle• chromosphere•CNO cycle• conduction• convection• convective zone• corona• coronal hole• coronal mass ejection• differential rotation• filament• granulation• granule• helioseismology• hydrogen fusion• hydrostatic equilibrium• limb darkening• luminosity (of the Sun)• magnetic-dynamo model• magnetogram• magnetic reconnection• negative hydrogen ion• neutrino• neutrino oscillation• photosphere•plage•plasma• positron•