Introduction To Modern Astronomy IIOur Star, the SunSlide 3Slide 4Basic FactsGuiding QuestionsThe Sun’s energy is generated by thermonuclear reactions in its coreSlide 8Proton-Proton Chain ReactionSlide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Sun’s Internal Structure: three layersAstronomers probe the solar interior using the Sun’s own vibrationsNeutrinos reveal information about the Sun’s core—and have surprises of their ownSlide 22The photosphere is the lowest of three main layers in the Sun’s atmospherePhotosphere: Limb Darkening EffectConvection in the photosphere produces granulesSlide 26The chromosphere (sphere of color)The corona ejects mass into space to form the solar windCorona is directly seen in EUV light, which is sensitive to 1 million Kelvin plasma emissionSlide 30Slide 31Slide 32Sunspots are low-temperature regions in the photosphereSlide 34Slide 35Slide 36Magnetic Field MeasurementMagnetogram: image of magnetic field22-year of solar magnetic cycleSlide 40Rotation of the SunDifferential Rotation of the Sun’s InteriorSolar Cycles are caused by the Sun’s differential rotation and convection: Babcock’s Magnetic Dynamo ModelActive Sun: Coronal LoopsSlide 45Slide 46Slide 47Slide 48Slide 49Key WordsASTR 113 – 003 Spring 2006Lecture 02 Feb. 01, 2006Review (Ch4-5): the FoundationGalaxy (Ch 25-27)Cosmology (Ch28-39)Introduction To Modern Astronomy IIStar (Ch18-24)1. Sun, Our star (Ch18)2. Nature of Stars (Ch19)3. Birth of Stars (Ch20)4. Evolution of Stars (Ch21)5. Death of Stars (Ch22)6. Neutron Stars (Ch23)7. Black Holes (Ch24)Extraterrestrial Life (Ch30)Our Star, the SunChapter EighteenASTR 113 – 003 Spring 2006Lecture 02 Feb. 01, 2006Basic Facts• Radius: 700,000 Km• Distance to Earth: 1 AU = 1.5 X 108 km • Light travel time: 8 minutes• Angular size: 30 arcmin • Effective Surface Temperature: 5800 KGuiding 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?The Sun’s energy is generated by thermonuclearreactions in its core•Sun’s total energy output: 1026 watts•Not chemical energy (only last 10,000 years)•Not gravitational contraction (only last 25 million years)•Energy from nuclear reaction–Corresponds to a reduction of mass according Einstein’s mass-energy equation E = mc2The Sun’s energy is generated by thermonuclearreactions in its core•Thermonuclear fusion occurs at very high temperatures•Hydrogen fusion occurs only at temperatures in excess of about 107 K•In the Sun, hydrogen fusion occurs in the dense, hot coreProton-Proton Chain Reaction•The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus; called proton-proton chain reactionProton-Proton Chain Reaction: Step 1Proton-Proton Chain Reaction: Step 2Proton-Proton Chain Reaction: Step 3Proton-Proton Chain Reaction4 H  He + energy + neutrinosMass of 4 H > Mass of 1 He•In every second, 600 million tons of hydrogen converts into helium to power the Sun•At this rate, the Sun can continue the hydrogen burning for more than 6 billion years.Theoretical Model of the Sun• Using physical equations to calculate the distribution of temperature, density, and pressure along the radius of the Sun from the core to the surface.•Based on physical conditions of1. Hydrostatic equilibrium: no expansion, no contraction2. Thermal equilibrium: no temperature change with time3. Energy transportation1. Conduction2. Convection3. Radiative diffusionHydrostatic EquilibriumTheoretical Model of the SunTheoretical Model of the Sun•From the center to the surface, luminosity increases, and mass increasesTheoretical Model of the Sun•From the center to the surface, temperature decreases, and density decreasesSun’s Internal Structure: three layers 1. Energy Core: Hydrogen fusion takes place, extending from the Sun’s center to about 0.25 solar radius2. Radiative Zone: extending to about 0.71 solar radius–In this zone, energy travels outward through radiative diffusion3. Convective Zone: an opaque zone at relatively low temperature and pressure–energy travels outward primarily through convectionAstronomers probe the solar interior usingthe Sun’s own vibrations •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 SunNeutrinos 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 so •Nuclear reaction is indeed occurring in the SunThe Sun’s Atmosphere•The Sun atmosphere has three main layers:1. Photosphere (400 Km thick, innermost)2. Chromosphere (2000 Km thick, above photosphere) 3. Corona (millions of Km, extended)•Everything below the solar atmosphere is called the solar interiorThe photosphere is the lowest of three main layersin the Sun’s atmosphere •The photosphere (sphere of light) is the visible surface of the Sun•It is only 400 km thick because of its opaqueness; Photons emitted below 400 km can not escape.•Temperature decreases upwardPhotosphere: Limb Darkening Effect• At the limb, one can not see as deeply as at the center• The gas at higher altitude is less hot (or lower temperature), and thus emit less energy• At the limb, appear dimmerConvection in the photosphere produces granules• Granulation is the direct evidence of convection• Each granule is about 1000 km•Granules from, disappear and reform in cycles lasting a few minutesSuper-granules in photosphere• Very large convection cell• Each super-granule is about 35,000 km• Moves slowly, lasting about one day•Hard to observe directly; better seen in Doppler