ASTR 113 003 Lecture 02 Spring 2006 Feb 01 2006 Introduction To Modern Astronomy II Review Ch4 5 the Foundation Star Ch18 24 Galaxy Ch 25 27 Cosmology Ch28 39 Extraterrestrial Life Ch30 1 2 3 4 5 6 7 Sun Our star Ch18 Nature of Stars Ch19 Birth of Stars Ch20 Evolution of Stars Ch21 Death of Stars Ch22 Neutron Stars Ch23 Black Holes Ch24 ASTR 113 003 Lecture 02 Spring 2006 Feb 01 2006 Our Star the Sun Chapter Eighteen Basic 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 K Guiding Questions 1 2 3 What is the source of the Sun s energy What is the internal structure of the Sun 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 thermonuclear reactions 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 mc2 The Sun s energy is generated by thermonuclear reactions 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 core Proton 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 reaction Proton Proton Chain Reaction Step 1 Proton Proton Chain Reaction Step 2 Proton Proton Chain Reaction Step 3 Proton Proton Chain Reaction 4 H He energy neutrinos Mass 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 of 1 Hydrostatic equilibrium no expansion no contraction 2 Thermal equilibrium no temperature change with time 3 Energy transportation 1 Conduction 2 Convection 3 Radiative diffusion Hydrostatic Equilibrium Theoretical Model of the Sun Theoretical Model of the Sun From the center to the surface luminosity increases and mass increases Theoretical Model of the Sun From the center to the surface temperature decreases and density decreases Sun s Internal Structure three layers 1 Energy Core Hydrogen fusion takes place extending from the Sun s center to about 0 25 solar radius 2 Radiative Zone extending to about 0 71 solar radius In this zone energy travels outward through radiative diffusion 3 Convective Zone an opaque zone at relatively low temperature and pressure energy travels outward primarily through convection Astronomers probe the solar interior using the 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 Sun Neutrinos reveal information about the Sun s core 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 Sun The 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 interior The photosphere is the lowest of three main layers in 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 upward Photosphere 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 dimmer Convection 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 minutes Super 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 images The chromosphere sphere of color Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere Chromosphere is best seen in spectral emission lines e g H line at 656 3 nm Hydrogen level 3 2 transition Spicules jets of rising gas extend upward from the photosphere into the chromosphere along the boundaries of supergranules The Corona the outermost The corona ejects mass into space to formlayer the solar wind of the Sun s atmosphere Corona is made of very high temperature gases at extremely low density It extends to several million Km Because of hot temperature it expands into the outer space forming solar wind Corona is directly seen in EUV light which is sensitive to 1 million Kelvin plasma emission The Sun s Atmosphere The Sun s Atmosphere The Sun s Magnetism The outer corona is much hotter than the inner chromosphere and photosphere The corona must be heated by a source other than the conduction or radiative diffusion from the underlying atmosphere because the energy transfer of conduction and radiative diffusion is always from high temperature to low temperature The corona heating is related to the ubiquitous presence of magnetic field in the Sun s atmosphere Sunspots are low temperature regions in the photosphere Sunspots are shaped dark regions in the photosphere mIt appears dark because it is cooler radiate less energy Sunspot Umbra core Sunspot Penumbra the brighter border Tracking the Sun s Rotation with Sunspots 11 year sunspot cycle or solar cycle The average number of sunspots