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Lecture Outlines Chapter 9 Astronomy: A Beginner’s Guide to the Universe 5th Edition Chaisson / McMillan © 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Chapter 9 The Sun Units of Chapter 9 The Sun in Bulk The Solar Interior The Solar Atmosphere The Active Sun The Heart of the Sun 9.1 The Sun in Bulk Basic Structure • • • • • • • • Radius= 696,000 km Mass= 1.99x10 Average Density = 1.4 g/cm3 71% Hydrogen (mass) 27% Helium (mass) 2% Metals (mass) Surface Temperature = 5780 K (9912°F) Core Temperature = 15,000,000 K 30 (26,993,105°F) • Rotation period= 25-36 days, faster at equator Basic Structure • Photosphere - "sphere of light", the visible surface of the Sun • Chromosphere - "sphere of color", visible during solar eclipses • Corona - the Sun's outermost atmosphere. The outflow of gas in this region is called the solar wind, which is protons and electrons that have escaped the Sun's gravity. 9.1 The Sun in Bulk Interior structure of the Sun: Outer layers are not to scale. The core is where nuclear fusion takes place. The Sun's Interior • Thermonuclear core - the central region of Sun where fusion takes place due to high temperatures and pressures. • Radiation zone - a region inside a star where energy is transported outward by the movement of photons. • Convection zone - a layer inside a star where energy is transported outward by means of heat flow through the gasses of the star (convection). 9.2 The Solar Interior Energy transport The radiation zone is relatively transparent; the cooler convection zone is opaque: 9.1 The Sun in Bulk Luminosity – total energy radiated by the Sun – can be calculated from the fraction of that energy that reaches Earth. Total luminosity is about 4 × 1026 W – the equivalent of 10 billion 1-megaton nuclear bombs per second. 9.2 The Solar Interior Mathematical models, consistent with observation and physical principles, provide information about the Sun’s interior. In hydrostatic equilibrium, inward gravitational force must be balanced by outward pressure: 9.2 The Solar Interior Solar density and temperature, according to the standard solar model: 9.2 The Solar Interior -the visible top layer of the convection zone is granulated -areas of upwelling material surrounded by areas of sinking material: 9.3 The Solar Atmosphere Spectral analysis can tell us what elements are present, but only in the chromosphere and photosphere: 9.3 The Solar Atmosphere The cooler chromosphere is above the photosphere -Difficult to see directly -Photosphere is too bright, unless Moon covers photosphere and not chromosphere during eclipse 9.3 The Solar Atmosphere Small solar storms in chromosphere emit spicules: 9.3 The Solar Atmosphere Solar corona can be seen during eclipse if both photosphere and chromosphere are blocked: 9.3 The Solar Atmosphere Corona is much hotter than layers below it – must have a heat source, probably electromagnetic interactions. 9.4 The Active Sun Sunspots: appear dark because slightly cooler than surroundings Usually occur in pairs 9.4 The Active Sun Sunspots come and go, typically in a few days. Sunspots are linked by pairs of magnetic field lines: 9.4 The Active Sun The rotation of the Sun drags magnetic field lines around with it, causing kinks 9.4 The Active Sun -11-year sunspot cycle -sunspot numbers rise (increased activity), fall, and then rise again -change in location (maximum closer to equator) 9.4 The Active Sun -really a 22-year cycle -the spots switch polarities between the N and S hemispheres every 11 years -Maunder minimum: few, if any, sunspots: 9.4 The Active Sun Areas around sunspots are active; large eruptions may occur in photosphere. Solar prominence is large sheet of ejected gas: lasts days or weeks (loop) 9.4 The Active Sun -Solar flare is a large explosion on Sun’s surface -emits a similar amount of energy to a prominence -lasts seconds, minutes, or hours 9.4 The Active Sun A coronal mass ejection emits charged particles that can affect the Earth: 9.4 The Active Sun Solar wind escapes Sun mostly through coronal holes, which can be seen in X-ray images 9.4 The Active Sun Solar corona changes along with sunspot cycle; is much larger and more irregular at sunspot peak: 9.5 The Heart of the Sun Nuclear fusion requires that like-charged nuclei get close enough to each other to fuse. (overcome EM repulsion) This can happen only if the temperature is extremely high – over 10 million K. fusion 4 Hydrogen Atoms 1 Helium Atom + 2 neutrinos gamma rays (energy) 9.5 The Heart of the Sun The process that powers most stars is a threestep fusion process. How it gets energy! Proton-proton chain reaction (Gamma rays) 4 protons → 1 helium 4 + 2 neutrinos +gamma rays 9.5 The Heart of the Sun The Sudbury neutrino observatory. Particles escaped directly from core of sun Summary of Chapter 9 • Sun is held together by its own gravity and powered by nuclear fusion • Outer layers of Sun: photosphere, chromosphere, corona. The corona is very hot. • Mathematical models and helioseismology give us a picture of the interior of the Sun • Sunspots occur in regions of high magnetic fields; darker spots are cooler Summary of Chapter 9 • Nuclear fusion converts hydrogen to helium, releasing energy • Solar neutrinos come directly from the solar core, although observations have told us more about neutrinos than about the Sun Nuclear Fusion Nuclear Fusion