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Lecture Slides CHAPTER 11: Our Star: The Sun. Understanding Our Universe S ECOND E DITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal. Prepared by Lisa M. Will, San Diego City College. Our Star: The Sun. Describe the structure of the Sun ’s interior and atmosphere.
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Lecture Slides CHAPTER 11: Our Star: The Sun Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College
Our Star: The Sun • Describe the structure of the Sun’s interior and atmosphere. • Explain how energy is produced and transported by the Sun. • Understand solar activity cycles.
Solar Interior • We only see the outer layers of the Sun, a G2 star. • Physics tells us about the interior. • The structure of the Sun is caused by a balance between forces due to pressure and gravity.
Solar Interior: Hydrostatic Equilibrium • The Sun must be in balance to exist for billions of years. • Hydrostatic equilibrium: outward pressure = inward force of gravity at every point in the Sun.
Solar Interior: Sun’s Surface • Density, pressure, and temperature decrease away from the center of the Sun. • Energy production peaks in the center – the core of the Sun.
Energy Production • Recall: Nuclei may consist of protons and neutrons. • Protons: positive electrical charge. • Neutrons: no electrical charge. • Protons are kept apart by electric repulsion.
Energy Production: Nuclear Fusion • The strong nuclear force binds the nucleus together. • Fusion requires ramming protons together at high speed (i.e., at high temperature). • Fusion also requires high densities to insure collisions.
Energy Production: Nuclear Fusion (Cont.) • They get close enough for the strong nuclear force to overpower electric repulsion
Energy Production: Sun’s Lifetime • The Sun has been around a long time, about 4.6 billion years. • The Sun must therefore generate a lot of energy over a long time. • Source: nuclear fusion of hydrogen to helium in the core of the Sun. • Fusion takes place in the core, where it is hot and dense enough.
More facts about the Sun • The core of the Sun extends from the center to about 20–25% of the Sun’s radius. • The Sun’s radius is 109 times bigger than the Earth’s radius! • The core of the sun has a temperature of about 15.7 million Kelvin! • The core of the sun has a density of 150 g/cm3 (but the average density of the Sun is only about 1.4g/cm3 ). Note that the density of water is 1g/cm3.
Fusion process: proton-proton chain. • Net result: 4 hydrogen nuclei turn into 1 helium nucleus, plus neutrinos, positrons and gamma rays! Positrons are just like electrons but have positive charge. Neutrinos have mass but have almost no interaction with matter—they easily go right through the Earth! • Why is energy produced?
Energy Production: Hydrogen Burning Process • Mass of 4 H nuclei is slightly greater than 1 He nucleus. It’s about 0.7% greater. • Thus, a little bit of mass is converted into energy as 4 H nuclei fuse into 1 He nucleus. • Relativity: mass and energy are equivalent: E = mc2 • Difference in mass is released as energy. • This nuclear fusion process is often called hydrogen burning. • This happens for all main-sequence stars.
Class Question • In which layer of the Sun does nuclear • fusion occur? • Convective Zone • Core • Corona • RadiativeZone
Energy Transport: Radiation Zone • Radiative zone: layer just outside of the core of the Sun. • Radiative transfer: photons travel from hotter to cooler regions.
Energy Transport: Convection Zone • Convection: rising/falling of hot/cool gas. • Convective zone: layer in between the radiative zone and the surface of the Sun.
Energy Transport • We can see evidence for the convective zone by looking at the surface of the Sun. • Convection is visible as bubbling of the surface. • Bubbles are large – the size of countries on Earth!
Energy Transport: Helioseismology • Helioseismology: sound waves move through the Sun, making surface and interior waves. • Doppler shifts give the speed of wave motion.
Energy Transport: Interior of the Sun • Speeds depend on the Sun’s composition and the depth of the convection zone. • Observations agree with models of the solar interior.
Surface • Photosphere: layer where light is emitted into space = apparent surface of the Sun. • Average temperature: 5800 K • Limb darkening: because we look through less material at the edges, it appears darker.
Atmosphere • Atmosphere: where the density drops very rapidly with increasing altitude. • The solar atmosphere is much less dense than the atmosphere of the Earth.
Atmosphere: The Solar Spectrum • Cooler outer layers of the Sun absorb some of the light from hotter, deeper layers. • This produces a complex absorption spectrum with more than 70 elements identified.
Atmosphere: Chromosphere • Chromosphere: layer directly above the photosphere. • Higher temperature than the photosphere. • Gives off a reddish emission-line spectrum due to hydrogen.
Atmosphere: Corona • Corona: layer above the chromosphere. • Very hot: T = 1 to 2 million K. • Emits X-rays. • Can extend for several solar radii. • This picture was taken during a total solar eclipse (the only time you can see the corona with your eyes)
Class Question Rank these layers of the Sun in the correct order, from coolest to hottest. Core, Corona, Photosphere Photosphere, Core, Corona Corona, Photosphere, Core Photosphere, Corona, Core
Magnetic Field • Sun’s magnetic field affects the structure of the atmosphere. • The solar wind: charged particles flowing away from the Sun through coronal holes, where magnetic field lines extend away from the Sun.
Magnetic Field: The Solar Wind Streams • Coronal material flows along the magnetic field away from the Sun. • This is the solar wind, which extends about 100 AU. • The Voyager 1 spacecraft is traveling across this boundary.
Solar Activity: Sunspots • Sunspots: cooler areas in the photosphere. • Sunspot structure: dark umbra with surrounding penumbra. • Sunspots are caused by the solar magnetic field.
Solar Activity: Sunspots (Cont.) • The number of sunspots varies as a function of time. • The latitude of sunspots also varies as a function of time.
Solar Activity: 11-year Sunspot Cycle • Sun shows an 11-year sunspot cycle. • Solar maximum: most sunspots and activity. • The Maunder minimum showed a distinct lack of sunspots between 1645 and 1715.
Class Question Why does the previous plot only date back to 1600? The Sun did not have sunspots pre-1600. There were previously too many sunspots to count. Sunspots were difficult to observe before the invention of the telescope.
Solar Activity: Magnetic Flip • The Sun’s magnetic field flips every 11 years => Solar magnetic field shows a 22 year cycle.
Solar Activity: Solar Prominences and Flares • Prominences: hot rising gas in the chromosphere constrained by magnetic fields. • Solar flares and coronal mass ejections are highly energetic eruptions.
Solar Activity: Solar Prominences and Flares (Cont.) • The explosive behavior of the Sun – prominences, flares, coronal mass ejections – is tied to the sunspot cycle. • Activity is greater at solar maximum.
