# The Sun. How are these quantities measured? Properties of the Sun. Chapter 14

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1 The Sun Chapter 14 The Role of the Sun in the Solar System > 99.9% of the mass Its mass is responsible for the orderly orbits of the planets Its heat is responsible for warming the planets It is the source of light in the Solar System Earth Properties of the Sun! Mass = 2.0x10 30 kg (333,000 Earth masses)! Diameter = 1.4x10 9 m (109 Earth Diameters)! Average Density = (Mass/Volume) = 1.4 g / cm 3! Luminosity (i.e., total power output) = 4x10 26 Watts! Surface Temperature = 5800 K! Rotation Period (at equator) = 25 days! Distance from Earth = 1 AU = 1.5 x m The Sun is an average star in almost every way How are these quantities measured? Mass (M*) and Distance (ae) Measured via radar ranging + (Venus - Earth) distance in AU, (av - ae ) P 2 = 4 2 a 3 GM (constants) 1/3 a = P 2 G M 4 2 (measured) (measured: radar) a V a E = M 1/3 1/3 G Mass 4 2 P 2/3 V P 2/3 E 1/3 a E = PE 2 G Distance M 4 2

2 How are these quantities measured? How are these quantities measured? Luminosity Measure energy / second / area received (solar constant = 1370 W/m 2 ) Distance + isotropic gives luminosity 4 x W This is 4,000,000,000,000,000,000,000, W lightbulbs A lightbulb takes up about 30 cm 2 of area Earth's area (radius = 6400 km) is 5 x cm 2 You can fit ~ 2 x lightbulbs on earth = 2 x W This is 20 million times dimmer than the Sun. Composition: Spectroscopy + Density Sun Emits in Non-Visible Wavelengths Too What Powers the Sun? T ime = Energy Luminosity = (Energy) (Energy/Second)!8

3 What Powers the Sun? Early ideas... Chemical Energy (binding energy of molecules) ~ 200 yrs Gravitational Energy ~ 10 7 yrs Too short. Something on the order of ~ yrs is needed!9 Thermonuclear Fusion Sun's energy via p-p (proton-proton) chain. Einstein: E = mc 2 Thermonuclear Fusion: combining 2 atomic nuclei at high Temperature to create new, more massive atom + release energy. Why is energy released? Mass of 4 Hydrogen = 4m proton Mass of Helium = 3.97m proton Converted to energy = 0.03 m proton Energy/Mass Book-Keeping 4 H nuclei fuse to become a 1 4 He nuclei P-P Chain However, the masses don t add up 4 H nuclei have a mass of 1 4 He nuclei has a mass of The missing link: the release of energy This energy is carried away by photons, positrons (anti electrons), and neutrinos. The efficiency of converting mass to energy is

4 Thermonuclear Fusion Fundamental Forces 4 fundamental forces in nature: High Temperature plasma Nuclei are positively charged, resulting in electromagnetic repulsion Higher temperature more kinetic energy nuclei get closer Strong nuclear force pulls them together Gravity: works over long distances, holds the Earth in orbit around the Sun and holds the Sun itself together. Strong and Weak Forces: hold nuclei together and participate in radioactive decay. Electromagnetic Force: hold atoms together. Two important points about Fusion 1) Fusion is the way by which elements heavier than hydrogen are built As stars evolve, they fuse different forms of light nuclei into heavier nuclei (such a Carbon & Iron) Two important points about Fusion, cont. 2) Hydrostatic equilibrium: the balance between the force of the (outward) radiation pressure from fusion reactions & the (inward) force of gravity is what keeps stars stable Thus, without fusion, there would be no planets like the earth

5 Two important points about Fusion, cont. 2) Hydrostatic equilibrium: the balance between the force of the (outward) radiation pressure from fusion reactions & the (inward) force of gravity is what keeps stars stable More on Fusion... We can create fusion on Earth: Such stability is important for life on planets The Sun will stay in its present state for The Sun is already about 5 billion years old, so it has 5 billion more years to go in its present state The difference is that in the Sun, it is controlled thermonuclear fusion. Note again that the stability of the Sun's energy output is important for the existence of life on earth. Cosmic Abundances of Major Elements The Sun is primarily Hydrogen & Helium The abundance of the Earth & Life (on Earth) is different from that of the Sun I.e., The Earth s crust is primarily Oxygen, Aluminum, & Calcium Life is primarily Hydrogen, Oxygen, Carbon, & Nitrogen Interior of the Sun Core: center of Sun (15x10 6 K) Radiative zone: region of sun where energy is transported via radiation Convective zone: region of the sun where energy is transported to the photosphere via blobs of warm, rising gas

6 Interior of the Sun Random Walk Photons generated near the center continually collide with electrons random walk. Time required to move energy from the core to the surface ~ million years!! Time for photon to traverse 7x10 8 m = 2.3 seconds Photon travels ~ 0.5 cm between absorptions, so it takes ~ 100,000 to 1 million years to reach the surface. This process is called radiative diffusion. Random walk animation General features of the Sun Photosphere: The region in the solar atmosphere from which most of the visible light escapes into space (5800 K) Sunspots: A region of the solar photosphere that is cooler than its surroundings & therefore appears dark (~4800 K) Sunspots Sunspots can be used to determine the sun s rotation period ~ days Sunspots were discovered by Galileo

7 Close-up of Sunspot The Photosphere (Video) Close-up of Photosphere Close-up of Photosphere Granulation: Caused by convective cells Granulation: Caused by convective cells

8 Corona Corona: The outer atmosphere of the Sun. It has temperatures in excess of a million degrees & extends for millions of kilometers into space Coronal gas expands & flows away from the Sun and forms the Solar Wind Note that a solar eclipse is the best time to see the corona directly Corona in Visible Light (Video) Hα Emission (Video) Much like gravity affects anything with mass, magnetic fields affect anything with an electric charge. Charged particles spin around magnetic field lines For the Sun, charged particles get trapped in magnetic fields, spiraling along then from one sunspot to another. Convective material is very hot (and thus comprised of ion & free electrons). This material cannot cross the field lines without being swept into magnetic fields Magnetic Fields magnetic field lines Sunspot granulation (convective cells) path of charged particle

9 X-ray image of the Sun The Nature of Sunspots These fields prevent convection from carrying as much heat into the sunspots The Sun rotates faster at its equator than its pole The magnetic field lines wind up as a result of differential rotation Sunspots occur when the magnetic fields poke through the photosphere Solar Cycle Solar Cycle The 22-year cycle in which the solar magnetic field reverses direction, consisting of two 11-year sunspot cycles The Aurora (i.e., dancing light in the earth s sky caused by charged particles entering our atmosphere) are more intense during the solar maxima. Cause of 22-year cycle: Winding of magnetic fields? The 22-year cycle in which the solar magnetic field reverses direction, consisting of two 11-year sunspot cycles The Aurora (i.e., dancing light in the earth s sky caused by charged particles entering our atmosphere) are more intense during the solar maxima. Cause of 22-year cycle: Winding of magnetic fields?

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