© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun: The Internal Physics 25 November 2008 Astronomy 7A

© 2005 Pearson Education Inc., publishing as Addison-Wesley Questions about The Sun 1.What produces the enormous light energy ? 2.How many years will the Sun continue shining? 3.Is the Sun’s light output constant, or variable? 4.What doesn’t the Sun contract, due to its gravity? 5.How did the Sun form ? 6.What are sunspots? And those loops on the surface? 7.What is the “sunspot cycle”? 8.What is the Sun made of? 9.Does the Sun have layers inside, like the Earth?

© 2005 Pearson Education Inc., publishing as Addison-Wesley Answers about The Sun 1.What produces the enormous light energy ? Nuclear reactions: 4H He 2. How many years will the Sun continue shining? 5 Billion years more 3.Is the Sun’s light output constant, or variable? Constant. Within 0.1% 4.What doesn’t the Sun contract due to its gravity? Gas Pressure pushes outward. 5.How did the Sun form ? A massive gas cloud collapsed by its own gravity. 6.What are sunspots? Dark regions with strong magnetic fields..

© 2005 Pearson Education Inc., publishing as Addison-Wesley Observable Properties of the Sun Distance: 1.5 x 10 8 km = 1 A.U. Mass: 2.0 x kg =300,000 x Earth Radius: 7.0 x 10 5 km = 110 Earths Density: 1.4 g/cm 3 ~ 40% more than Water Luminosity: 3.8 x watts -- - light spreads out spherically 1 sec of Lum. supplies 500,000 yrs worth of energy for humanity.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Properties of the Sun Density: 1.4 g/cm 3 ~ 40% more than Water No hard surface Implies: Sun is Gaseous compressed by gravity

© 2005 Pearson Education Inc., publishing as Addison-Wesley You can measure the amount of different atoms from darkness of the absorption lines. Dark spectral lines are caused by absorption of light by atoms in the Sun’s atmospshere. Composition of the Sun Magnesium Sodium Calcium Iron

© 2005 Pearson Education Inc., publishing as Addison-Wesley Composition of the Sun (by Mass) 70% 28% 0.2% 0.3% C, N, O, Ne, Fe, Others: 2% Hydrogen He Representative of the Universe as a whole: Hydrogen and Helium Dominate. (But not for Earth.)

© 2005 Pearson Education Inc., publishing as Addison-Wesley Gravity Balanced by Pressure Gas pressure supports the star against the inward force of gravity. At Sun’s center, pressure is huge. (Weight of material above is huge.) Huge Pressure Huge temperature and densities at Sun’s center.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Nuclear Reactions Fusion of Hydrogen to Helium Center of the Sun Temperature = 15 Million K Collisions between atoms so violent: electrons removed from atoms. leaving bare nucleus of each atom. Computer models (balancing gravity with pressure) show: Nuclei of atoms collide & react

© 2005 Pearson Education Inc., publishing as Addison-Wesley Fusion occurs only in the Sun’s core Nuclear fusion a reaction where Hydrogen combines (fuses) to form Helium nuclei. Electric force: nuclei repel each other. Nuclei have positively charged protons For fusion to occur, nuclei must be moving fast enough to overcome electric repulsion This requires high temperatures When nuclei touch, the nuclear force binds them together At low speeds, electric repulsion prevents protons from coming close. At high speeds, protons overcome electric repulsion. Come close. Nuclear reaction!

© 2005 Pearson Education Inc., publishing as Addison-Wesley Neutrons Neutrons are not stable! They do not exist alone for long! n p+ p+ + e- e- + e (  -decay) p+p+ n + e + + e (inverse  -decay) e is a neutrino ---- a weakly interacting particle which has almost no mass and travels at nearly the speed of light. ¯ e- = electron e+ = positron (anti-electron) Note: Charge conserved.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Nuclear Fusion in the Sun: Proton-Proton Chain IN: 6 H, (2 e - ) OUT: He, 2 H, 2 e, 4  4 H nuclei are converted into 1 He nucleus and energy is released.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Quiz What is this object? a) deuterium b) tritium c) helium d) 2 H

© 2005 Pearson Education Inc., publishing as Addison-Wesley Neutrinos from the Sun Neutrinos are created in the proton-proton reaction. We have detected them, proving that the theory of nuclear fusion reactions is correct! But we only detect about 30% of the neutrinos predicted by theoretical models. Reason: Three types of neutrinos: electron ( e ), muon (  ), and tau (  ) our neutrino detectors can register only electron neutrinos Neutrinos can change type after being created, allowing us to detect only 1/3 of them

© 2005 Pearson Education Inc., publishing as Addison-Wesley Mass Input: 4 p + 2 e- Mass Output: 1 He (2p + 2n) Look up Masses of particles: Mass Input > Mass Output Mass Input = Mass Output Mass, m, is missing ! Converted to Energy: E = mc 2 Mass Accounting Where c is speed of light, 3x10 8 m/s.

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Solar Thermostat Higher Temp causes faster collisions: - Reactions proceed faster. - More energy is produced. Added energy heats Sun to higher temperature. The Sun expands ! Expansion causes gases to cool, and gas density to be lower. Atoms move more slowly and are farther apart. Reaction rate declines. Sun cools Back to normal Temp. Suppose the Sun Heats Up at little accidentally Is there a negative feedback to bring temperature back ? Sun’s energy output (luminosity) remains stable: Thermostat

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Solar Luminosity has Risen 30% in Past 4 Billions years During the past 4.6 billion years: 4 Hydrogen atoms fused into Helium Core now has fewer atoms. Lower pressure: the Sun’s core contracts, causing it to heat up The fusion rate increases (until higher pressure balances gravity) A new equilibrium is reached at a higher energy output Thus, the Sun’s luminosity increases. Computer Models indicate the Sun’s luminosity has increased 30% since it formed 4.6 billion years ago. From 2.9 x watts to today’s 3.8 x watts

© 2005 Pearson Education Inc., publishing as Addison-Wesley “Observing” the Solar Interior The Sun’s interior is opaque… we can not see directly into it with light We can construct mathematical computer models of it. the models are a grid of temperature, pressure, & density vs. depth these values are calculated using known laws of physics they are tested against the Sun’s observable quantities We can directly measure sound waves moving through the interior we observe “sunquakes” in the photosphere by using Doppler shifts motion of sound waves can be checked against interior conditions predicted by models There is another way to see directly into the core…neutrinos!

© 2005 Pearson Education Inc., publishing as Addison-Wesley T = 15 million K; Depth = inner 1/4 of Sun Where the Sun’s energy is generated. Interior Zones Interior Zones Energy is transported from center outward. The interior is divided into two zones: Radiation Zone (energy carried by light) Convection Zone (energy carried by rising hot gas) Boundary between them is at: T = 2 x 10 6 K; Distance from center: 0.70 R Sun Core Core

© 2005 Pearson Education Inc., publishing as Addison-Wesley Layers of the Sun Core Radiation Zone Convective Zone photosphere Corona Solar Wind

© 2005 Pearson Education Inc., publishing as Addison-Wesley Photon Transport of Energy “Radiation Transport”

© 2005 Pearson Education Inc., publishing as Addison-Wesley Energy Transport by Photons (Light) Radiation Zone Energy travels as photons of light, which continually collide with particles Photons scatter, changing direction (random walk), and change wavelengths This is called radiative diffusion This is a slow process! It takes about 1 million years for energy to travel from the core to the surface. Path of photon, scattered by electrons and atoms.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Layers of the Sun Core Radiation Zone Convective Zone photosphere Corona Solar Wind

© 2005 Pearson Education Inc., publishing as Addison-Wesley Convective Transport of Energy Wait 10 sec For flame

© 2005 Pearson Education Inc., publishing as Addison-Wesley Convective Energy Transport Convection: Hot air rises; carries heat with it. The bottom of the convection zone is heated … hot gas rises to the top cooler gas sinks to the bottom…similar to boiling a pot of water! Energy is brought to the surface via bulk motions of matter

© 2005 Pearson Education Inc., publishing as Addison-Wesley Convection Visible at Surface of the Sun

© 2005 Pearson Education Inc., publishing as Addison-Wesley Layers of the Sun Core Radiation Zone Convective Zone photosphere Corona Solar Wind

© 2005 Pearson Education Inc., publishing as Addison-Wesley Photosphere T = 5,800 K; depth = 400 km This is the yellow “surface” that we see.

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Photosphere: Visible Surface of the Sun Photosphere: opaque “surface” human eye sees. Granulation (convection) Sunspots

© 2005 Pearson Education Inc., publishing as Addison-Wesley Journey Into the Sun Photosphere Convection Zone Radiation Zone Core: proton-proton nuclear reactions: Helium

© 2005 Pearson Education Inc., publishing as Addison-Wesley Photospheric Features Granulation: the tops of convection cells seen “bubbling” on the Solar surface Sunspots: dark spots on the surface where the temperature is cooler. National Solar Observatory/AURA/NSF

© 2005 Pearson Education Inc., publishing as Addison-Wesley Sunspots and Convection at Surface of the Sun

© 2005 Pearson Education Inc., publishing as Addison-Wesley Layers of the Sun Core Radiation Zone Convective Zone photosphere Corona Solar Wind Chromosphere

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Chromosphere

© 2005 Pearson Education Inc., publishing as Addison-Wesley Chromosphere Temp = 10,000 K Hydrogen Emission n = 3 to 2.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Chromosphere T = 10,000 K; Depth: Thin and patchy over surface A thin hot layer above the photosphere where most of the Sun’s UV light is emitted. SOHO UV image of the Sun Light emitted from Helium at 20,000 K

© 2005 Pearson Education Inc., publishing as Addison-Wesley Prominences from the Chromosphere Hydrogen Alpha: Electrons drop from 3rd - 2nd level. Wait 10 sec For movie.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Prominences – Gas trapped in the magnetic fields is heated and elevated above the photosphere and chromosphere. X-ray images from NASA’s TRACE mission. Movie. Click to launch.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Flares: Magnetic Explosions

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Flares: Magnetic Explosions

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Corona

© 2005 Pearson Education Inc., publishing as Addison-Wesley Corona

© 2005 Pearson Education Inc., publishing as Addison-Wesley Corona T = 2 Million K Thickness  Radius of Sun (700,000 km) The hot, ionized gas which surrounds the Sun. –it emits mostly X-rays It can be seen in visible light during an eclipse. X-ray image (YOHKOH telescope) Visible image

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Wind The stream of electrons, protons, Helium nuclei and other ions which flow out from the Sun. It extends out beyond Pluto. X-ray image of corona UV image of solar wind Visible image of solar wind comet SOHO-6 (fell into Sun) Sagittarius

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Wind electrons, protons, He nuclei expelled by flares Interact with Earth’s magnetic field to cause…

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Aurorae A strong Solar wind can affect human technology by: interfering with communications knocking out power grids damage electronics in space vehicles the Northern & Southern Lights

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Magnetic Activity The photosphere of the Sun is covered with sunspots. Sunspots are not constant; they appear & disappear. They do so in a cycle, lasting 11 years. Sun’s magnetic field switches polarity (N-S) every 11 yrs So the entire cycle repeats every 22 yrs

© 2005 Pearson Education Inc., publishing as Addison-Wesley Sunspots: Cool, Magnetic Regions Umbra, Penumbra

© 2005 Pearson Education Inc., publishing as Addison-Wesley What causes a sunspot? Magnetic field slows down convection; Less heat is transported to surface; so that part of photosphere is cooler

© 2005 Pearson Education Inc., publishing as Addison-Wesley 11-year Sunspot Cycle

© 2005 Pearson Education Inc., publishing as Addison-Wesley Magnetic Activity changes with Time : 11-year Cycle (Last Maximum in Year 2000)

© 2005 Pearson Education Inc., publishing as Addison-Wesley Sunspot Cycle

© 2005 Pearson Education Inc., publishing as Addison-Wesley Rotation Period of Sun: 30 days

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun: How long will it Shine ? Until it burns up its available Hydrogen (in the core where T > 2 million degrees) At Current Rate of Energy production: 5 billion more years

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Death of the Sun in 5 Billion Years Core becomes pure helium! No Hydrogen burning possible. The Helium core begins to collapse. –H shell (around Helium) heats up and H fusion begins there. –Outer layers of the Sun expand. –The Sun enters giant phase of its life. Original Sun Expanding: “Giant Star”

© 2005 Pearson Education Inc., publishing as Addison-Wesley Giant Star Phase The He core collapses until it heats to 10 8 K – He fusion begins ( 3 He C) Carbon forms! The star, called a Giant, is once again stable. – Gravity balanced by pressure, from He fusion reactions – Giant stars create, and release, most of the Carbon in the universe: Key ingredient for organic molecules and life. The Dying Sun: 5 billions years from now