© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun: Our Star Lecture 21 How does the Sun work ? What is the energy source for the Sun ?

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun Homework Read Chapter 14: The Sun MasteringAstronomy: Assignment Chapter 14 Due Friday, Nov 16

© 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 : Fusion 2. How many years will the Sun continue shining? 5 Billion years 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 Quiz: The average human being uses roughly 1000 Watts (Ten 100 Watt light bulbs) on average. How much energy per second do humans use in the entire world? a)5x10 8 W b)5x10 12 W c)5x10 16 W d)5x10 20 W Approach: Find approximate answer 1.6 billion people (6 x 10 9 ) watts per person 3.Total power: 6 x 10 9 x1000 = 6x10 12 Watts

© 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 atmosphere. 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: Atoms are Ionized 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 Which particle has the greatest mass? a)helium b)electron c) proton d)deuterium

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun: Our Star Lecture 20 Section 2 The Nuclear Reaction that Powers the Sun Four protons Helium

© 2005 Pearson Education Inc., publishing as Addison-Wesley Proton - proton Reaction makes Deuterium P + P D + positron + neutrino D = 2 H = Deuterium = proton+neutron 2 H: “2” is number of Protons + neutrons

© 2005 Pearson Education Inc., publishing as Addison-Wesley D + P 3 He +   photon

© 2005 Pearson Education Inc., publishing as Addison-Wesley 3 He + 3 He He + 2P

© 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 Fission: Nuclear Reactors on Earth work by Fission A heavy nucleus, such a Uranium 235 is split apart by a fast-moving neutron. The resulting nuclei have lower energy. The extra energy comes out as fast-moving particles, notably neutrons that provide the energy of the reactor, heating the material to high temperatures.

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Sun: Our Star Lecture 20 Section 3 Energy Transport within the Sun: Photons carry energy Convection carries energy

© 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 Radiative 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 Radiative Zone Convective Zone 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 in the Sun: Hot gases rise, cool gases fall

© 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 The Sun: Our Star End of Lecture 20