1. 26 July 2005AST 2010: Chapter 151 The Sun: A Nuclear Powerhouse

2. 26 July 2005AST 2010: Chapter 152 Happy Sun

3. 26 July 2005AST 2010: Chapter 153 Why Does the Sun Shine? The Sun gives off energy The energy must come from somewhere — there’s no free lunch Conservation of energy is a fundamental tenet of physics Where does the energy come from? Until the 20th century only 2 possibilities were known: Chemical reactions Gravity

4. 26 July 2005AST 2010: Chapter 154 The Sun’s Energy Output How bright is the Sun? The Sun produces 4x10 26 watts The watt is the unit for the rate of energy use, commonly seen on light bulbs and appliances Our largest power plants produce around 5 x 10 9 watts of power (5,000 megawatts) Sun’s power = 8 x 10 16 of these power plants (10,000 trillion) Anyway you look at it, the Sun gives off a lot of energy

5. 26 July 2005AST 2010: Chapter 155 Is the Sun Powered by Chemical Reactions? What are chemical reactions? Examples: Rearrange the atoms in molecules, as in 2H 2 +O 2  2H 2 O This reaction combines hydrogen and oxygen (gases) to produce water plus energy Reverse the process: 2H 2 O  2H 2 + O 2 By adding energy, we can dissociate water into hydrogen and oxygen The energy factor is often left out of chemical-reaction formulas, for convenience If the Sun is powered by burning coal or oil, how long could its fuel last? Only a few thousand years! A process that uses fuel more efficiently is needed — something that gets more energy out of every kilogram of material

6. 26 July 2005AST 2010: Chapter 156 Gravity Squeeze? The Sun’s interior experiences contraction due to its own gravity This gravitational contraction converts gravitational potential energy into heat energy Drop a book  noise (gravitational potential energy turns into sound energy) A contraction of 40 m per day would account for the Sun’s energy output Efficiency ~ 1/10,000 % Gravity could power the Sun for about 100 million years but the Sun is thought to be at least 4 billion years old! So gravity cannot be the Sun's main energy source although it did help ignite the Sun when it formed

7. 26 July 2005AST 2010: Chapter 157 Nuclear Physics and Theory of Relativity To understand the way the Sun produces its energy, we need to learn a little about nuclear physics and the special theory of relativity Nuclear physics deals with the structure of the nuclei of atoms The special theory of relativity deals with the behavior of things moving at close to the speed of light

8. 26 July 2005AST 2010: Chapter 158 Converting Mass to Energy E = m c 2 Out of the special theory of relativity comes the most famous equation in science: E = m c 2 This equation tells us that mass (m) is just another form of energy (E)! The c 2 is the square of the speed of light For example, 1 gram of matter is equivalent to the energy obtained by burning 15,000 barrels of oil

9. 26 July 2005AST 2010: Chapter 159 …But There Are Rules We can’t simply convert atoms into energy We rearrange the protons and neutrons in nuclei to get a lower-mass configuration The difference between initial mass and final mass is converted to energy Chemical energy comes from rearranging atoms to configurations of lower energy (mass) Nuclear energy comes from rearranging nuclei to configurations of lower mass (energy) In each case, we get out the energy difference

10. 26 July 2005AST 2010: Chapter 1510 Elementary Particles 5 particles play a fundamental role inside the Sun Protons and neutrons make atomic nuclei Electrons orbit nuclei of atoms Photons are emitted by the Sun Neutrinos are also emitted Particle name Mass (MeV/c 2 ) Charge (e) Proton938.272 +1 Neutron939.565 0 Electron0.511 Neutrino<10 -6 0 Photon00

11. 26 July 2005AST 2010: Chapter 1511 Atomic Nucleus Two ways to rearrange nuclei and get energy: Fission produces energy by breaking up massive nuclei like uranium into smaller nuclei like barium and krypton is used in A-bombs and nuclear reactors needs uranium-235 and plutonium-238 cannot occur inside the Sun: it has no uranium or plutonium Fusion produces energy by fusing light nuclei like hydrogen to make more massive nuclei like helium is used in H-bombs can occur inside the Sun: it has lots of hydrogen!!

12. 26 July 2005AST 2010: Chapter 1512 How Does Fusion Work? Nuclear fusion is a process by which two light nuclei combine to form a single, larger nucleus However, nuclei are positively charged Like charges repel Two nuclei naturally repel each other and thus cannot fuse spontaneously For fusion, electrical repulsion must be “overcome” When two nuclei are very close, the strong nuclear force takes over and holds them together How do two nuclei get close enough?

13. 26 July 2005AST 2010: Chapter 1513 Fusion Needs Fast-Moving Nuclei Fast moving nuclei can overcome the repulsion They get a running start Lots of fast moving nuclei implies high temperatures The core of the Sun has a temperature of 15 million kelvin Low speed High speed

14. 26 July 2005AST 2010: Chapter 1514 Fusion Powers the Sun Temperatures in the cores of stars are estimated to be above the 8 million K needed to fuse hydrogen nuclei together Calculations have shown that the observed power output of the Sun is consistent with the power produced by the fusion of hydrogen nuclei The observed neutrinos from the Sun produced are expected as one of the byproducts of fusion reactions We can, therefore, hypothesize: all stars produce energy by nuclear fusion

15. 26 July 2005AST 2010: Chapter 1515 Proton-Proton Chain Fuse two hydrogen ( H= 1 proton) to make deuterium ( 2 H= 1 proton+1 neutron), neutrino, and positron Fuse one deuterium and one hydrogen to make helium-3 ( 3 He= 1 proton+2 neutrons) and a gamma ray (energetic photon) Fuse two helium-3 to make helium-4 ( 4 He ) and two hydrogen

16. 26 July 2005AST 2010: Chapter 1516 Why a Complicated Chain? Fusion would be simpler if four protons would collide simultaneously to make one helium nucleus That is simpler, but less likely rare for four objects to collide simultaneously with high enough energy chance of this happening are very, very small rate too slow to power the Sun The proton-proton chain: each step involves collision of two particles chance of two particles colliding and fusing is much higher so nature slowly builds up the helium nucleus

17. 26 July 2005AST 2010: Chapter 1517 Fusion and Solar Structure Fusion occurs only in Sun's core This is the only place that is hot enough Heat from fusion determines the Sun's structure

18. 26 July 2005AST 2010: Chapter 1518 Heat from Core Determines Sun's Size There is a force equilibrium inside the Sun, called hydrostatic equilibrium, which is a balance between thermal pressure from the hot core pushing outward gravity contracting the Sun toward its center The nuclear-fusion rate — how often fusion can occur — is very sensitive to temperature A slight increase/decrease in temperature causes the fusion rate to increase/decrease by a large amount

19. 26 July 2005AST 2010: Chapter 1519 Gravity and Pressure Force equilibrium Newton's 1st law states that an object’s acceleration is zero if forces on the object balance Gravity tries to pull the 1/4 pounder toward Earth’s center Newton’s 3rd law implies that pressure from the table opposes gravity Hydrostatic equilibrium in the Sun The “cloud of gas” is like 1/4 pounder Gravity pulls it toward the center Pressure from below opposes gravity The heat from fusion in the hot core increases the pressure Thus the energy output of the Sun controls its size! pressure from table weight from gravity cloud pressure from hot gas weight from gravity

20. Temperature and Pressure The temperature of a gas corresponds to the random motion of atoms in the gas The pressure of a gas is the amount of force per unit area on a surface in contact with the gas In general, pressure increases with increasing temperature

21. 26 July 2005AST 2010: Chapter 1521 Balancing Fusion, Gravity, and Pressure If the fusion rate increases, then thermal pressure increases causing the star to expand the star expands to a new point where gravity would balance the thermal pressure the expansion would reduce the pressure inside the core the temperature in the core would drop the nuclear fusion rate would subsequently slow down the thermal pressure would then drop the star would shrink the temperature would then rise again and the nuclear fusion rate would increase stability would be re-established between the nuclear- reaction rates and the gravity compression

22. 26 July 2005AST 2010: Chapter 1522 Hydrostatic Equilibrium The balance between the fusion rate, thermal pressure, and gravity determines the Sun's size Bigger stars have cooler cores Smaller stars have hotter cores and, therefore, are more compressed

23. 26 July 2005AST 2010: Chapter 1523 Other Particles Helium is not the only product in the fusion of hydrogen Two other particles are produced Positrons Neutrinos

24. 26 July 2005AST 2010: Chapter 1524 Gamma-Ray Propagation in the Sun The positrons emerging from the fusion reactions in the core quickly annihilate the electrons near them The annihilation produces pure electromagnetic energy in the form of gamma-ray photons These photons take about a million years to move from the core to the surface This migration is slow because they scatter off the dense gas particles The photons move on average about only a centimeter between collisions In each collision, they transfer some of their energy to the gas particles As they reach the photosphere, the gamma-ray photons have become visible-light photons because the photons have lost some energy in their journey through the Sun

25. 26 July 2005AST 2010: Chapter 1525 Neutrinos These particles have no charge and are nearly massless They rarely interact with ordinary matter Neutrinos travel extremely fast at almost the speed of light if their mass is tiny Neutrinos pass from the core of the Sun to its surface in only two seconds They take less than 8.5 minutes to travel from the Sun to the Earth

26. 26 July 2005AST 2010: Chapter 1526 Neutrino Abundance & Counting The Sun produces a lot of neutrinos In one second several million billion neutrinos pass through your body Do you feel them? Not to worry! The neutrinos do not damage anything The great majority of neutrinos pass right through the entire Earth as if it weren’t there In principle, we can use the number of solar neutrinos received on Earth to get clues about the Sun’s energy output, but neutrinos have a very low probability of interacting with ordinary matter they could pass through a light year of lead and not be stopped by any of the lead atoms!

27. 26 July 2005AST 2010: Chapter 1527 Detecting Neutrinos Increase the odds of detecting neutrinos by using a large amount of a material that reacts with neutrinos in a measurable way A chlorine isotope changes to a radioactive isotope of argon when hit by a neutrino A gallium isotope changes to a radioactive isotope of germanium Neutrinos can interact with protons and neutrons and produce an electron The electron can be detected

28. 26 July 2005AST 2010: Chapter 1528 Neutrino Detectors Neutrino detectors use hundreds of thousands of liters of these materials in a container buried under many tens of meters of rock to shield the detectors from other energetic particles from space called cosmic rays Even the largest detectors can detect only a few neutrinos per day