Chapter 15: The Sun: A Nuclear Powerhouse

1. Chapter 15: The Sun: A Nuclear Powerhouse Astronomy 2010

2. Happy Sun Astronomy 2010

3. Why Does the Sun Shine? • The Sun gives off energy (duh)! • 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 Astronomy 2010

4. The Sun’s Energy Output • How bright is the Sun? • The Sun produces 4x1026 watts • 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 109 watts of power (5,000 megawatts or 5 gigawatts) • Sun = 8 x 1016 of these power plants (80,000 trillion) • Anyway you look at it, the Sun gives off a lot of energy. Astronomy 2010

5. Is the Sun Powered by Chemical Reactions? • What are chemical reactions? • Rearrange the atoms in molecules, as in 2H2 + O2 2H2O + energy. This reaction combines hydrogen and oxygen gas to produce water plus energy. • Reverse the process: 2H2O + energy  2H2 + O2. By adding energy we can dissociate water into hydrogen and oxygen. • The energy factor is often left out of chemical reaction formulas, for convenience. Astronomy 2010

6. Is the Sun Powered by Chemical Reactions? • 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. Astronomy 2010

7. Gravity Squeeze? • Gravitational contraction: falling layers of the Sun's material compress the Sun  heat energy • drop a book  noise! Gravitational potential energy. • A contraction of 40m per day would account for the Sun’s energy output. • Efficiency ~ 1/10000 % • Gravity could power the Sun for about 100 million years  but the Sun is at least 4 billion years old! • Gravity can't be the Sun's main energy source • But it did help ignite the Sun when it formed Astronomy 2010

8. 15.2 Mass, Energy, and the Special Theory of Relativity • To understand the way the Sun produces 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. Astronomy 2010

9. 15.2.1 Converting Mass to Energy • Out of the special theory of relativity comes the most famous equation in science: • This equation tells us that mass (m) is just another form of energy (E)! • The c2 is the square of the speed of light. • 1 gram of matter is equivalent to the energy obtained by burning 15,000 barrels of oil. Astronomy 2010

10. …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. Astronomy 2010

11. Astronomy 2010

12. Elementary Particles (condensed) 5 particles play a fundamental role in the Sun. Protons and neutrons make atomic nuclei Electrons orbit nuclei of atoms Photons are emitted by the Sun Neutrinos are also emitted Astronomy 2010

13. 15.2.3 The Atomic Nucleus • Two ways to rearrange nuclei and get energy: • Fission • produces energy by breaking up massive nuclei like Uranium into less massive nuclei like Barium and Krypton • A-bombs, nuclear reactors • needs Uranium 235, Plutonium 238 • Problem: no Uranium or Plutonium on the Sun • Fusion • produces energy by combining light nuclei like Hydrogen to make more massive nuclei like Helium. • H-bomb, tokamak, internal confinement fusion • Sun has lots of Hydrogen!! Astronomy 2010

14. How Does Fusion Work? • Nuclear fusion: • 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? Astronomy 2010

15. Fusion needs fast moving nuclei Low speed High speed • Fast moving nuclei can overcome the repulsion - they get a running start. • Lots of fast moving nuclei means high temperature. • The core of the Sun has a temperature of 15 million degrees Kelvin. (ouch!) Astronomy 2010

16. Fusion Power on Earth • Fusion is the source of energy for hydrogen bombs. • We are trying to harness fusion to generate electricity: • tokamak - magnetic confinement machine as envisioned for ITER shown to the right • inertial confinement fusion - Lawrence Livermore National Lab ITER reactor Astronomy 2010

17. Fusion Powers the Sun • Temperatures in the cores of stars are above the approximately 8 million K needed to fuse hydrogen nuclei together. • Calculations: observed power output of the Sun consistent with fusion of hydrogen nuclei. • Observation: neutrinos from Sun produced by fusion reactions. • Hypothesis: all stars produce energy by nuclear fusion. Astronomy 2010

18. proton-proton chain • fuse two hydrogen, H (1 proton) to make deuterium, 2H(1 proton, 1 neutron), neutrino and positron • fuse one deuterium and one hydrogen to make helium-3 3He(1 proton, 2 neutrons), gamma ray (energetic photon) • fuse two helium-3 to make helium 4He plus two hydrogen Astronomy 2010

19. Why a complicated chain? • Fusion would be simpler if four protons would collide simultaneously to make one helium nucleus • 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 • 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. Astronomy 2010

20. Fusion and Solar Structure • fusion only in Sun's core • only place its hot enough • heat from fusion determines Sun's structure Astronomy 2010

21. Heat from Coredetermines Sun's Size • Force equilibrium • Hydrostatic equilibrium: balance between • thermal pressure from the hot core pushing outwards • gravity squeezes the star collapse to the very center • Nuclear fusion rate is very sensitive to temperature. • A slight increase/decrease in T causes fusion rate to increase/decrease by a large amount. Astronomy 2010

22. Gravity and Pressure pressure from table • force equilibrium • Newton's second law: F = ma • static equilibrium: no acceleration if forces on object balance • gravity tries to pull 1/4 pounder to center of the Earth • pressure from table opposes gravity • hydrostatic equilibrium on Sun • "cloud of gas" (like 1/4 pounder) • gravity pulls cloud to the center • pressure from gas below opposes gravity • heat from fusion in the hot core increases pressure • energy output controls size of sun! weight from gravity pressure from hot gas cloud weight from gravity Astronomy 2010

23. Temperature and Pressure • temperature: random motion of atoms in a gas • pressure: amount of force per unit area on piston from gas • generally pressure increases with increasing temperature Astronomy 2010

24. Balancing Fusion, Gravity and Pressure If the rate of fusion increases, then: • thermal pressure increases causing the star to expand. • star expands to a new point where gravity would balance the thermal pressure. • the expansion would reduce compression of 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 rise again and the nuclear fusion rate would increase • Stability would be re-established between the nuclear reaction rates and the gravity compression Astronomy 2010

25. Hydrostatic Equilibrium • balance between pressure, heat from fusion and gravity determines Sun's size • big stars have cooler cores, small stars have hotter cores – more compressed Astronomy 2010

26. Other Particles • Helium is not the only product in the fusion of hydrogen. • Two other particles produced • Positron • Neutrino Astronomy 2010

27. Gamma Ray Propagation in the Sun • Positrons quickly annihilate with electrons. • Photons produced in core of the Sun take about a million years to move to the surface. • Slow migration because they scatter off the dense gas particles • move about only a centimeter between collisions. • In each collision, they transfer some of their energy to the gas particles. • As they reach the photosphere, gamma rays have become visible photons. Astronomy 2010

28. Neutrinos • Nearly massless particles with no charge. • Rarely interact with ordinary matter. • Neutrinos travel extremely fast • Almost at the speed of light if small mass. • Neutrinos pass from the core of the Sun to the surface in only two seconds. • They take less than 8.5 minutes to travel the distance from the Sun to the Earth. Astronomy 2010

29. Neutrino Counting • In principle: • Use neutrino count at Earth as indicator of the Sun’s energy output • The problem: • Neutrinos have a very low probability of interacting with matter. • Could pass through a light year of lead and not be stopped by any of the lead atoms! Astronomy 2010

30. Neutrino Abundance • 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 wasn’t there. Astronomy 2010

31. 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. Astronomy 2010

32. Neutrino Detectors Ray Davis • 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 detect only a few neutrinos per day. Nobel Prize in 2003 Kamiokande Astronomy 2010

33. Solar Neutrino Production • Number of neutrinos produced in the Sun is directly proportional to the number of nuclear reactions taking place in the Sun's core. • Same principle with neutrinos produced via the Carbon-Nitrogen-Oxygen chain. • The more reactions there are, the more neutrinos are produced and the more that should be detected here on the Earth.  Astronomy 2010

34. Solar Neutrino Production (cont’d) • Physicists find that the number of neutrinos coming from the Sun is smaller than expected. • Early experiments detected only 1/3 of the expected number of neutrinos. • These experiments used hundreds of thousands of liters of cleaning fluid (composed of chlorine compounds) or very pure water. Astronomy 2010

35. Solar Neutrino Production (cont’d) • Later experiments using many tons of gallium were able to detect the more abundant low-energy neutrinos. • However, those experiments also found the same problem: too few neutrinos (the gallium experiments found about 2/3 the expected number). • The puzzling lack of neutrinos from the Sun is called the solar neutrino problem. Astronomy 2010

36. Possible Explanations: • Nuclear fusion is not the Sun's power source.  • Not supported by observations, not likely to be the correct reason. • The experiments were not calibrated correctly.  • Unlikely that all carefully-tuned experiments were tuned in the same wrong way. Experiments independently verified by many other scientists; astronomers think that the results are correct. • The nuclear reaction rate in the Sun is lower than what our calculations say.  • Possible but many people have checked and re-checked the physics of the reaction rates. • Strong constraints in how much one can lower the temperature in the core of the Sun to slow down the reactions. Astronomy 2010

37. Solar Neutrino Solution • Three types of neutrinos exist. • The Sun produces only one type, called electron neutrinos • The experiments detect only electron-type. • On their way from the Sun, neutrinos can transform from one type to another  we only detect 1/3 of the mix at Earth. • This also implies that neutrinos have mass, very small, but not zero. Astronomy 2010

38. Neutrino Oscillations • The Sun produces only e neutrinos (green). • This becomes one of the types 1, 2, or 3 on its way to Earth. • These 3 mix on their way to Earth. • When we look at the neutrinos on Earth, some of the original green is now blue or yellow. Astronomy 2010