The Sun: A Nuclear Powerhouse

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

2. Happy Sun AST 2010: Chapter 15

3. 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 AST 2010: Chapter 15

4. The Sun’s Energy Output • How bright is the Sun? • The Sun produces 4x1026 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 109 watts of power (5,000 megawatts) • Sun’s power = 8 x 1016 of these power plants (10,000 trillion) • Anyway you look at it, the Sun gives off a lot of energy AST 2010: Chapter 15

5. Is the Sun Powered by Chemical Reactions? • What are chemical reactions? Examples: • Rearrange the atoms in molecules, as in 2H2+O2 2H2O • This reaction combines hydrogen and oxygen (gases) to produce water plus energy • Reverse the process: 2H2O 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 • 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 AST 2010: Chapter 15

6. 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 AST 2010: Chapter 15

7. 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 AST 2010: Chapter 15

8. Converting Mass to Energy • Out of the special theory of relativity comes the most famous equation in science:E = m c2 • This equation tells us that mass (m) is just another form of energy (E)! • The c2 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 AST 2010: Chapter 15

9. …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 AST 2010: Chapter 15

10. 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 Elementary Particles AST 2010: Chapter 15

11. 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!! AST 2010: Chapter 15

12. 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? AST 2010: Chapter 15

13. Low speed High speed 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 AST 2010: Chapter 15

14. 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 observedneutrinos 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 AST 2010: Chapter 15

15. 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) and a gamma ray(energetic photon) • Fuse two helium-3 to make helium-4 (4He) and two hydrogen AST 2010: Chapter 15

16. 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 AST 2010: Chapter 15

17. 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 AST 2010: Chapter 15

18. 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 AST 2010: Chapter 15

19. Gravity and Pressure pressure from table • 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! weight from gravity pressure from hot gas cloud weight from gravity AST 2010: Chapter 15

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. 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 AST 2010: Chapter 15

22. 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 AST 2010: Chapter 15

23. Other Particles • Helium is not the only product in the fusion of hydrogen • Two other particles are produced • Positrons • Neutrinos AST 2010: Chapter 15

24. 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 • Thesephotons 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 AST 2010: Chapter 15

25. 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 AST 2010: Chapter 15

26. 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! AST 2010: Chapter 15

27. 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 AST 2010: Chapter 15

28. 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 AST 2010: Chapter 15