Astronomy 111 – Lecture 18

1. Astronomy 111 – Lecture 18 Our Sun - An Overview

2. Basic Concepts • Size and Composition of Sun • Age and Energy Problem • Models of the Sun • The Atmosphere of the Sun • The Future of the Sun Astronomy 111 - Lecture 18

3. Basic Sun Data • Distance to Earth • Mean Distance : 1 AU = 149,598,000 km (light travel time to Earth = 8.32 min) • Maximum : 152,000,000 km • Minimum : 147,000,000 km • Mean angular diameter : 32 arcmin • Radius : 696,000 km = 109 Earth radii • Mass : 1.9891 x 1030 kg = 3.33 x 105 Earth Masses Astronomy 111 - Lecture 18

4. Basic Sun Data • Composition (by mass) • 74% Hydrogen, 25% Helium, 1% other elements • Composition (by number of atoms) • 92.1% Hydrogen, 7.8% Helium, 0.1% other elements • Mean Density : 1.4 g cm-3 • Mean Temperatures • Surface 5800 K, Centre 15.5 Million K (!) • Total Luminosity : 3.86 x 1026 W (1 sec  enough energy for 1 million years for humankind) Astronomy 111 - Lecture 18

5. How old is the Sun ? • Really problematic : Sun does not come with a time-stamp, shows no wrinkles etc… • Start with Earth & Solar System • Date geological formations, rocks • Various geochronometrical methods using radioactive decay properties  >4.2 billion years • Find oldest meteorites  4.5-4.7 billion years • Conclusion : Sun at least that old ! Astronomy 111 - Lecture 18

6. What fuel does the Sun use ? • A truly gigantic energy crisis emerges: • Sun is billions of years old. • Life on Earth as well. •  Sun must shine roughly like today for already a long time. • How does it create all the energy ? • What keeps the Sun so hot ? Astronomy 111 - Lecture 18

7. The Solar Energy Source • First guesses (early 18th century) : • Chemical Processes (‘burning’ fuel) • Maximum burning time : 10,000 years ! • Using gravitational energy (Kelvin-Helmholtz mechanism, mid-1800s) • Slow contraction of Sun releases energy, heats gas up -> energy radiated into space • Maximum contraction time : 25 million years ! • Only important at very early stage (birth of Sun) Astronomy 111 - Lecture 18

8. A new Energy Source • Problem : • Need more energy released per atom • Solution : • Albert Einstein : E = mc2 • m = mass, c = speed of light, E = energy • Mass and energy are equivalent. • Why does that help ? • Speed of light is a large number ! Astronomy 111 - Lecture 18

9. Thermonuclear Fusion • Turning mass into energy : • Transforming Hydrogen into Helium • Mass balance : 4 1H atoms 6.693 x 10-27 kg 1 4He atom 6.645 x 10-27 kg ------------------------------------ Mass lost : 0.048 x 10-27 kg • Extremely efficient process : • Fusing 1 kg of Hydrogen releases same energy as burning 20,000 metric tons of coal ! Astronomy 111 - Lecture 18

10. Thermonuclear Fusion • How much hydrogen must be converted to give solar luminosity ? • 600 million metric tons per second ! • Sounds enormous, but….. • Sun has enough fuel for at least 9 billion years • Solution to energy crisis ! • Problem : How does fusion work in detail ? Astronomy 111 - Lecture 18

11. Thermonuclear Fusion • ‘nuclear’ – regarding nuclei of atoms • ‘fusion’ – putting together (NOT fission) • ‘thermo’ – need enormous temperatures as nuclei do not fuse easily due to electric repulsion Astronomy 111 - Lecture 18

12. Thermonuclear Fusion • Introducing the Proton-Proton Chain Astronomy 111 - Lecture 18

13. neutrino positron photon 2H 3He 4He 3He 2H photon positron neutrino

14. Models of the Sun • Solar surface temperature ~ 5800 K • Temperature required for fusion ~ 10 million K • Fusion can only occur at core of the Sun ! • Can we understand and describe the conditions inside the Sun ? • Use theoretical models ! • Start with well-known principles and laws of physics Astronomy 111 - Lecture 18

15. Models of the Sun • Observational Fact 1 : • The Sun does not change size over long periods of time, keeps its shape quite well. • Principle of Hydrostatic Equilibrium • Pressure and Gravity maintain a balance. Astronomy 111 - Lecture 18

16. Gravity Gas Pressure Hydrostatic Equilibrium Astronomy 111 - Lecture 18

17. Models of the Sun • Conclusions : • Pressure increases with depth • Temperature increases with depth Astronomy 111 - Lecture 18

18. Models of the Sun • Observational Fact 2 : • The Sun does not heat up or cool down over long periods of time, keeps its temperature quite well. • Principle of Thermal Equilibrium • Energy Generation and Energy Transport maintain a balance. Astronomy 111 - Lecture 18

19. Models of the Sun • Modes of Energy Transport : • Heat conduction  very inefficient for gases • Convection : ‘circulation of fluids’, upwelling of hot gases • Radiative diffusion : Photons carrying away energy Astronomy 111 - Lecture 18

20. cooler water sinks Convection Hot blob rises Astronomy 111 - Lecture 18

21. Photon Random Walk Astronomy 111 - Lecture 18

22. Models of the Sun Astronomy 111 - Lecture 18

23. Models of the Sun • Construct computer model to describe state of solar material from core to atmosphere • How can we test those models ? • Do they describe the interior well ? • How can we ‘see’ into the Sun ? Astronomy 111 - Lecture 18

24. Testing the Models • Test 1 : Helioseismology • Sun vibrates (‘rings like a bell’) • Use waves to test interior structure • Same Principle as Geologist/Geophysicists with Earthquakes • Nice analogy : Ripeness of melons Astronomy 111 - Lecture 18

25. Astronomers probe the solar interior usingthe Sun’s own vibrations • Helioseismology is the study of how the Sun vibrates • These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun Astronomy 111 - Lecture 18

26. Testing the Models • Test 2 : Catching solar neutrinos • By-products of fusion • Unlike photons, neutrinos escape solar interior very easily • ‘Ghost-like’ particles : very, VERY hard to detect • 1014 solar neutrinos every second through 1m2 on Earth Astronomy 111 - Lecture 18

27. Catching Neutrinos Astronomy 111 - Lecture 18

28. The Solar Atmosphere • Core of Sun hidden because gases become opaque • Outermost layers show remarkable structures (‘atmosphere’) • Tiny and much less dense than interior • Nonetheless most important for life on our planet Astronomy 111 - Lecture 18

29. The Solar Atmosphere Astronomy 111 - Lecture 18

30. The Photosphere • Almost all of the visible light emanates from that small layer of gas. • Temperature decreases upwards in photosphere. • Sun darker at the edge ? Astronomy 111 - Lecture 18

31. The Photosphere Astronomy 111 - Lecture 18

32. The Photosphere • Cooler layers further out  Notice absorption lines in spectrum ! • Cool ? • Still about 4400 K at the upper edge of the atmosphere. • Comparison : Siberian winter night to hot tropical day in Hawaii. Astronomy 111 - Lecture 18

33. The Photosphere • Solar Granulation : Convection cells of gas in photosphere • 4 million granules cover the solar surface • Each granule covers area ~ Texas & Oklahama combined Astronomy 111 - Lecture 18

34. Astronomy 111 - Lecture 18

35. Astronomy 111 - Lecture 18

36. The Solar Chromosphere • Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere • Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules Astronomy 111 - Lecture 18

37. The Solar Chromosphere • Spectrum : Emission lines ! •  Temperature must rise again : • Top of chromosphere : 25,000 K • Approximately 300,000 spicules exist at one time, each last about 15 minutes • Covering ~ 1 % of solar surface • Phenomenon related to Sun’s magnetic field Astronomy 111 - Lecture 18

38. The Solar Corona • The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density • The solar corona blends into the solar wind at great distances from the Sun Astronomy 111 - Lecture 18

39. The corona ejects mass into space to form the solar wind Astronomy 111 - Lecture 18

40. Activity in the corona includes coronal mass ejections and coronal holes Astronomy 111 - Lecture 18

41. Sunspots are low-temperature regions inthe photosphere Astronomy 111 - Lecture 18

42. Astronomy 111 - Lecture 18

43. Astronomy 111 - Lecture 18

44. Sunspots are produced by a 22-year cyclein the Sun’s magnetic field Astronomy 111 - Lecture 18

45. Astronomy 111 - Lecture 18

46. The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field Astronomy 111 - Lecture 18

47. Astronomy 111 - Lecture 18

48. The future of the Sun • Equilibrium holds due to energy generation • What happens when Sun runs out of fuel ? • Drama at the End of the Solar Life • A story for another day…. • When will that happen ? • ~ 4.0 – 5.0 billion years from now • Puuuh, we are safe ! Astronomy 111 - Lecture 18

49. Astronomy 111 - Lecture 18