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ASTRO 101

ASTRO 101. Principles of Astronomy. Instructor: Jerome A. Orosz (rhymes with “ boris ” ) Contact:. Telephone: 594-7118 E-mail: orosz@sciences.sdsu.edu WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ Office: Physics 241, hours T TH 3:30-5:00.

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ASTRO 101

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  1. ASTRO 101 Principles of Astronomy

  2. Instructor: Jerome A. Orosz (rhymes with “boris”)Contact: • Telephone: 594-7118 • E-mail: orosz@sciences.sdsu.edu • WWW: http://mintaka.sdsu.edu/faculty/orosz/web/ • Office: Physics 241, hours T TH 3:30-5:00

  3. Text: “Discovering the Essential Universe, Fifth Edition”by Neil F. Comins

  4. Course WWW Page http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html Note the underline: … ast101_fall2012.html … Also check out Nick Strobel’s Astronomy Notes: http://www.astronomynotes.com/

  5. Homework/Announcements • Homework due Tuesday, November 13: Question 4, Chapter 8 (Describe the three main layers of the Sun’s interior.) • Chapter 9 homework due November 20: Question 13 (Draw an H-R Diagram …)

  6. Next:How Does the Sun Work?

  7. How Does the Sun Work? • Some useful numbers: • The mass of the Sun is 2x1030 kg. • The luminosity of the Sun is 4x1026 Watts. • The first question to ask is: What is the energy source inside the Sun?

  8. Energy Sources • A definition: Efficiency = energy released/(fuel mass x [speed of light]2) • Chemical energy (e.g. burning wood, combining hydrogen and oxygen to make water, etc.). • Efficiency = 1.5 x 10-10 • Solar lifetime = 30 to 30,000 years, depending on the reaction. Too short!

  9. Energy Sources • A definition: Efficiency = energy released/(fuel mass x [speed of light]2) • Gravitational settling (falling material compresses stuff below, releasing heat). • Efficiency = 1 x 10-6 • Solar lifetime = 30 million years. Too short, although this point was not obvious in the late 1800s.

  10. Energy Sources • A definition: Efficiency = energy released/(fuel mass x [speed of light]2) • Nuclear reactions: fusion of light elements (as in a hydrogen bomb). • Efficiency = 0.007 • Solar lifetime = billions of years.

  11. Ways to Transport Energy

  12. Ways to Transport Energy • Conduction: direct contact • Radiation: via photons • Convection: via mass motions

  13. The Phases of Matter

  14. Phases of Matter • Matter has three “phases” • Solid. Constant volume and constant shape. • Liquid. Constant volume but variable shape. • Gas. Variable volume and variable shape.

  15. The Gas Phase • In a gas, the atoms and/or molecules are widely separated and are moving at high velocities: • Relatively heavy molecules such as CO2 move relatively slowly. • Relatively light molecules like H2 move relatively quickly. • The average velocities of the gas particles depend on the temperature of the gas.

  16. Heating a Gas • The velocity of a gas particle depends on the mass of the particle and its temperature. Image from Nick Strobel (http://www.astronomynotes.com)

  17. Ideal Gas • For a fixed volume, a hotter gas exerts a higher pressure: Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

  18. The 4 “Forces” in Nature

  19. The 4 “Forces” of Nature • There are 4 “fundamental forces” in nature: • Gravity: relative strength = 1, range = infinite. • Electromagnetic: rel. str. = 1036, range = infinite. • “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. • “Strong” nuclear: rel. str. = 1038, range = 10-15 meter. • Gravity is an attractive force between all matter in the Universe. The more mass something has, the larger the net gravitational force is.

  20. The 4 “Forces” of Nature • There are 4 “fundamental forces” in nature: • Gravity: relative strength = 1, range = infinite. • Electromagnetic: rel. str. = 1036, range = infinite. • “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. • “Strong” nuclear: rel. str. = 1038, range = 10-15 meter. • The electromagnetic force can be repulsive (+,+ or -,-) or attractive (+,-). Normal chemical reactions are governed by this force.

  21. The 4 “Forces” of Nature • There are 4 “fundamental forces” in nature: • Gravity: relative strength = 1, range = infinite. • Electromagnetic: rel. str. = 1036, range = infinite. • “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. • “Strong” nuclear: rel. str. = 1038, range = 10-15 meter. • The weak force governs certain radioactive decay reactions. • The strong force holds atomic nuclei together.

  22. The Strong Force. p = proton, positive charge n = neutron, no charge p n n p • Here is a helium nucleus. The protons are held together by the strong • force.

  23. The 4 “Forces” of Nature • There are 4 “fundamental forces” in nature: • Gravity: relative strength = 1, range = infinite. • Electromagnetic: rel. str. = 1036, range = infinite. • “Weak” nuclear: rel. str. = 1025, range = 10-10 meter. • “Strong” nuclear: rel. str. = 1038, range = 10-15 meter. • Gravity is the most important force over large scales since positive and negative charges tend to cancel.

  24. How to get energy from atoms • Fission: break apart the nucleus of a heavy element like uranium. • Fusion: combine the nuclei of a light element like hydrogen.

  25. More Nuclear Fusion • Each atomic nucleus has a “binding energy” associated with it. The curve is increasing as you go up to iron from small nuclei, and decreasing as you go down from large nuclei. • The tendency of Nature is to increase binding energy, much like the tendency of water to flow downhill. Image from Vik Dhillon (http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy213/phy213_fusion1.html)

  26. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • Fission of elements heaver than iron can release energy (leads to higher BE’s).

  27. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • Why does this produce energy? • Before: the mass of 4 protons is 4 proton masses. • After: the mass of 2 protons and 2 neutrons is 3.97 proton masses. • Einstein: E = mc2. The missing mass went into energy! 4H ---> 1He + energy

  28. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • Extremely high temperatures and densities are needed! Images from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

  29. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • Extremely high temperatures and densities are needed! The temperature is about 15,000,000K at the core of the Sun.

  30. More Nuclear Fusion • Fusion of elements lighter than iron can release energy (leads to higher BE’s). • As you fuse heavier elements up to iron, higher and higher temperatures are needed since more and more electrical charge repulsion needs to be overcome. • Hydrogen nuclei have 1 proton each • Helium nuclei have 2 protons each • Carbon nuclei have 6 protons each • ….. • The Sun is presently only fusing hydrogen since it is not hot enough to fuse helium.

  31. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • Why does this produce energy? • Before: the mass of 4 protons is 4 proton masses. • After: the mass of 2 protons and 2 neutrons is 3.97 proton masses. • Einstein: E = mc2. The missing mass went into energy! 4H ---> 1He + energy

  32. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • The details are a bit complex: Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

  33. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • The details are a bit complex:

  34. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • The details are a bit complex: • In the Sun, 6 hydrogen nuclei are involved in a sequence that produces two hydrogen nuclei and one helium nucleus. This is the proton-proton chain.

  35. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • The details are a bit complex: • In the Sun, 6 hydrogen nuclei are involved in a sequence that produces two hydrogen nuclei and one helium nucleus. This is the proton-proton chain. • In more massive stars, a carbon nucleus is involved as a catalyst. This is the CNO cycle.

  36. Nuclear Fusion • The CNO cycle (left) and pp chain (right) are outlined.

  37. Nuclear Fusion • Summary: 4 hydrogen nuclei (which are protons) combine to form 1 helium nucleus (which has two protons and two neutrons). • Why doesn’t the Sun blow up like a bomb? There is a natural “thermostat” in the core.

  38. Controlled Fusion in the Sun

  39. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature.

  40. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature. • Rate ~ (temperature)4 for p-p chain.

  41. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature. • Rate ~ (temperature)4 for p-p chain. • Rate ~ (temperature)15 for the CNO cycle.

  42. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature. • Rate ~ (temperature)4 for p-p chain. • Rate ~ (temperature)15 for the CNO cycle. • Small changes in the temperature lead to large changes in the fusion rate.

  43. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature. • Rate ~ (temperature)4 for p-p chain. • Rate ~ (temperature)15 for the CNO cycle. • Small changes in the temperature lead to large changes in the fusion rate. • Suppose the fusion rate inside the Sun increased:

  44. Controlled Fusion in the Sun • First, note that the rate of the p-p chain or CNO cycle is very sensitive to the temperature. • Rate ~ (temperature)4 for p-p chain. • Rate ~ (temperature)15 for the CNO cycle. • Small changes in the temperature lead to large changes in the fusion rate. • Suppose the fusion rate inside the Sun increased: • The increased energy heats the core and expands the star. But the expansion cools the core, lowering the fusion rate. The lower rate allows the core to shrink back to where it was before.

  45. Models of the Solar Interior • The interior of the Sun is relatively simple because it is an ideal gas, described by three quantities: • Temperature • Pressure • Mass density

  46. Models of the Solar Interior • The interior of the Sun is relatively simple because it is an ideal gas, described by three quantities: • Temperature • Pressure • Mass density • The relationship between these three quantities is called the equation of state.

  47. Ideal Gas • For a fixed volume, a hotter gas exerts a higher pressure: Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

  48. Hydrostatic Equilibrium • The Sun does not collapse on itself, nor does it expand rapidly.

  49. Hydrostatic Equilibrium • The Sun does not collapse on itself, nor does it expand rapidly. Gravity and internal pressure balance: Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

  50. Hydrostatic Equilibrium • The Sun does not collapse on itself, nor does it expand rapidly. Gravity and internal pressure balance. This is true at all layers of the Sun. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

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