A100 Exploring the Universe: How Stars Work. Martin D. Weinberg UMass Astronomy

Transcription

1 A100 Exploring the Universe: How Stars Work Martin D. Weinberg UMass Astronomy October 07, 2014 Read: Chaps 14, 15 10/07/12 slide 1

2 Exam scores posted in Mastering Questions returned and keys posted after make-ups Read: Chaps 14, 15 10/07/12 slide 2

3 Exam scores posted in Mastering Questions returned and keys posted after make-ups Brief discussion of selected exam questions Today: More on stars Finish: How stars work? What is fusion? How is energy transported? How have we tested our theories? Read: Chaps 14, 15 10/07/12 slide 2

4 Exam scores posted in Mastering Questions returned and keys posted after make-ups Brief discussion of selected exam questions Today: More on stars Finish: How stars work? What is fusion? How is energy transported? How have we tested our theories? Questions? Read: Chaps 14, 15 10/07/12 slide 2

5 Maxwell distribution & exam grades What do they have in common?? Read: Chaps 14, 15 10/07/12 slide 3

6 Maxwell distribution & exam grades Distribution of scores Read: Chaps 14, 15 10/07/12 slide 3

7 Maxwell distribution & exam grades Fraction with given speed cold warm hot Speed Read: Chaps 14, 15 10/07/12 slide 3

8 Maxwell distribution & exam grades What do they have in common?? Fraction with given speed Speed cold warm Let s roll some dice! hot 1. Choose some number of dies (1,2,3,4... ) 2. Roll and add up faces 3. Repeat Step 2 and make a histogram Read: Chaps 14, 15 10/07/12 slide 3

9 Gravitational equilibrium Energy provided by fusion maintains the pressure Rate of fusion very sensitive to temperature Mass of protons and neutrons converted to energy as larger nuclei are built Mass less than sum of parts E = mc 2 Read: Chaps 14, 15 10/07/12 slide 4

10 P-P fusion reaction 3 steps in a fusion reaction Read: Chaps 14, 15 10/07/12 slide 5

11 P-P fusion reaction 3 steps in a fusion reaction Read: Chaps 14, 15 10/07/12 slide 5

12 P-P fusion reaction 3 steps in a fusion reaction Read: Chaps 14, 15 10/07/12 slide 5

13 Mass, energy and fusion P-P fusion reaction Certain combinations of protons and nuclei are less than their sum of parts Can liberate energy by assembling a more massive nucleus! m lost = u u 0.03 u E lost = m lost c 2 = ( m/s) 2 E lost J 28 MeV = 28,000,000 ev Read: Chaps 14, 15 10/07/12 slide 6

14 Mass, energy and fusion P-P fusion reaction Certain combinations of protons and nuclei are less than their sum of parts Can liberate energy by assembling a more massive nucleus! m lost = u u 0.03 u E lost = m lost c 2 = ( m/s) 2 E lost J 28 MeV = 28,000,000 ev Read: Chaps 14, 15 10/07/12 slide 6

15 Mass, energy and fusion P-P fusion reaction Certain combinations of protons and nuclei are less than their sum of parts Can liberate energy by assembling a more massive nucleus! m lost = u u 0.03 u E lost = m lost c 2 = ( m/s) 2 E lost J 28 MeV = 28,000,000 ev Read: Chaps 14, 15 10/07/12 slide 6

16 Mass, energy and fusion P-P fusion reaction Certain combinations of protons and nuclei are less than their sum of parts Can liberate energy by assembling a more massive nucleus! m lost = u u 0.03 u E lost = m lost c 2 = ( m/s) 2 E lost J 28 MeV = 28,000,000 ev Read: Chaps 14, 15 10/07/12 slide 6

17 Mass, energy and fusion P-P fusion reaction Elements below iron (Fe) can from by fusion Elements below iron (Fe) cannot (radioactive decay) Most elements heavier than helium are made in stars! Fusion elements made in stars Fission elements made when star explodes Read: Chaps 14, 15 10/07/12 slide 7

18 Fusion rate acts a theromstat If excess energy produced, star expands As star expands, core of star cools As core cools, fusion rate drops quickly As energy production drops, pressure drops Star contracts and heats by falling Equilibrium is maintained! Read: Chaps 14, 15 10/07/12 slide 8

19 How does energy get to surface? Fusion temperature: 10 million degrees Outer Sun: 5000 degrees Read: Chaps 14, 15 10/07/12 slide 9

20 Three mechanisms for energy transport 1. Conduction: heat flows from hot to cold Energy transferred from atom to atom E.g. handle of frying pan on stove Read: Chaps 14, 15 10/07/12 slide 10

21 Three mechanisms for energy transport 1. Conduction: heat flows from hot to cold Energy transferred from atom to atom E.g. handle of frying pan on stove 2. Radiative diffusion: high energy photons interact with matter give up some of their energy, replenishing local heat supply Random walk [movie] Takes 10 7 (10 million) years for photon generated to get out! Photons can also provide some of the pressure to support star against its own gravitational pull Read: Chaps 14, 15 10/07/12 slide 10

22 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. Hot blobs of gas rise, release energy Cool blobs of gas fall Example: coffee cup with milk... Read: Chaps 14, 15 10/07/12 slide 11

23 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. [movie] Read: Chaps 14, 15 10/07/12 slide 11

24 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. [movie] Read: Chaps 14, 15 10/07/12 slide 11

25 3. Convection: energy carried from hotter regions below to cooler regions above by bulk buoyant motions of the gas. Instant coffee with soy milk [movie] Read: Chaps 14, 15 10/07/12 slide 11

26 Fusion: radii < 0.25R Radiative diffusion: 0.25R < radii < 0.71R Convection: 0.71R < radii < 1.00R Solar wind: R < radii Read: Chaps 14, 15 10/07/12 slide 12

27 Radius: m (109 Earth radii) Mass: kg (300,000 Earth masses) Luminosity: watts Read: Chaps 14, 15 10/07/12 slide 13

28 What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly B. The core would expand and cool C. The Sun would blow up like a hydrogen bomb Read: Chaps 14, 15 10/07/12 slide 14

29 What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly B. The core would expand and cool C. The Sun would blow up like a hydrogen bomb Read: Chaps 14, 15 10/07/12 slide 14

30 How do we know what is happening inside the Sun? Read: Chaps 14, 15 10/07/12 slide 15

31 How do we know what is happening inside the Sun? Making mathematical models Observing solar vibrations Observing solar neutrinos Read: Chaps 14, 15 10/07/12 slide 15

32 Patterns of vibration on surface tell us about what Sun is like inside Read: Chaps 14, 15 10/07/12 slide 16

33 Patterns of vibration on surface tell us about what Sun is like inside Data on solar vibrations agree very well with mathematical models of solar interior Read: Chaps 14, 15 10/07/12 slide 16

34 from fusion fly directly through the Sun Solar neutrinos can tell us whats happening in core Read: Chaps 14, 15 10/07/12 slide 17

35 Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number Read: Chaps 14, 15 10/07/12 slide 18

36 Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number More recent observations find the right number of neutrinos, but some have changed form Read: Chaps 14, 15 10/07/12 slide 18

37 Surface of the Sun Three regions: Photosphere most of Sun s luminosity Chromosphere Above the photosphere Superheated region above the Chromosphere Read: Chaps 14, 15 10/07/12 slide 19

38 Surface of the Sun Three regions: Photosphere most of Sun s luminosity Above: photons stream without interacting Below: photons interact with solar material Chromosphere Above the photosphere Superheated region above the Chromosphere Read: Chaps 14, 15 10/07/12 slide 19

39 Surface of the Sun Three regions: Photosphere most of Sun s luminosity Above: photons stream without interacting Below: photons interact with solar material Chromosphere Above the photosphere Lower density, tenuous atmosphere. cooler. Slightly Superheated region above the Chromosphere Read: Chaps 14, 15 10/07/12 slide 19

40 Surface of the Sun Three regions: Photosphere most of Sun s luminosity Above: photons stream without interacting Below: photons interact with solar material Chromosphere Above the photosphere Lower density, tenuous atmosphere. cooler. Slightly Superheated region above the Chromosphere Origin of the solar wind, protons that have escaped the Sun. Read: Chaps 14, 15 10/07/12 slide 19

41 Photosphere Sun appears darker around the edge Photons from limb from greater height in atmosphere Upper photosphere/lower chromosphere is cool Read: Chaps 14, 15 10/07/12 slide 20

42 Photosphere Sun appears darker around the edge Photons from limb from greater height in atmosphere Upper photosphere/lower chromosphere is cool Read: Chaps 14, 15 10/07/12 slide 20

43 Image of Sun during total eclipse Material streams away from Sun Narrow transition between chromosphere and corona Read: Chaps 14, 15 10/07/12 slide 21

44 Read: Chaps 14, 15 10/07/12 slide 22

45 [movie] Read: Chaps 14, 15 10/07/12 slide 22

46 are regions of high magnetic field Zeeman splitting Read: Chaps 14, 15 10/07/12 slide 23

47 Consequences: magnetic fields! Generated by moving charges: electric currents Continuous loops of lines of force that have both tension and pressure (like rubber bands) Can be strengthened by stretching them, twisting them, and folding them back on themselves. This stretching, twisting, and folding is done by the fluid flows within the Sun! Read: Chaps 14, 15 10/07/12 slide 24

48 Magnetic field is trapped in ionized gas (plasma) Rotating Sun winds up the field lines Read: Chaps 14, 15 10/07/12 slide 25

49 TRACE solar mission (NASA) SOHO solar mission (ESA) UV image Read: Chaps 14, 15 10/07/12 slide 26

50 Solar Components Solar Wind: a flow of charged particles from the surface of the Sun Read: Chaps 14, 15 10/07/12 slide 27

51 Solar Components : outermost layer of the solar atmosphere, T 1 million K Read: Chaps 14, 15 10/07/12 slide 27

52 Solar Components Chromosphere: middle layer of the solar atmosphere, T K Read: Chaps 14, 15 10/07/12 slide 27

53 Solar Components Photosphere: visible surface of the Sun, T 6,000 K Read: Chaps 14, 15 10/07/12 slide 27

54 Solar Components Convection zone: energy transported upwards by rising gas Read: Chaps 14, 15 10/07/12 slide 27

55 Solar Components Radiation zone: energy tranported upward by photons Read: Chaps 14, 15 10/07/12 slide 27

56 Solar Components Core: energy generated by nuclear fusion, T 15 million K Read: Chaps 14, 15 10/07/12 slide 27