14.1 A Closer Look at the Sun

Transcription

1 14.1 A Closer Look at the Sun Our goals for learning: Why does the Sun shine? What is the Sun's structure?

2 Why does the Sun shine?

3 Is it on FIRE?

4 Is it on FIRE? NO! Chemical energy content Luminosity ~ 10,000 years

5 Is it CONTRACTING?

6 Is it CONTRACTING? NO! Gravitational potential energy Luminosity ~ 25 million years

7 NUCLEAR ENERGY! Nuclear potential energy (core) Luminosity ~ 10 billion years

8 Weight of upper layers compresses lower layers.

9 Gravitational equilibrium: Energy supplied by fusion maintains the pressure that balances the inward crush of gravity.

10 Energy Balance: The rate at which energy radiates from the surface of the Sun must be the same as the rate at which it is released by fusion in the core.

11 Gravitational contraction: Provided the energy that heated the core as Sun was forming Contraction stopped when fusion began.

12 What is the Sun's structure?

13 Radius: m (109 times Earth) Mass: kg (300,000 Earths) Luminosity: watts

14 Solar wind: A flow of charged particles from the surface of the Sun

15 Corona: Outermost layer of solar atmosphere ~1 million K

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17 Chromosphere: Middle layer of solar atmosphere ~ K

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20 Photosphere: Visible surface of Sun ~ 6000 K Light emerges from the core

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22 Convection Zone: Energy transported upward by rising hot gas

23 Radiation Zone: Energy transported upward by photons

24 Core: Energy generated by nuclear fusion ~ 15 million K

25 By the time photons reach the surface of the Sun, they are mostly infrared light. visible light. ultraviolet light. X rays. gamma rays.

26 By the time photons reach the surface of the Sun, they are mostly infrared light. visible light. ultraviolet light. X rays. gamma rays.

27 What have we learned? Why does the Sun shine? Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

28 What have we learned? What is the Sun's structure? From inside out, the layers are: Core Radiation zone Convection zone Photosphere Chromosphere Corona

29 14.2 Nuclear Fusion in the Sun Our goals for learning: How does nuclear fusion occur in the Sun? How does the energy from fusion get out of the Sun? How do we know what is happening inside the Sun?

30 Fission Big nucleus splits into smaller pieces. (Example: nuclear power plants) Fusion Small nuclei stick together to make a bigger one. (Example: the Sun, stars)

31 High temperatures enable nuclear fusion to happen in the core.

32 The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

33 The proton proton chain is how hydrogen fuses into helium in Sun.

34 IN 4 protons OUT 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower.

35 Thought Question 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.

36 Thought Question 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. The solar thermostat keeps burning rate steady.

37 Solar Thermostat Decline in core temperature causes fusion rate to drop, so core contracts and heats up. Rise in core temperature causes fusion rate to rise, so core expands and cools down.

38 How does the energy from fusion get out of the Sun?

39 Energy gradually leaks out of radiation zone in form of randomly bouncing photons.

40 Convection (rising hot gas) takes energy to surface.

41 Bright blobs on photosphere show where hot gas is reaching the surface.

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43 If fusion in the solar core ceased today, worldwide panic would break out tomorrow as the Sun began to grow dimmer. Yes, because Earth would quickly freeze over. Yes, because Earth would no longer be bound to the solar system and would drift into space. Yes, because the Sun would collapse and the planets would soon follow. No, it takes thousands of years for photons created in nuclear reactions at the solar core to reach the surface. No, the Sun would continue to glow brightly for billions of years because of gravitational contraction.

44 If fusion in the solar core ceased today, worldwide panic would break out tomorrow as the Sun began to grow dimmer. Yes, because Earth would quickly freeze over. Yes, because Earth would no longer be bound to the solar system and would drift into space. Yes, because the Sun would collapse and the planets would soon follow. No, it takes thousands of years for photons created in nuclear reactions at the solar core to reach the surface. No, the Sun would continue to glow brightly for billions of years because of gravitational contraction.