1. The Sun Astronomy 311 Professor Lee Carkner Lecture 23

2. What is responsible for creating a comet’s tail? a)The solar wind b)The comet’s motion c)Light pressure d)a and c only e)a, b, and c

3. Which of the following would you not expect to find in a comet? a)Ammonia b)Carbon Dioxide c)Iron d)Silicates e)Carbon compounds

4. Helios -- The God of the Sun   The Sun was often worshiped by ancient people

5. WARNING !  NEVER LOOK DIRECTLY AT THE SUN  NEVER LOOK AT THE SUN WITH BINOCULARS OR A TELESCOPE  PERMANENT EYE DAMAGE CAN RESULT

6. Observing the Sun   The solar surface is nearly featureless   Much of our information about the Sun comes from multiwavelength observations, indirect measurements, and modeling

7. The Sun From the Inside Out   The solar interior is the source of solar energy which is transported to the surface  Nuclear fusion and magnetic fields play key roles in the energetics and structure of the Sun

8. Why Does the Sun Shine?   What could power the Sun for this length of time?  Chemical energy (burning) --  Kelvin-Helmholtz contraction (gravitational energy) --  Nuclear Fusion Reactions --

9. The Core  At the core of the Sun the temperature and pressure are very high due to the weight of the outer layers   At these conditions the hydrogen atoms are moving so fast and are packed together so tightly that they can fuse together to make helium:  Each reaction converts some mass to energy (E=mc 2 )

10. Hydrogen Fusion

11. How Does the Energy at the Core Get Out?  Energy in the Sun is transported in two ways:  Radiation -- by photons (light particles)   Transports the energy from the core to about 75% of the way to the surface   Convection -- Hot gas rises, cool gas sinks  Like baseboard heating   Cooler outer layers are less ionized and so absorb the radiation and heat up and rise

12. The Inner Structure of the Sun

13. The Photosphere  It takes about 170,000 years for the energy to reach the surface of the Sun   The visible surface of the Sun is called the photosphere   The photosphere is the top of the convection zone   Each about 1000 km across

14. Granules

15. Sunspots in the Photosphere   Sunspots look dark because they are cooler than the rest of the photosphere  Sunspots are regions where the Sun’s magnetic field inhibits the flow of warmer material

16. Sunspots

17. Sunspot Cycles  Sunspots exist for a maximum of a few months   There is a sunspot cycle of 11 years   The year 2000 was a sunspot maximum   Just after sunspot minimum the spots appear at about 30 degrees north or south

18. Sunspot Maximum and Minimum

19. The Sunspot Cycle

20. Sunspot Cycles and Differential Rotation   The equator moves faster than the poles   Over time the wrapping gets tighter   The sunspots are caused by the magnetic field “kinks”  The magnetic field prevents warmer material from flowing into the sunspots, so the sunspots are cooler than the rest of the photosphere

21. The Twisted Magnetic Field of the Sun

22. The Sun’s Magnetic Field   It is twisted and tangled producing loops of magnetic force   The Sun’s magnetic field is responsible for the structure and heating of the outer layers of the atmosphere

23. The Solar Wind   This flow of charged particles is called the solar wind  Interacts with the Earth’s magnetic field   Very strong solar wind bursts can cause power and communication failures

24. The Chromosphere   It has a density of 1/10000 of the photosphere so it is hard to see  It has a temperature of about 20000 K   It is too thin for the atoms to collide and lose energy

25. The Corona   The light from the photosphere normally washes out the corona, but you can see it during an eclipse   T~10 million K   This plasma emits strongly at ultraviolet and X-ray wavelengths

26. Flares and Magnetic Activity   Sometimes a large outburst of material is seen, called a coronal mass ejection   They are both examples of magnetic activity  Changes in magnetic activity also seem to effect climate 

27. Structure of the Sun Core Radiative Zone Convective Zone Photosphere Chromosphere Corona

28. Summary: Structure of the Sun  Core  fusion converts H to He and power the Sun  Radiative Layer  transports energy from the core  Convective Layer  transports energy to the photosphere  Photosphere  visible surface of the Sun  Chromosphere  hot middle atmospheric layer  Corona  very hot outer layer

29. Summary: Solar Energetics  Thermonuclear Fusion  Energy is produced at the core of the Sun by converting hydrogen to helium  Energy Transport  Energy is transported via radiation where the opacity is low and via convection where the opacity is high  Magnetic Fields  The outer layers of the Sun are composed of hot plasma in magnetic loops