1. The Sun Astronomy 311 Professor Lee Carkner Lecture 23

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

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

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

5. 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

6. 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 --

7. 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 )

8. Hydrogen Fusion

9. How Does the Energy at the Core Get Out?  Energy in the Sun is transported in two ways:  Radiation --  Like a heat lamp   Gas in this area is very hot and ionized and so doesn’t absorb the radiation very well  Convection --  Like baseboard heating   Cooler outer layers are less ionized and so absorb the radiation and heat up and rise

10. The Inner Structure of the Sun

11. 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  The surface of the sun is covered in granules where hot gas from the interior of the Sun is rising  Each about 1000 km across

12. Granules

13. Sunspots in the Photosphere  The photosphere sometimes has small dark regions called sunspots   Sunspots are regions where the Sun’s magnetic field inhibits the flow of warmer material

14. Sunspots

15. 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

16. Sunspot Maximum and Minimum

17. The Sunspot Cycle

18. Sunspot Cycles and Differential Rotation  The Sun rotates differentially   The magnetic field gets “wrapped-up” around the equator   Eventually the magnetic field is squeezed so tight it reconnects and cancels itself out   The magnetic field prevents warmer material from flowing into the sunspots, so the sunspots are cooler than the rest of the photosphere

19. The Twisted Magnetic Field of the Sun

20. 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

21. 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

22. 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

23. The Corona  The corona is the outer layer of the Sun’s atmosphere   It is thinner and hotter than the chromosphere   The high temperatures produce a hot ionized gas called a plasma  This plasma emits strongly at ultraviolet and X-ray wavelengths

24. Flares and Magnetic Activity   Sometimes a large outburst of material is seen, called a coronal mass ejection   They are both examples of magnetic activity   During the Maunder Minimum in sunspot activity (1645-1715) it was very cold in Europe (The Little Ice Age)

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

26. 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

27. 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