THE SUN

1. THE SUN The star we see by day

2. Goals • Summarize the overall properties of the Sun. • What are the different parts of the Sun? • Where does the light we see come from? • The scientific method: solar neutrinos.

3. The Sun, Our Star • The Sun is an average star. • From the Sun, we base our understanding of all stars in the Universe. • No solid surface.

4. Vital Statistics • Radius = 100 x Earth (696,000 km) • Mass = 300,000 x Earth (1.99 x 1030 kg) • Surface temp = 5,800 K • Core temp = 15,000,000 K • Luminosity = 4 x 1026 Watts • Solar “Day” = • 24.9 Earth days (equator) • 29.8 Earth days (poles)

5. Structure • ‘Surface’ • Photosphere • ‘Atmosphere’ • Chromosphere • Transistion zone • Corona • Solar wind • ‘Interior’ • Convection zone • Radiation zone • Core

6. Interior Properties • Core = 20 x density of iron • Surface = 10,000 x less dense than air • Average density = Jupiter • Core = 15,000,000 K • Surface = 5800 K

7. Do you see the light? • Everything in the solar system reflects light. • Everything also absorbs light and heats up producing blackbody radiation. • Q: Where does this light come from? • A: The Sun. • But where does the Sun’s light come from?

8. Density = 20 x density of Iron Temperature = 15,000,000 K Hydrogen atoms fuse together. Create Helium atoms. In The Core

9. Nuclear Fusion • 4H  He • The mass of 4 H nuclei (4 protons): 4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg • The mass of He nuclei: = 6.643 x 10-27 kg • Where does the extra 4.7 x 10-29 kg go? • ENERGY!  E = mc2 • E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2 • E = hc/l l = 4.6 x 10-14 m (gamma rays) • So: 4H  He + light!

10. The Radiation Zone • This region is transparent to light. • Why? • At the temperatures near the core all atoms are ionized. • Electrons float freely from nuclei • If light wave hits atom, no electron to absorb it. • So: Light and atoms don’t interact. • Energy is passed from core, through this region, and towards surface by radiation.

11. The Convection Zone • This region is totally opaque to light. • Why? • Closer to surface, the temperature is cooler. • Atoms are no longer ionized. • Electrons around nuclei can absorb light from below. • No light from core ever reaches the surface! • But where does the energy in the light go? • Energy instead makes it to the surface by convection.

12. Convection • A pot of boiling water: • Hot material rises. • Cooler material sinks. • The energy from the pot’s hot bottom is physically carried by the convection cells in the water to the surface. • Same for the Sun.

13. Solar Cross-Section • Progressively smaller convection cells carry the energy towards surface. • See tops of these cells as granules.

14. The Photosphere • This is the origin of the 5,800 K blackbody radiation we see. • Why? • At the photosphere, the density is so low that the gas is again transparent to light. • The hot convection cell tops radiate energy as a function of their temperature (5800 K). l = k/T = k/(5800 K)  l = 480 nm (visible light) • This is the light we see. • That’s why we see this as the surface.

15. HOT You COOL The Solar Atmosphere • Above the photosphere: • Thin cool gas • Mostly transparent to light again • Unlike radiative zone, here atoms not totally ionized. • Therefore, there are electrons in atoms able to absorb light. • Absorption lines in solar spectrum are from these layers in the atmosphere.

16. The Chromosphere • Hydrogen most common. • Brightest hydrogen line – Ha. • Chromosphere = color

17. Ha Sun Photo by Big Bear Solar Observatory

18. Prominences

20. Corona • Magnetic activity carry energy up to the Transition Zone. • 10,000 km above photosphere. • Temperature climbs to 1,000,000 K • Remember photosphere is only 5800 K • The hot, low density, gas at this altitude emits the radiation we see as the Corona. • But corona very faint compared to photosphere.

22. Solar Wind • At and above the corona: • Gas is very hot • Very energetic • Like steam above our boiling pot of water, the gas ‘evaporates’. • Wind passes out through Coronal Holes • Solar Wind carries away a million tons of Sun’s mass each second! • Only 0.1% of total Sun’s mass in last 4.6 billion years.

23. Aurorae • The solar wind passes out through the Solar System. • Consists of electrons, protons and other charged particles stripped from the Sun’s surface. • When charged particles and magnetic fields interact: light!

25. Solar Cycle • Increase in solar wind activity - Coronal Mass Ejections • Increase in Auroral displays on Earth • Increase in disruptions on and around Earth. Courtesy of SOHO/LASCO/EIT consortium.

26. Magnetic fields and Sunspots • At kinks, disruption in convection cells. • Sunspots form.

27. 11-year sunspot cycle. • Center – Umbra: 4500 K • Edge – Penumbra: 5500 K • Photosphere: 5800 K Sunspots

28. Magnetic fields and Sunspots • Where magnetic fields “pop out” of Sun, form sunspots. • Sunspots come in pairs.

30. Solar Neutrino Problem • We observe: • Sun’s luminosity (total light radiated). • We hypothesize: • 4H  He + light + neutrinos • We can test: • Observe number neutrinos reaching Earth • Does or test agree with hypothesis? • No

31. What to Do? • For 30 years: • Theorists certain of nuclear reaction. • Observers positive of observations. • Detected only 1/3 the hypothesized neutrinos. • What to do?

32. Neutrino Flavors • 3 types of neutrinos • Electron neutrino • Tau neutrino • Muon neutrino • Nuclear reactions produce only electron neutrino. • Previous detectors only detected electron neutrinos.

33. Neutrino Fluctuations • New detector (2002) gives number of all three flavors. • Total number agrees with number predicted in core of Sun. • Conclusion: • Nuclear hypothesis is correct. • Neutrinos change flavor. • Neutrinos have mass (used to be thought massless). • Problem solved  new science discovered.