THE SUN

1. THE SUN The star we see by day

2. 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.

3. 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

4. Scientific Method • How do we know this stuff? • Three examples: • Fusion in the core (core temperature). • Different zones in interior. • Solar activity and Earth

5. 1. The Core • Scientific Method: • Observations • Make hypothesis (a model) • Models make predictions • Test predictions • Compare results of predictions with observations • Revise model if necessary.

6. Testing the Core • Observe Sun’s: • Mass (how?) • Composition (how?) • Radius • Luminosity (total energy output) • Use physics to make a model Sun. • Predict: • Surface temp/density (how do you test?) • Surface Luminosity (how do you test?) • Core temp/density  Fusion Rate  neutrino rate (test?)

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

8. 2. Helioseismology • Continuous monitoring of Sun. • Ground based observatories • One spacecraft (SOHO) • Surface of the Sun is ‘ringing’ • Sound waves cross the the solar interior and reflect off of the surface (photosphere).

9. Solar Interior • Core • Only place with fusion • Radiation Zone • Transparent • Convections Zone • Boiling hot

10. 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.

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

12. The Photosphere • This is the origin of the 5,800 K thermal radiation we see. 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.”

13. 3. Solar Activity and Earth • Is there a connection between Solar Activity and Earth’s Climate? • Observation: • Little Ice Age • Maunder Minimum

14. What is Solar Activity? • Sunspots • Magnetic Fields • Coronal Mass Ejections • Solar Wind • Magnetic Storms • Aurora • Other effects?

15. 11-year sunspot cycle. • Center – Umbra: 4500 K • Edge – Penumbra: 5500 K • Photosphere: 5800 K Sunspots

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

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

18. Prominences Hot low density gas = emission lines

19. Stereo

21. Corona and Solar Wind • Hot, low density, gas emits the radiation we see as the Corona: 1,000,000 K • Solar Wind: Like steam above our boiling pot of water, the gas ‘evaporates’. • 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. 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.

25. Aurora • The solar wind passes out through the Solar System. • Consists of electrons, protons and other charged particles stripped from the Sun’s surface. • Magnetic fields herd charged particles into atmosphere at poles. • Charged particles excite electrons in atoms.  Light!

28. 2003 CME Oklahoma 10/29/2003 Credit: E. Woldt