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: Use physics to make a model Sun.
Mass (how?)
Composition (how?)
Radius
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. In The Core Density = 20 x density of Iron Temperature = 15,000,000 K
Hydrogen atoms fuse together.
Create Helium atoms.

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 Radiation Zone Convections Zone
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. Sunspots 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

20. 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.

22. 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.

23. 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!

26. 2003 CME
Oklahoma 10/29/2003
Credit: E. Woldt

27. Homework #7
Due Friday 24-Sept, Read Exoplanet articles plus the website:
Why do we believe there are planets around other stars? How have we detected them?