1. The star we see but seldom notice
THE SUN
The star we see but seldom notice

2. Goals Summarize the overall properties of the Sun.
What are the different parts of the Sun and how do we know this?
Where does the light we see come from?
Solar activity and magnetic fields.

3. The Sun, Our Star The Sun is an average star.
From the Sun, we base our understanding of all stars in the Universe.
Like Jovian Planets it’s a giant ball of gas.
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 = 5780 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’ ‘Atmosphere’ ‘Interior’ Photosphere Chromosphere
Transistion zone
Corona
Solar wind
‘Interior’
Convection zone
Radiation zone
Core

6. The Solar Interior How do we know what’s inside the Sun?
Observe the outside.
Theorize what happens on the inside.
Complex computer programs model the theory.
Model predicts what will happen on the outside.
Compare model prediction with observations of the outside.
Scientific Method!

7. 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).

8. 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 = 5780 K

9. 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?

10. Our Journey through the Sun
Journey from the Sun’s core to the edge of its ‘atmosphere.’
See where its light originates.
See what the different regions of the Sun are like.
See how energy in the core makes it to the light we see on Earth.

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

12. Nuclear Fusion 4H  He The mass of 4 H atoms:
4 x (1.674 x10-27 kg) = x kg
The mass of He atom: = x kg
Where does the extra 4.8 x kg go?
ENERGY!  E = mc2
E = (4.8 x kg ) x (3.0 x 108 m/s)2
E = hc/l  l = 4.6 x m (gamma rays)
So: 4H  He + light!

13. 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.

14. 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.

15. 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.

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

17. The Photosphere
This is the origin of the 5800 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.

18. The Solar Atmosphere Above the photosphere, transparent to light.
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.

19. Atmospheric Composition
Probably same as interior.
Same as seen on Jupiter.
Same as the rest of the Universe.

20. The Chromosphere Very low density But also very hot
Same as the gas tubes we saw in class and lab.
Energy from below excites the atoms and produces emission from this layer.
Predominant element – Hydrogen.
Brightest hydrogen line – Ha.
Chromosphere = color

21. Spicules and Prominences
Emission from the atmosphere is very faint relative to photosphere.
Violent storms in the Chromosphere.
Giant curved prominances
Long thin spicules.

22. Prominences

24. Ha Sun
Photo by Robert Gendler

25. Corona
Spicules and other 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.

27. The X-Ray Sun
Q: At 1,000,000 K where does a blackbody spectrum have its peak?
A: X-rays
Can monitor the Solar Coronasphere in the X-ray spectrum.
Monitor Coronal Holes

29. 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.

30. The Aurora The solar wind passes out through the Solar System.
Consists of electrons, protons and other charged particles stripped from the Sun’s surface.
Interaction with planetary magnetic fields gives rise to the aurora.

31. The Active Sun Solar luminosity is nearly constant.
Very slight fluctuations.
11-year cycle of activity.

32. Solar Cycle Increase in Coronal holes Increase in solar wind activity
- Coronal Mass Ejections
Increase in Auroral displays on Earth
Increase in disruptions on and around Earth.

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

34. Can see that Sun doesn’t rotate as a solid body?
Equator rotates faster.
This differential rotation leads to complications in the Solar magnetic field.

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

36. Magnetic fields and Sunspots
Sunspots come in pairs.
Opposite orientation in North and South.
Every other cycle the magnetic fields switch.

37. Sunspot Numbers

38. Active Regions
Areas around sunspots give rise to the prominences