1. Please press “1” to test your transmitter.
The Sun
Our Star
Please press “1” to test your transmitter.

2. General Properties Average star
Average star
Only appears so bright because it is so close.
109 times Earth’s diameter
333,000 times Earth’s mass
Consists entirely of gas (av. density = 1.4 g/cm3)
Central temperature = 15 million 0K
Surface temperature = K

3. Which parts of the sun can only be seen during a total solar eclipse?
Prominences
The solar corona
Sun spots
1 and 2
All of the above.

4. Structure of the Sun Only visible during solar eclipses
Only visible during solar eclipses
Apparent surface of the sun
Heat Flow
Temp. incr. inward
Solar interior

5. The Sun’s Interior Structure
Photosphere
Energy transport via convection (explained soon)
Flow of energy
Energy transport via radiation
Energy generation via nuclear fusion
Temp, density and pressure decr. outward

6. Do we have a direct view of the sun’s energy source?
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Yes, because the sun is just a transparent gas ball.
Yes, because most of the energy is produced very close to the surface.
Yes, because the sun’s center is so bright that the light is shining through any material.
No, because the sun has a non-transparent solid surface.
No, because the radiation produced in the center is scattered around many times on its way towards the surface. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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7. How is energy produced in an H bomb?
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(Chemical) Burning of hydrogen.
Nuclear fusion of hydrogen into heavier elements.
Nuclear fission of hydrogen.
Nuclear fission of heavier elements into hydrogen.
Nuclear fission of heavier elements into elements heavier than hydrogen. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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8. Energy generation in the Sun: Fusion of Hydrogen into Helium
Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons).
Basic reaction:
4 1H → 4He + energy
4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.
T ≥ 107 K = 10 million K
Energy gain = Dm*c2
= 0.43*10-11 J
per reaction.
Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

9. How do we know that the sun is made mostly of Hydrogen?
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Space probes have taken samples of solar material and analyzed it.
The sun’s spectrum shows strong emission lines from Hydrogen.
The sun’s spectrum shows strong absorption lines from Hydrogen.
Hydrogen is very easily flammable; this explains the sun’s brightness.
Nonsense! The sun is actually made mostly of Nitrogen and Oxygen. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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10. Absorption Lines

11. Analyzing absorption spectra
Each element produces a specific set of absorption (and emission) lines.
Comparing the relative strengths of these sets of lines, we can study the composition of gases.
By far the most abundant elements in the Universe

12. Which Hydrogen Lines appear in visible light?
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The Balmer Lines (from/to the first excited state)
The Balmer Lines (from/to the ground state)
The Lyman Lines (from/to the first excited state)
The Lyman Lines (from/to the ground state)
The Einstein Lines (from/to the second excited state) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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13. The Balmer Lines Ha Hb Hg …
Transitions from 2nd to higher levels of hydrogen
n = 1
n = 4
n = 5
n = 3
n = 2 Ha Hb Hg
The only hydrogen lines in the visible wavelength range.
2nd to 3rd level = Ha (Balmer alpha line)
2nd to 4th level = Hb (Balmer beta line) …

14. The Cocoon Nebula (Ha emission)

15. Energy Transport Radiative energy transport Convection g-rays
Energy generated in the sun’s center must be transported to the surface.
Inner layers:
Radiative energy transport
Outer layers (including photosphere):
Convection
Cool gas sinking down
Bubbles of hot gas rising up
Gas particles of solar interior
g-rays

16. Granulation
… is the visible consequence of convection

17. Which every-day phenomenon is another example of convective energy transport?
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Gas bubbles rising up in a soda drink.
Gas bubbles rising up in boiling water.
Giant waves moving onto the sea shore.
Earthquakes.
All of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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18. Which every-day phenomenon is another example of radiative energy transport?
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The heat of a bonfire warming you when you’re sitting close to it.
Heating food in the microwave oven.
The air around a light bulb heating up when the light is on.
All of the above.
None of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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19. The Sun’s Interior Structure
Photosphere
Energy transport via convection (explained soon)
Flow of energy
Energy transport via radiation
Energy generation via nuclear fusion
Animation
Temp, density and pressure decr. outward

20. Very Important Warning:
Never look directly at the sun through a telescope or binoculars!!!
This can cause permanent eye damage – even blindness.
Use a projection technique or a special sun viewing filter.

21. Cooler regions of the photosphere (T ≈ 4240 K).
Sun Spots
Active Regions
Visible
Ultraviolet
Cooler regions of the photosphere (T ≈ 4240 K).

23. Considering that sunspots are cooler regions on the photosphere with a temperature of ~ 4240 K, how would you think a sunspot would appear if you could put it on the night sky without the sun surrounding it?
It would be invisible.
It would glow very faintly, similar to the faint red glow of the eclipsed moon.
It would appear moderately bright, comparable to the brightest stars.
It would appear very bright – even brighter than the full moon.
It would be almost as bright as the sun itself.

24. Solar Activity, seen in soft X-rays

25. What can we infer from the fact that we see the gas above active regions (sun spots) mostly in ultraviolet light and X-rays?
The gas must be very dense.
The gas must be very dilute.
The gas must be very hot.
The gas must be very cold.
The gas must consist mostly of Helium.

26. Magnetic field in sun spots is about 1000 times stronger than average.
Sun Spots (III)
Magnetic North Poles
Magnetic South Poles
Related to magnetic activity.
Magnetic field in sun spots is about 1000 times stronger than average.
In sun spots, magnetic field lines emerge out of the photosphere.

27. Magnetic Field Lines Magnetic North Pole Magnetic South Pole
Magnetic North Pole
Magnetic South Pole
Magnetic Field Lines
Mass ejections from the sun often follow magnetic field loops.

28. After 11 years, the direction of magnetic fields is reversed
The Solar Cycle
Solar Maxima
11-year cycle
Full 22-year cycle
Reversal of magnetic polarity (during solar minima)
After 11 years, the direction of magnetic fields is reversed
=> Total solar cycle = 22 years

29. Are we currently near a solar maximum or a solar minimum?
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30. The Solar Corona
Very hot (T ≥ 1 million 0K), low-density gas

31. Prominences
Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

32. Eruptive Prominences
Extreme events, called coronal mass ejections (CMEs) and solar flares, can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis).
(Ultraviolet images)

33. (Ultraviolet images)
Eruptive Prominences