1. Sun: General Properties

2. The Sun General Properties Spectral type G2V Age: ~ 4.52 Gyr
General Properties
Spectral type G2V
Age: ~ 4.52 Gyr
Absolute visual magnitude MV = 4.83
Absolute bolometric magnitude Mbol = 4.76
Initial abundances:
X = 0.73, Y = 0.25, Z = 0.02
Central temperature = 15 million 0K
Effective (surface) temperature = K

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

4. Interior Structure of the Sun
Site of main energy production
dL/dr
0.1
0.2
0.3
0.4
0.5
r/R0

5. Interior Structure of the Sun
Mass composition
11H
0.8
Mass fraction
0.7
0.6
0.5
0.4
42He
0.3
0.2
0.1
32He (x100)
0.1
0.2
0.3
0.4
0.5
0.6
r/R0

6. The Sun’s Atmosphere Only visible during solar eclipses
Only visible during solar eclipses
Apparent surface of the sun
Heat Flow
Temp. incr. inward
Solar interior

7. Granulation … is the visible consequence of convection
… is the visible consequence of convection

8. Sun Spots (I)

9. Cooler regions of the photosphere (T ≈ 4000 K).
Sun Spots (II)
Active Regions
Visible
Ultraviolet
Cooler regions of the photosphere (T ≈ 4000 K).

10. Solar Activity, seen in soft X-rays
Solar Activity, seen in soft X-rays

11. Magnetic Fields in Sun Spots
Magnetic fields on the photosphere can be measured through the Zeeman effect
→ Sun Spots are related to magnetic activity on the photosphere

12. 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.

13. Magnetic Loops Magnetic field lines
Magnetic field lines
Mass ejection from the sun often follow magnetic field loops.

14. The Solar Cycle 11-year cycle Reversal of magnetic polarity
11-year cycle
Reversal of magnetic polarity
After 11 years, North/South order of leading/trailing sun spots is reversed
=> Total solar cycle = 22 years

15. The Sun’s Magnetic Cycle
After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged.
→ New magnetic field structure is similar to the original one, but reversed!
→ New 11-year cycle starts with reversed magnetic-field orientation

16. The Solar Cycle (II) Maunder Butterfly Diagram
Maunder Butterfly Diagram
Sun spot cycle starts out with spots at higher latitudes on the sun
Evolve to lower latitudes (towards the equator) throughout the cycle.

17. The sun spot number also fluctuates on much longer time scales:
The Maunder Minimum
The sun spot number also fluctuates on much longer time scales:
Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

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

19. 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)

20. Eruptive Prominences
(Ultraviolet images)

21. Sound waves produced by a solar flare
~ 5 minutes
Solar Aurora
Sound waves produced by a solar flare
Coronal mass ejections

22. The Chromosphere
Region of sun’s atmosphere just above the photosphere.
Chromospheric structures visible in Ha emission
Absorption and emission lines from singly ionized metals (He II, Fe II, Si II, Cr II, Ca II)
T: 4400 K → 25,000 K
n: 1011 cm-3 → 107 cm-3

23. Spicules = filaments of hot gas, visible in Ha emission.
The Chromosphere (II)
Spicules = filaments of hot gas, visible in Ha emission.
Streams extend up to ~ 10,000 km above photosphere

24. The Transition Zone Transition from moderate to high ionization
Transition from moderate to high ionization
h ~ 2300 – 2600 km above photosphere
T ~ 25,000 → 106 K
n ~ 107 → 105 cm-3
Observe selective heights in (UV) emission lines of ionized metals
Ly a → ~ 20,000 K
O VI 1032 → ~ 300,000 K
CIII 977 → ~ 90,000 K
Mg X 625 → ~ 1,400,000 K

25. The Solar Corona Very hot (T ≥ 106 K), low-density (n ≤ 105 cm-3) gas
Very hot (T ≥ 106 K), low-density (n ≤ 105 cm-3) gas

26. Active vs. Quiescent Corona
Near Solar Maximum
Near Solar Minimum

27. X-ray images of the sun reveal coronal holes.
X-ray images of the sun reveal coronal holes.
These arise at the foot points of open field lines and are the origin of the solar wind.