1. The Sun
Chapter 10

2. The Sun - Our Star “Average” star Made entirely of gas
too hot for liquids or solids
Surface temp = 5000 K
Center temp = 10 million K
Composition (by mass):
70% H
28% He
2% everything else (C, N, O, etc.)

3. Major Regions of the Sun
Interior
Core
Radiative Zone
Convective Zone
Atmosphere
Photosphere
Chromosphere
Corona

4. Photosphere Effective “surface” of sun Thin atmospheric layer
not solid
region we see by eye
Thin atmospheric layer
Few hundred km
Low density gas
0.01% Earth’s atmosphere
Close-up shows granulation
convection cells
Sunspots
Cooler than surrounding gas
appear dark

5. Atmospheric Features Sunspots Larger than Earth Occur in groups
Associated with magnetic fields
Galileo used to measure solar rotation
25 days at equator
longer at poles
Number of sunspots
Cyclical
11 yr period
Correlated with solar activity
greatest activity at sunspot maximum
least at minimum

6. Most Detailed Sunspot Image Ever!
Swedish Vacuum Telescope

7. Solar Rotation

8. Atmospheric Features Sunspots Larger than Earth Occur in groups
Associated with magnetic fields
Galileo used to measure solar rotation
25 days at equator
longer at poles
Number of sunspots
Cyclical
11 yr period
Correlated with solar activity
greatest activity at sunspot maximum
least at minimum

9. Atmospheric Features Prominences Loops of hot gas Base near sunspots
Trace magnetic fields

10. Chromosphere Region just above photosphere Emission-line spectrum
indicates higher temperature
reddish color from hydrogen line
Temperature increases outward

11. Transition Region Between chromosphere and corona
Temperature increases
from 10,000 K to 106 K
Spicules (“spikes”)
gas jets poking up from photosphere
duration ~10 minutes
heights 5,000-20,000 km

12. Corona Outermost region of atmosphere Seen during eclipse
Extends millions of km
Seen during eclipse
Very high temperature
millions of degrees K
heated by solar magnetic field

13. Solar Wind Gas flows away from sun
10 million tons/yr
protons & electrons
speed km/s
Material goes outward into solar system
Hits Earth’s atmosphere
guided by magnetic field
makes atmosphere glow (aurorae)

14. Solar Flare

15. Solar Flares Eruptions caused by magnetic fields
may last 5-10 min
release huge amounts of energy
gas heated to 107 K
produces X-rays and UV radiation
Coronal Mass Ejections
very large flares
large mass of gas ejected from corona

16. Solar Flares & Effects on Earth
CME hits Earth’s Magnetic Field
Aurorae intensified
affects magnetic field
produces power surges
’89 Quebec/Montreal power outage
electrical interference
Disrupts radio communications
Cell phone outage
Danger to satellites/astronauts
Space Weather at:

17. Solar Variability & Earth’s Climate
Solar Luminosity Varies ~1%
Highest at sunspot maximum
Lowest at minimum activity
Maunder minimum
Very few sunspots
“Little Ice Age” in Europe
extreme cold temps
shorter growing season
Still learning how Sun affects Earth’s climate

18. Solar Interior Interior is gaseous Constant battle: Star shrinks
Gravity pulls in
Pressure pushes out
Star shrinks
becomes hotter, until
Pressure balances gravity
Hydrostatic Equilibrium

19. Why Does Sun Shine? Sun in Hydrostatic Equilibrium Hot objects
Pressure requires high temperature
Sun is hot
Hot objects
Radiate (emit light)
lose energy
Solar Luminosity
1026 Watts
Age of Sun
greater than 4.5 billion years (age of earth)
What is energy source?
Sun shines because it is hot

20. Sun’s Energy Source Thermal energy (chemical reactions)?
wood or fuel burning
Entire Sun used in a few thousand years NO
Gravitational contraction?
energy released as Sun contracts
would suffice for 100 million years NO
Nuclear Energy?
Einstein: E = mc2
Can convert mass into energy
1 gram of matter = energy of 15,000 barrels of oil
Need 4 million tons/sec
Will suffice for 10 billion years YES

21. Nuclear Reactions Fusion: Fission: two light nuclei joined into one
Powers Sun
Fission:
massive nucleus splits in two
Powers nuclear reactors (Davis-Besse)

22. Converting Mass to Energy
Nuclear reactions change mass
Mass increase consumes energy
Mass decrease releases energy
Mass decreases in
Fission of heavy nuclei
Can occur spontaneously
natural radioactivity
Fusion of light nuclei
Like charges repel
Fusion requires high speed
Temperature > 10 million K

23. Fusion: Sun’s Energy Source
Sun’s core:
Temperature = 15 million K
Large enough to fuse hydrogen
p-p chain (proton-proton)
Series of fusion reactions
Converts: 4 hydrogen to 1 helium nucleus
mass of 4 H > mass of 1 He
mass decrease is energy source

24. p-p Chain in the Sun