Sun – chapter 14
NOT chapter 16
Composition, Energy source,
Structure, “normal life” changes
Are you here?
1. Yes
2. No
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Goals & Outcomes
• Explain how electromagnetic radiation and astronomical instruments
are used to reveal the properties of stars and galaxies.
• Diagram the process of nuclear fusion.
• Learn about some problems astronomers and astrophysicists are
trying to solve, and understand the methods scientists are using to
try to solve these problems.
• Develop a sense of what scientists know about the overall universe,
its constituents, and our location
• Diagram and explain radiative transport in the Sun. Explain the causes
of surface features on the Sun.
Major Sun facts
• What’s a star?
– Big Hot Ball of Gas
– Hot, so it must emit all forms of light.
Must protect your body if in space!
• Which gases?
– 70%=H,
28%=He,
2%=other
– Other: O=1%, C=0.4%, Fe=0.14%, N=0.1%, Si, Mg,
Ne, S 0.040.1%.
• How do we know these numbers?
Sun’s Age
• Easy to measure: brightness & distance
• Can figure out ______________
• Geology & biology tell us age must be
few billion years
• Meteorites tell us 4.6 Gyr
• What can make the Sun shine so bright for so
long?
Sun
The picture shown to the
right is taken using the
Hydrogen 3  2 transition
(red colored, called H-alpha
Show Sun movies!
Power Sources and how long Sun could shine
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This is discussed on page 495-496.
Fire (chemical burning)  few thousand yrs
Cooling off  25 Myr
Shrinking and heating up  25 Myr
Until 1920, NO idea what makes the Sun shine.
See common
misconception
p. 498
– Einstein’s E = mc2
• Nuclear fusion. (Do humans use fusion?)
• 4H  1 He + 2 neutrinos + 2 e+ + energy
–
(For more details, see figure 14.7 – not on test)
– Every sec: Fuses 600 Megatons H into 596 Megatons He.
– Can do this for a total of 10 billion years
– How old is Sun. How much time left?
• Review: what’s H atom made of? He atom?
Why is fusion difficult for people & Sun?
• What is nuclear fusion? What’s in the nucleus?
• Why would they be difficult to combine?
– Fig 14.6, top, page 499
• What must happen to hold protons together?
• Strong force “turns on” when protons touch
– Fig 14.6, bottom also page 499
• Why difficult to make protons touch?
• How do you overcome this?
• Where are conditions right?
– 15 million K, density = 150x water
• Humans use more energy than fusion releases,
with current technology. Wait 30 years!
At what temperature does helium fuse
with another helium?
1. Also at 15 million K
2. Less than 15 million K
3. More than 15 million K
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Calif. Elementary School Science Standards
for nuclear processes
•
From California Science Standards, high school
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Students know protons and neutrons in the nucleus are
held together by nuclear forces that overcome the
electromagnetic repulsion between the protons
Students know the energy release per gram of material
is much larger in nuclear fusion or fission reactions than
in chemical reactions. The change in mass (calculated by
E = mc2 ) is small but significant in nuclear reactions.
Students know the Sun is a typical star and is
powered by nuclear reactions, primarily
the fusion of hydrogen to form helium.
Structure & evolution of the Sun
• Start with a clicker question.
According to what you know, is the Sun
shrinking?
1. Yes
2. No
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Star lives – a constant struggle
Figure 14.2, page 496
• Gravity tries to shrink the Sun
• Shrinking wins until …
• Some source of pressure resists gravity
– What is the Sun? (What is any star?)
– Big sphere of ~70%H, ~28% He GAS.
– Gas particles collide with each other – causes
pressure.
• Think of an example of balls of gas in your everyday life.
• What do you do to resist shrinking (i.e. increase the gas
pressure)?
Gravity & Pressure imbalance
(pp. 502-503) Figure 14.8
• If gravity is stronger than gas pressure, what
happens?
– Gas ___________ and becomes _____ when it
contracts.
– Fusion in the core will _________. Star _____
• If gas pressure is stronger than gravity, what
happens?
– Gas ___________ and becomes _____ when it
expands.
– Fusion in the core will _________. Star _____
Review question: How does the mass of
the Sun change over its lifetime?
1. Not at all
2. A little bit, but not much
3. The Sun’s mass changes a lot
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Sun’s structure –
Radiation transport
• See figure 14.3, page 497.
• Core – where fusion occurs
– 15 million K
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Radiation zone – where energy carried by light
Convection zone – energy carried by moving gas
Photosphere – “surface” where light escapes
Corona – 1 million+ K thin outer layer – see pic above
– How could we see this?
• Solar wind comes off the Sun.
– Charged particles, fast.
– Causes Northern/Southern Lights (aurora borealis/australis).
– Affect atmospheres, magnetism
Radiation & Convection Zones
• Core is hot – thermal emitter
• Radiation Zone
– Photons “randomly walk” outwards
• Collide often, change directions. Figure 14.9, page 504
– “Drunk photons”
• Takes ~1 million years to “walk” from core  outside
• Convection Zone
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Outer layers are colder, stuff gets opaque
Photons get “stuck”
Hot gas rises. Cold gas sinks. Sideways @ top
Movement is visible through ECC scopes! Show movie!
See figure 14.10 page 504
• Photosphere
– Light escapes from gas. Outside temp: 5800 K.
– Quick review question: how long does it take light to get here
once it leaves the Sun?
Evil scientist magically turns off fusion.
Would you notice 6 minutes later?
1. Yes
2. No
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Evil scientist magically turns off fusion.
Would you notice 10 minutes later?
1. Yes
2. No
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The amount of Helium in the Sun is:
1. Increasing
2. Decreasing
3. Staying the same
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What’s happening to the number of
particles in the Sun’s core?
1. Increasing
2. Decreasing
3. Staying the same
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Slow evolution of the Sun
• What happens to core gas pressure as a result of
combining 4 particles into 1 particle?
• What happens to the core? It _____ and _____.
• That causes what?
• Balance restored!
• What happens to fusion rate?
• What happens to the Sun’s luminosity?
Studying Sun’s interior
• Helioseismology
– See figure 14.11 page 505
– Waves on surface tell us about insides
– Strongest clues about interior of the Sun
– Figure 14.12 shows results
Solar Neutrinos
• Result of fusion (turning proton into neutron).
– 3% of Sun’s energy in neutrino form. 97% light.
• Neutrinos rarely interact with anything
– Fly straight out of the Sun, don’t random walk
• Occasionally they interact with stuff
– Detectable. See figure 14.14 page 506
– 60 billion/cm2/sec. New million ton detectors “see” 13
neutrinos per day. [New versions: looking for dark matter!]
• We saw 1/3 to ½ as many as expected from fusion
• HUGE puzzle
• Solution (May 2001): neutrinos changed “flavors”
– Other flavors weren’t detectable in past.
Another
neutrino
detector
• Japanese Super
Kamiokande
• filled with
50,000 tons of
water
• 13,000 light
detectors
(seen on wall)
Link to Earth - spots
• What are the spots?
• Hotter or colder than
the rest of the
photosphere?
• Like most other
strange features,
they’re caused by
magnetic fields
Magnetic fields
• Results:
–Sunspots
–Flares & storms
–Heating up the corona (probably)
• Differential Rotation: figure 14.24
page 512
Solar Cycle
• Sunspots go through 11 year cycle – few spots to
many back to few.
– 2007 “Solar minimum”
– When is the next “Solar maximum”
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Cycle isn’t a perfect clock
Some cycles stronger than others
Figures 14.22 & 14.23
There might be a link to Earth weather / climate.
But no strong evidence either way.
Summary
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We know the Sun fuses based on its age
Fusion energy source & productS
Composition (general)
Why fusion is difficult (repulsion, strong force)
Gravity vs. pressure. How currently balanced?
Different “layers” of Sun, a little about each
Neutrinos – what, where, problem?
Magnetic fields
Solar Cycle – time, observable results
A PUZZLE: north pole is warmer than south pole by 80,000K
all the time. (Does not flip when magnetic field does.) Stay
tuned.