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Our Star
A Closer Look at the Sun
• Our goals for learning
• Why was the Sun’s energy source a major
mystery?
• Why does the Sun shine?
• What is the Sun’s structure?
Why was the Sun’s energy source
a major mystery?
• Scientists knew the Sun had existed and shone for
millions of years.
• Could calculate the Sun’s daily energy output
• Could not work out what fuel source could provide
so much power for so long
Is it on FIRE? … NO!
Chemical Energy Content
Luminosity
~ 10,000 years
Is it CONTRACTING? … NO!
Gravitational Potential Energy
Luminosity
~ 25 million years
It can be powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
Luminosity
E=mc2
~ 10 billion years
Why does the Sun shine?
Composition of the Sun
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The Sun is a plasma - an extremely hot gas
The atoms’ electrons are ionized - stripped away
from the nucleus
The Sun is made mostly of:
Hydrogen, the smallest element (~70%)
Helium, the 2nd smallest element (~30%)
Atom
Ion
Atom
Ion
Fusion occurs in the innermost
part of the Sun because the
weight of upper layers
compresses then crushes lower
layers
Gravitational
contraction:
Provided energy
that heated core
as Sun was
forming. Intense
heat eventually
forced fusion.
Contraction
stopped when
fusion began
Stable Star
• Sun’s core fuses hydrogen into helium, releasing
energy into the outer layers of the Sun.
• The energy causes gas expansion,
pushing outward against the
gravity pulling inward. This
expansion is called thermal
pressure.
• When the pressures balance, a star
is in gravitational equilibrium.
What have we learned?
• Why was the Sun’s energy source a major
mystery?
– Chemical and gravitational energy sources
could not explain how the Sun could sustain its
luminosity for more than about 25 million years
• Why does the Sun shine?
– The Sun shines because gravitational
equilibrium keeps its hydrogen core hot and
dense enough to release energy through nuclear
fusion.
Nuclear Fusion in the Sun
• Our goals for learning
• How does nuclear fusion occur in the Sun?
• How does the energy from fusion get out of
the Sun?
• How do we know what is happening inside
the Sun?
How does nuclear fusion occur in
the Sun?
Fission
Fusion
Big nucleus splits into
smaller pieces
Small nuclei stick
together to make a
bigger one
(Nuclear power plants)
(Sun, stars)
Fusion occurs in the innermost
part of a star because the weight
of upper layers compresses then
crushes lower layers
High temperatures
cause particles to
smash together.
Enables nuclear
fusion to happen in
the core
Sun releases energy by fusing four hydrogen nuclei into one helium
nucleus
The Complete Fusion Reaction
Proton-proton chain is how hydrogen fuses into helium in Sun
IN
4 protons
OUT
4He
nucleus
2 gamma rays
2 positrons
2 neutrinos
ENERGY
Total mass is
0.7% lower
Solar Thermostat
• The energy released from fusion causes the sun’s gases to
expand (thermal expansion).
• The balancing act between thermal expansion pushing out
and gravity contracting inward, forces the temperature to
stay constant.
Solar Thermostat
If temperature falls because
fusion rate drops :
Thermal pressure drops,
gravity gains
=>core contracts and
heats back up
If temperature rises because
fusion rate increases:
Thermal pressure increases
over gravity.
=>core expands, which
causes core to cool down
Solar Thermostat
Therefore, gravitational equilibrium, causes thermal
equilibrium, as long as a star’s core continues to fuse.
Sun is estimated to do this for another 5 billion years!
What is the net fusion reaction that
produces energy in the core of the
Sun?
a) 4 hydrogen nuclei form 1 helium nucleus plus energy.
b) 2 hydrogen nuclei form 1 helium nucleus plus energy.
c) 6 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus
plus energy.
d) 3 hydrogen nuclei form 1 helium nucleus plus energy.
e) 4 hydrogen nuclei form 1 helium nucleus, 1 carbon nucleus,
plus energy.
© 2014 Pearson Education, Inc.
What balances the inward push of
gravity inside the Sun?
a)
b)
c)
d)
the rigidity of the solid central core
electron degeneracy pressure
thermal pressure of the gas
neutron degeneracy pressure
© 2014 Pearson Education, Inc.
What have we learned?
• How does nuclear fusion occur in the Sun?
– The core’s extreme temperature and density are
just right for nuclear fusion of hydrogen to
helium through the proton-proton chain
– Gravitational equilibrium acts as a thermostat to
regulate the core temperature because fusion
rate is very sensitive to temperature
What is the Sun’s structure?
Sun Contains
•Core
•Radiation Zone
•Convection Zone
•Photosphere
interior
•Chromosphere
•Corona
•Solar wind
Atm.
Core:
Energy generated
by nuclear fusion
~ 15 million K
Radiation Zone:
Energy transported
upward by photons
Convection Zone:
Energy transported
upward by rising
hot gas
Photosphere:
Visible surface of
Sun
~ 6,000 K
Chromosphere:
Middle layer of
solar atmosphere
~ 104 - 105 K
Absorption lines due to the Sun’s
atmosphere
Corona:
Outermost layer
of solar
atmosphere
~1 million K
Emits x-rays
and streamers
that follow the
magnetic lines
Solar wind:
The corona
dissipates,
becoming a
flow of charged
particles from
the surface of
the Sun
How does the energy from fusion
get out of the Sun?
Energy moves randomly around the radiation zone in form of
randomly bouncing photons. Eventually it makes it to the edge of
the radiative zone and into the convective zone
Convection (rising hot gas) takes energy to surface
Bright blobs on photosphere are where hot gas is reaching
the surface (effect is called granules, or granulation)
How we know what is happening
inside the Sun?
We learn about inside of Sun by …
• Making mathematical models
• Observing solar vibrations
• Observing solar neutrinos
Patterns of
vibration on
surface tell us
about what Sun is
like inside
The vibrations
reflect, reinforce or
cancel each other
which changes the
patterns and
frequencies seen at
the Sun’s surface.
Data on solar
vibrations agree
very well with
mathematical
models of solar
interior
Neutrinos created
during fusion fly
directly through the
Sun
Observations of
these solar neutrinos
can tell us what’s
happening in core
Which of the following layers of
the Sun is farthest from the core?
a)
b)
c)
d)
convection zone
photosphere
chromosphere
corona
© 2014 Pearson Education, Inc.
What have we learned?
• What is the Sun’s structure?
– From inside out, the layers are:
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Core
Radiation Zone
Convection Zone
Photosphere
Chromosphere
Corona
Solar wind
What have we learned?
• How does the energy from fusion get out of
the Sun?
– Randomly bouncing photons carry it through
the radiation zone
– Rising of hot plasma carries energy through the
convection zone to photosphere
• How do we know what is happening inside
the Sun?
– Mathematical models agree with observations
of solar vibrations and solar neutrinos
The Sun-Earth Connection
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Our goals for learning
What causes solar activity?
How does solar activity vary with time?
How does solar activity affect humans?
What causes solar activity?
Solar activity is like “weather”
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Sunspots
Solar Flares
Coronal Mass Ejections (CMEs)
Solar Prominences
• All are related to magnetic fields
Sunspots
Sunspots
Are cooler
than other
parts of the
Sun’s surface
(4000 K)
Are regions
with strong
magnetic
fields
Charged particles spiral along magnetic field lines,
emitting light as they go: x-ray light.
Sun at Multiple Wavelengths
Microwave
Visible
Ultra-violet
X-Ray
Magnetic field lines pierce the surface
Loops of bright
gas often connect
sunspot pairs
The Corona
appears bright in
X-ray photos in
places where
magnetic fields
trap hot gas
Magnetic activity
can cause solar
prominences that
erupt high above
the Sun’s surface
The particles
associated with
the prominance
can be flung out
into space
making a
Coronal Mass
Ejection
Magnetic activity
also causes solar
flares that send
bursts of X-rays
into space
How does solar activity vary with
time?
Number of sunspots rises and falls in 11-year cycle
Sun in
X-Rays
over
Solar
Cycle
Sunspot cycle has something to do with winding and twisting of Sun’s
magnetic field
The magnetic lines
erupt out and can snap.
The snapping lines
fling particles or
energy (or both) out
through the solar
system
Bursts of charged
particles are Coronal
Mass Ejections
Bursts of x-rays are
Solar Flares
Coronal
Mass
Ejection or
Solar Flare
How does solar activity affect
humans?
Solar Flare Headed toward Earth
Charged particles streaming from Sun can disrupt electrical power grids
and can disable communications satellites
Alaska
Aurora
What have we learned?
• What causes solar activity?
– Stretching and twisting of magnetic field lines
near the Sun’s surface causes solar activity.
– Activity detectable as sunspots
• How does solar activity vary with time?
– Activity rises and falls with an 11-year period
• How does solar activity affect humans?
– Bursts of charged particles from the Sun can
disrupt communications, satellites, and
electrical power generation
Live Solar Observation at the
Big Bear Solar Observatory
http://www.bbso.njit.edu/
Daily Space weather and
geostorm warning
http://www.spaceweather.com