Our Star
Why does the Sun shine?
Is it on FIRE?
Is it on FIRE?
Chemical Energy Content
Luminosity
~ 10,000 years
Is it on FIRE? … NO!
Chemical Energy Content
Luminosity
~ 10,000 years
Is it CONTRACTING?
Is it CONTRACTING?
Gravitational Potential Energy
Luminosity
~ 25 million years
Is it CONTRACTING? … NO!
Gravitational Potential Energy
Luminosity
~ 25 million years
E=
2
mc
—Einstein, 1905
It is powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
Why does the Sun shine?
– Chemical & Gravitational energy can’t explain how
Sun could shine for > 25 million years.
– Sun must be stable, balancing gravity pulling in
with forces pushing out from light and heat.
Chemical
Energy?
Gravitational
Energy?
Why does the Sun shine?
Gravitational equilibrium =
balance of forces in & out
Sun shines because gravitational
equilibrium keeps core hot &
dense & generate energy through
nuclear fusion.
Weight of upper layers
compresses lower layers
Gravitational
equilibrium:
Energy provided
by fusion
maintains the
pressure.
What is the Sun’s structure?
What is the Sun’s structure?
—From what we can see going outwards…
• Photosphere
• Chromosphere
• Corona
These we CAN see – & measure
with Visible-light, UV, X-ray
telescopes in orbit & on Earth
Photosphere
Radius:
6.9  108 m
(109 times Earth)
Mass:
2  1030 kg
(300,000 Earths)
Luminosity:
3.8  1026 watts
Photosphere:
Visible surface
~ 6,000 K
Seen in Visible
Light
Shows
•Dark “limb”
•Granules
•Sunspots!
•Absorption
lines
Granules seen
in the
Photosphere
Granules seen in the Photosphere are the
visible “top” of a current of hot gas
Sunspots seen in
the Photosphere
The
Chromosphere
– seen during a
total solar
eclipse
Credit: Chromosphere image courtesy Luc Viatour and Windows to the
Universe, http://www.windows.ucar.edu
Chromosphere:
Middle layer of
solar atmosphere
~ 104–105 K
Seen in UV Light
Shows emission
lines
Features:
Prominences,
Flares, Spicules
Spicules seen in the Chromosphere (UV)
Prominences
seen in the
Chromosphere
Prominences
seen in the
Chromosphere
Look
carefully –
you can see
Chabot!
Corona:
Outermost
layer of solar
atmosphere
~1 million K
Seen in X-rays
Shows
emission lines
+ faint
continuous
spectrum
The Corona – Visible Light
The Corona – X-ray Light
Solar wind:
A flow of
charged
particles
(electrons,
protons, some
helium nuclei)
from the
surface of the
Sun
Creates Aurora
How do we know what is
happening inside the Sun?
What is the Sun’s structure?
—Going *inwards* to layers we cannot see…
• Convection zone
• Radiation zone
• Core
These we can’t see – we model with computer simulations,
observations of surface features to suggest structure, and
observations of neutrinos to hint at fusion reactions taking
place in the core.
Convection zone:
Beneath
Photosphere
Creates granules
we see
Energy transported
upward by rising
hot gas
Bright blobs on photosphere where hot gas reaches the surface
Convection (rising hot gas) takes energy to the surface.
Patterns of
vibration on the
surface tell us
about what the
Sun is like inside.
Radiation zone:
Energy transported
upward by Gamma
Ray & X-ray
photons
Determine extent
based on computer
models
Core:
Energy generated
by nuclear fusion
~ 15 million K
Generates gamma
rays
Inner 10% of sun
Data on solar
vibrations agree
with mathematical
models of solar
interior.
How does nuclear fusion occur in
the Sun?
Fusion is the UNITING of light atomic nuclei into
heavier nuclei,
releasing binding energy in the form of gamma ray
radiation and other particles
Fission
Fusion
Big nucleus splits into
smaller pieces
Small nuclei stick
together to make a
bigger one
(Nuclear power plants)
(Sun, stars)
High temperatures
enable nuclear
fusion to happen in
the core.
Sun releases energy by fusing four hydrogen nuclei into one
helium nucleus.
Proton–proton chain is how hydrogen fuses into helium in Sun
IN
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower.
Neutrinos created
during fusion fly
directly through the
Sun.
Observations of
these solar neutrinos
can tell us what’s
happening in the
core.
Solar neutrino problem:
Early searches for solar
neutrinos failed to find
the predicted number.
Solar neutrino problem:
Early searches for solar
neutrinos failed to find
the predicted number.
More recent observations
find the right number of
neutrinos, but some have
changed form.
Thought Question
What would happen inside the Sun if a slight rise in
core temperature led to a rapid rise in fusion energy?
A. The core would expand and heat up slightly.
B. The core would expand and cool.
C. The Sun would blow up like a hydrogen bomb.
Thought Question
What would happen inside the Sun if a slight rise in
core temperature led to a rapid rise in fusion energy?
A. The core would expand and heat up slightly.
B. The core would expand and cool.
C. The Sun would blow up like a hydrogen bomb.
Solar thermostat keeps burning rate steady
Solar Thermostat
Decline in core temperature
causes fusion rate to drop, so
core contracts and heats up
Structure of the Sun
Rise in core temperature
causes fusion rate to rise, so
core expands and cools down
What causes solar activity?
Solar activity is like “weather”
• Sunspots
• Solar flares
• Solar prominences
All are related to magnetic fields.
Sunspots…
Are cooler
than other
parts of the
Sun’s
surface
(4,000 K)
Are regions
with strong
magnetic
fields
Zeeman
Effect
We can
measure
magnetic
fields in
sunspots by
observing
the splitting
of spectral
lines
Charged particles spiral along magnetic field lines.
Loops of bright gas often connect sunspot pairs.
Magnetic activity
causes solar flares
that send bursts of
X-rays and
charged particles
into space.
Magnetic activity
also causes solar
prominences that
erupt high above
the Sun’s surface.
The corona
appears bright in
X-ray photos in
places where
magnetic fields
trap hot gas.
How does solar activity affect
humans?
Coronal mass
ejections send
bursts of energetic
charged particles
out through the
solar system.
Charged particles streaming from the Sun can disrupt electrical power
grids and disable communications satellites.
How does solar activity vary with
time?
The number of sunspots rises and falls in 11-year cycles.
The sunspot cycle has something to do with the winding and
twisting of the Sun’s magnetic field.
Hans Bethe Video Clips
• http://www.youtube.com/watch?v=apgB_N
R59ss
• http://www.youtube.com/watch?v=c7iJtT41
yy8
• http://www.webofstories.com/play/4536?o=
MS