Chapter 13: Our Star –
The Sun
The Sun Today
Visible light
Hydrogen Alpha light
The Sun nearing the peak of its 11-year cycle. This cycle
seems to be a low activity one.
The Sun in other wavelengths
X-Ray
Extreme UV
Hydrogen Alpha
These show what the Sun can look like when it is active
Basic Solar Facts
Diameter: 1.4 million km
Mass: 330,000 x Earth
Age: 4.5 billion years
Average Density: 1.41 gm/cm3
Distance from Earth: 149.6 million km
Average Solar Wind Speed: 3 million km/hr
Luminosity: 3.9x1026 watts
Temperature at surface: 5,770 K
Temperature at Core: 15,000,000 K
Rotation Period at Equator: 25 Earth days
Rotation Period at Poles: 35 Earth days
What is the interior of the Sun
like? How do we know?
We make models based on
hydrostatic equilibrium
The models tell us the
conditions inside the Sun
We compare what the models
predict to what we can see
We observe the Sun in as many
different wavelengths to learn as
much as possible about how it works
Where does the Sun get its
energy?
To the ancients who believed the
Earth was the center of the
universe, the Sun was made of
quintessence, an element whose
property was to glow. The
concept of “energy” wasn’t even
invented until the late 1600’s.
By the 1700’s the best ideal for
the source of the Sun’s energy
was chemistry
If Sun was highest
quality chemical fuel
(i.e. pure carbon
coal) it would exhaust
its fuel in less than
10,000 years!
Later ideas for source of Sun’s
energy: Gravitational Collapse
Kelvin-Helmholtz
Contraction
Whenever anything
shrinks it heats up.
This could produce the
observed solar output
for about 25 million
years. This was the
original source of
energy as the Sun was
forming.
In 1905 Einstein proposed
a new way to get energy:
from matter
The
answer
came
from his
famous
equation:
E = mc2
By the 1920’s
Sir Arthur
Eddington
proposed the
Sun produced
energy by
fusion
The same source of energy as the
hydrogen bomb
The Sun converts hydrogen into
helium in a multi step process
Hans Bethe worked
out the details of
hydrogen fusion in
the 1930’s
It starts with two normal hydrogen nuclei fusing to form a
heavy hydrogen nuclei: a deuterium. The reaction also
produces a positron and a neutrino. When the positron
annihilates with an electron two gamma rays are produced
The second step uses the
product of the first step
plus another hydrogen
A deuterium and a hydrogen fuse to form a
helium-3 plus a gamma ray photon
Final step of the ProtonProton cycle
Two helium-3 fuse to form a normal
helium plus two hydrogen nuclei
Overall Proton – Proton Cycle
4 1H  4He + 2e+ + 2g + 2n
Releases 4.3x10-12 Joules per helium atom produced
The Sun converts 600,000,000 tonnes of H into
596,000,000 tonnes of He every second! The
difference in mass is the energy produced according
to E = mc2. This is only a 0.67% efficient conversion!
The Sun has enough hydrogen in its’ core to last
another 5 billion years before it runs out
Energy is only produced in the core region where the
temperature and pressure are high enough
Watch ClassAction Proton-Proton Animation in Sun
and Solar Energy Module
Fusion
requires high
temperatures
to overcome
the electric
repulsion of
protons
The electric force between the protons is repulsive. The
strong nuclear force between them is attractive but it is
a very short range force so they have to get very close
The solar neutrino problem
Step 1 in the Proton-Proton cycle
Early measurements only detected 1/3 as many as
predicted by solar models. Either the models were
wrong or we didn’t really understand the neutrino
Detecting neutrinos is not
easy
Neutrinos come in three “flavors” and early experiments
could only detect one flavor
We now know the neutrino
changes “flavor”
Current experiments can detect all three flavors and we
are now finding just as many as our solar theories predict
How does the energy get from
the core to the surface?
Most of the energy released in
the core is in the form of gamma
ray photons
Something must happen between
the core and the surface!
Most of the
energy released
at the surface is
in the form of
visible photons
Heat Transfer: Energy can
move by one of three methods
Conduction: atomic & molecular
vibrations in solids.
Example…cast iron skillet
Convection: large scale motions in
liquids and gasses
Example…boiling water
Radiation: electromagnetic radiation
Example…heat lamp
Which method works best is
determined by pressure,
density and temperature
Once again, we
use hydrostatic
equilibrium
models to
determine
which method
works best at
each layer of
the Sun
The Solar Interior
The models tell us
that radiation is the
means of heat
transport for the
first 70% of the way
then convection
takes over near the
surface before
going back to
radiation at the
surface.
How can we verify our
models for the interior of
the Sun?
GONG
stands for
Global
Oscillating
Network
Group
Solar Surface Oscillations
The surface
of the Sun
oscillates in
many
different
ways. This
shows one of
the ways it
oscillates
(extremely
exaggerated)
Different ways sound
bounces around inside Sun
How different
waves travel
depends on the
density and
temperature of
the gas it is
travelling
through
Internal Structure from
Surface Waves
Actual Image of Oscillations
Internal Differential Rotation
The Photosphere
The Sun is a ball of
gas so there is no
surface. The
“surface” is a layer
of gas that is only
about four hundred
kilometers thick.
The density of the gas has to be just right to emit a
good blackbody spectrum. Too dense and the light
can’t get out. Not dense enough and not enough light
is produced
Solar Granulation
Each granule is
~1000 km across
and lasts a minute
or two
Granulation Cells are
Convection Cells
The photosphere is the top of the convection layer of
the Sun where convection changes back to radiation
Note the decrease in
temperature in the photosphere
The Chromosphere
Spicules
The
temperature in
the
chromosphere
climbs slowly
but then jumps
up in the
corona
The Corona
The corona eventually fades into the solar wind.
There are other components to the solar wind as
well but the corona contributes the main wind.
The corona is heated by
coronal loops
Note the size of Earth for scale
The solar
wind moves
outwards at
1 to 3
million
kilometers
per hour
At those speeds it
can take two to six
days to reach Earth
Eventually the Sun’s magnetic
field plows into the galactic field
Sunspots
A Sunspot Close-up
Penumbra
Umbra
The umbra can be up to two thousand degrees cooler
than the surrounding photosphere
Under a sunspot
Heat flow is
stopped by the
sunspot like a
cork in a bottle.
The heat has to go
somewhere,
though, so it
squirts out around
the edges
The Solar Cycle
The number of sunspots increases and
decreases with about an eleven year cycle
The
location
of
sunspots
changes
during the
solar
cycle
The cause of sunspots lies
in differential rotation
Solar Rotation Period versus latitude and depth
Sunspots are created by kinking
in the Sun’s magnetic field
It starts with a global magnetic field but
the differential rotation causes it to wrap
around and get kinked up after a few
rotations
Activity Associated with Sunspots
Filaments & Plages
Solar Prominence
Watch YouTube Magnificent Eruptive Solar Prominence
at http://www.youtube.com/watch?v=rQ2Ad2nK_VM
Solar Flare
Coronal Mass Ejection (CME)
CME’s can also occur when there are no sunspots
Coronal Holes
Surprisingly, the Sun is
brighter during periods of
maximum sunspots
The
difference is
about 0.3%
If the sunspots go away,
Earth gets cold
During the Maunder
Minimum Europe suffered
through a “Little Ice Age”