Chapter 14
• Our Star
p. 111
Ideas About the Sun’s Energy Production
• Pre-19th century
– Fire due to wood or
coal
– Ruled out after we
knew the actual size and
distance to the Sun
• 19th Century
– Gravitational
Contraction
• Could maintain energy
output for 25 million
years
– Ruled out when we
discovered that the
earth was far older than
25 million years
So Why Does the Sun shine?
• E=mc2
– Conversion of
matter into energy
through nuclear
fusion
• For fusion to
occur, the Sun
must generate
extreme
temperature
internally without
flying apart
Nuclear Fission vs. Nuclear Fusion
Fission
Fusion
Big nucleus splits into
smaller pieces.
Small nuclei stick
together to make a
bigger one.
(Example: nuclear
power plants)
(Example: the Sun, stars)
The Sun releases energy by fusing four hydrogen nuclei
into one helium nucleus.
The energy comes from the fact that the Helium nucleus
has slightly less mass (0.7%) than the sum of two
protons and two neutrons.
It appears as kinetic energy and the energy of the
gamma-ray photons
Proton-Proton Cycle
Basic reaction:
4 1H  4He + energy
4 protons have 0.048*10-27
kg (= 0.7 %) more mass than
4He.
 Energy gain = m*c2
= 0.43*10-11 J
per reaction.
Sun needs 1038 reactions,
transforming 5 million tons
of mass into energy every
second, to resist its own
gravity.
Need large proton speed ( high
temperature) to overcome Coulomb
barrier (electromagnetic repulsion
between protons).
T ≥ 107 K = 10
million K
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.
The solar thermostat keeps burning rate steady.
Gravitational Equilibrium
Contraction of the sun
due to its own gravity
provided the energy that
heated the core as Sun
was forming.
Once nuclear fusion
began, it generated
pressure that halted the
contraction.
The Sun exists is a state
of gravitational
equilibrium (also called
hydrostatic equilibrium
Solar Thermostat
Decline in core temperature
causes fusion rate to drop, so
core contracts and heats up.
Rise in core temperature
causes fusion rate to rise, so
core expands and cools down.
• Calculations show that the Sun has enough
hydrogen in its core to maintain gravitational
equilibrium for 10 billion years.
• We will investigate what happens after that in a later
chapter
Sun’s Basic Structure
Insert TCP 6e Figure 14.3
Core:
Energy
generated by
nuclear fusion
~ 15 million K
Radiation Zone:
Energy
transported
upward by
photons until
they reach a
region where the
temperature has
dropped to
about 2 million
K. Photons get
absorbed there.
Convection
Zone:
Absorbed
photons heat the
gas at the
bottom of the
convection zone.
Energy is
transported
upward by
rising hot gas
Photosphere:
Visible surface
of Sun
~ 5800 K
Chromosphere:
Middle layer of
solar
atmosphere
~ 104–105 K
Corona:
Outermost
layer of solar
atmosphere
~1 million K
Solar wind:
A flow of
charged
particles
from the
surface of the
Sun
Radiative Diffusion
Most of the energy from the Sun works its way out from the core as
photons. They execute a random walk to make their way out.
It takes thousand of years for energy liberated in the core to get to the
top of the radiation zone.
Energy Transport in the Sun
• Follow a single gamma ray as it heads
toward the surface.
– The ray is scattered by electrons in the core until it
reaches cooler gas
– The cooler gas absorbs the gamma photon and emits
two x-ray photons, each of less energy than the gamma
photon
– The x-rays migrate toward cooler regions where they are
absorbed and emitted as still longer-wavelength
photons.
– This process repeats until the single gamma in the core
has resulted in about 1800 photons of lower energy.
Convection Zone
Photons absorbed at the plasma near the surface heats the plasma and
results in a convection zone (rising hot gas) taking energy to surface.
The Photosphere: Granules
• Bright central regions are hotter than the edges so they
appear brighter.
• Doppler shifts show that hot gas is rising in the center
and sinking at the edges
• Each granule is about the size of Texas
• They last about 10-20 minutes
Supergranules
• Supergranules which are a little over twice
Earth’s diameter, include about 300 granules
each.
– These supergranules are regions of very slowly rising
currents that last a day or two.
– They appear to be produced by larger currents of rising gas
deeper under the photosphere.
Photosphere Spectrum
• Below the photosphere, the gas is dense and hot and
therefore radiates a continuous spectrum of light.
• Atoms in the photosphere absorb photons of specific
wavelengths—producing the absorption lines you
see.
Chromospherre
• The chromosphere lies above the photosphere.
• Visible to unaided eye only during a total
eclipse
• Pink color due to red, blue, and violet Balmer
emission lines of hydrogen.
Chromosphere Spectrum
• The chromosphere produces an emission spectrum.
• Atoms in the lower chromosphere are ionized, and
atoms in the higher layers of the chromosphere are
even more highly ionized.
– Can be used to determine the temperature of various
parts of the chromosphere.
Temperature Profile
• Just above the photosphere, the temperature falls to a
minimum of about 4,500 K and then rises rapidly to the
extremely high temperatures of the corona.
The Solar Corona
• The outermost part of the sun’s atmosphere is called
the corona, after the Greek word for crown.
• You can see the inner parts of the corona during a
solar eclipse.
Chronograph
• Observations made with specialized telescopes called
coronagraphs on Earth or in space can block the light of the
photosphere and record the corona out beyond 20 solar radii.
• Such images reveal that
magnetic fields link the
sunspots with features in
the chromosphere and
corona.
Coronal Spectrum
• Complex coronal spectrum
– Sunlight reflected from dust particles produces a spectrum
with absorption lines just like the sun
– When sunlight from the photosphere is scattered off free
electrons in the ionized coronal gas, it produces a
continuous spectrum without absorption lines.
• Fast moving electrons produce photons with a large Doppler shift
smearing out the absorption lines
– Emission lines of highly ionized gas are superimposed on
this continuous spectrum.
• Higher in the corona means more ionization – implies the
temperature rises
Coronal Temperature Profile
• Just above the chromosphere, the temperature is about
500,000 K.
• In the outer corona, it can be as high as 2 million K or
more.
Heating the Corona
• Magnetic fields extend from the
photosphere to the corona.
– Turbulence in the photosphere whips the field
around
– As the gas of the chromosphere and corona has a very
low density, it can’t resist movement in the magnetic
fields.
– This motion heats the gas
How we know what is happening inside the
Sun?
We learn about the inside of the Sun by …
• making mathematical models
• observing solar vibrations
• observing solar neutrinos
Mathematical Models
• Use basic physics to develop equation to
describe the measured properties of the Sun
– Using a computer we can calculate the
temperature, density, and pressure at any depth
within the Sun
– From these we predict the rate of fusion can
predict many measurable properties of the Sun.
– If these models correctly predict the known
properties, then we gain confidence that we really
do understand what is going on in the solar
interior.
Helioseismology
•
Random motions in the sun
constantly produce vibrations
like resonance in organ pipes.
•
These are sound waves of very
long wave period.
•
These vibrations are observed
by Doppler shifts.
•
Interpretation requires a lot of data to reconstruct the subsurface
features.
•
GONG, Global Oscillation Network Group, uses telescopes
spread around the world to observe the sun continuously.
Data on solar
vibrations agree
very well with
mathematical
models of solar
interior.
Solar Neutrinos
• Neutrinos created
during fusion fly
directly through the
Sun.
• Observations of
these solar
neutrinos can tell
us what’s
happening in core.
• Neutrinos interact very rarely with matter. Trillions pass
through you every second. So they are very difficult to
detect.
Searching for Neutrinos
• Raymond Davis filled a 100,000-gallon tank
with the cleaning fluid perchloroethylene
(C2Cl4).
– Theory predicted that, about once per day, a solar neutrino
would convert a chlorine atom in the tank into radioactive
argon.
– This could be detected later by its radioactive decay.
– Results showed only one event every 3 days
• There were two possible explanations
– We didn’t correctly understand how the sun and stars make
their energy.
– There was something about neutrinos that we did not
understand.
Solar Neutrino Problem
•
Early searches for solar neutrinos
failed to find the predicted
number, but they were looking
only for electron neutrons, the
kind produced by fusion in the
Sun
•
We discovered that some neutrinos
change into other types (muon
and tau neutrinos) in a process
called oscillation.
•
More recent observations find the
right number of neutrinos
Illustrates the great synergy between astrophysics and subatomic physics
• This solution to the solar neutrino problem is exciting
because neutrinos can’t oscillate unless they have
mass.
– Neutrinos were long thought to be massless.
– However, if they have even a small mass, they are so
common their gravity could affect the evolution of the
universe as a whole.
Solar Activity
Solar activity is like “weather”.
•
•
•
•
Spicules
Sunspots
Solar flares
Solar prominences
All these phenomena are related to magnetic
fields.
They can at times affect our daily lives.
Spicules
–Spicules are small plasma burst
from the photosphere into the
chromosphere.
– They are created by periodic
sound waves leaving the sun.
– The plasma is concentrated to
a small thread like jet due to
strong magnetic fields.
– that last 5 to 15 minutes
Sunspots
• Cooler than other
parts of the Sun’s
surface (4000 K)
• Regions with
strong magnetic
fields
• They appear dark
by comparison
Observing the Sun
• In the early 17th century, Galileo
observed the sun and saw spots on its
surface.
– Day by day, he saw the spots moving
across the sun’s disk.
– These are sunspots.
– He rightly concluded that the sun was
rotating.
Sunspot Rotation
http://www.youtube.com/watch?v=U0Lt3SgiEQ8
Sunspots tend to occur in
pairs connected by magnetic
fields, represented as lines in
this drawing.
The TRACE satellite can
detect the hot gas trapped in
the magnetic fields arching
above the sunspot group.
(NASA/TRACE
Magnetic Loops
• Sunspots tend to
occur in pairs
resembling a bar
magnet
– Polarity different
on opposite sides
of the solar
equator
– At the end of an
11-year cycle, the
new spots appear
with reversed
magnetic polarity.
Magnetic field lines
Zeeman Effect
We can measure
magnetic fields
in sunspots by
observing the
splitting of
spectral lines.
Solar Prominences
Gas in the
chromosphere and
corona can
become trapped in
the magnetic
fields of sunspots
resulting in large
prominences that
can rise far above
the surface.
Prominences
Relatively cool gas
(60,000 – 80,000 oK)
May be seen as dark
filaments against the
bright background of
the photosphere
Looped prominences: gas ejected from the sun’s
photosphere, flowing along magnetic loops
Prominence in UV
Hot plasma trapped in
the magnetic field
loop above the
chromosphere into
the lower corona.
In the visible region
they look pink due to
the three visible
Balmer lines. From
above these look dark
against the surface
and are called
filaments
Filaments
Solar Flares
Magnetic fields can
become ‘twisted.’ At
some point the field will
suddenly reorganize
itself and in the process
release excess energy
heating the plasma to
100 million K releasing
a burst of X-rays and
accelerating charged
particles to nearly the
speed of light
The Solar Wind
• Some magnetic field lines extend far out into space
• Gas from the solar atmosphere follows along the
magnetic fields that point outward and flows away from
the sun in a breeze called the solar wind.
• This X-ray
photograph shows
dark regions called
coronal holes from
which much of the
solar wind escapes.
Auroras
Solar winds, guided by the Earth’ magnetic field, and low
density gases at ~130 km above the surface create conditions
like the gas discharge tubes.
Coronal Mass Ejections
• Flares and other solar storms sometimes eject
large numbers of charged particles from the
solar corona.
– Called a coronal mass ejection
– If aimed toward Earth, they can create a
geomagnetic storm,
•
•
•
•
Particularly strong aurora
Disrupt power delivery
Interfere with communications
Damage satellites
Variation in Sunspot Activity with Time: The
Sunspot Cycle
• Averages 11 years, min to min, but varies from 7 to 15 years
• Sunspot minimum
– Few, if any, sunspots visible
– We just emerged from one a year or so ago
• Sunspot maximum
– May see dozens of sunspots simultaneously
– The frequency and intensity of solar flares, prominences, and
CME’s, follow the sunspot cycle
Maunder Butterfly Diagram
• Sunspots appear at higher latitudes (farther from the
equator) early in the cycle, and at lower latitudes later
in the cycle.
Maunder Minimum
The Sun’s Magnetic Cycle
• The Babcock mode
– As the electrons in the ionized gas carry any magnetic field.
The field is is ‘frozen’ into the gas.
• Where these magnetic tubes burst through the sun’s
surface, sunspot pairs occur.
– After about 11 years the field is so tangled that it begins to
rearrange itself into a simpler structure, but with opposite
polarity