The Sun . Astro 129: Chapter 1a The Solar Constant What is the Solar flux arriving at Earth? Use...
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- Slide 1
- The Sun
- Slide 2
- Astro 129: Chapter 1a
- Slide 3
- Slide 4
- The Solar Constant What is the Solar flux arriving at Earth?
Use relation between luminosity and flux L = 3.910 26 W, 1AU =
15010 6 km What is the source of the Suns energy?
- Slide 5
- Astro 129: Chapter 1a The Source of the Suns Energy
1.Kelvin-Helmholz contraction doesnt work 2.Chemical reactions dont
work. To produce the observed solar flux from chemical reactions
the sun would have burned out in about 10,000 years. (a chemical
reaction releases roughly 1x10 -19 Joules per atom) We need a
process that can liberate more energy per unit mass than what can
be achieved by chemical reactions. Albert Einstein discovered such
a process.
- Slide 6
- Astro 129: Chapter 1a The Source of the Suns Energy There are
two avenues by which fusion of hydrogen proceeds in stars. T T (T =
16 million K) proton proton chain (example: 4 1 H 4 He + neutrinos
+ gamma-ray photons) T >> T CNO cycle (a carbon nucleus
absorbs protons and finally emits a helium nucleus)
- Slide 7
- Astro 129: Chapter 1a The Source of the Suns Energy
Proton-Proton Chain
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- Astro 129: Chapter 1a The Proton-Proton Chain Proton-Proton
Chain The proton-proton chain has four branches PP1, PPII, PPIII,
and PPIV. The PPI and PPII branches produce about 86% and 13.9%,
respectively, of the Suns energy. PPI Branch PPII Branch
- Slide 9
- Astro 129: Chapter 1a The Proton-Proton Chain Proton-Proton
Chain We can summarize the thermonuclear fusion of hydrogen as
follows: 4 1 H 4 He + neutrinos + gamma-ray photons Mass of 1 H
proton = m p = 1.67210 -27 kg Mass of 4 He = m He = 6.64510 -27 kg
m = 4m p m He = 0.043510 -27 kg How much energy is released for
every PPI reaction? Hint use E = mc 2
- Slide 10
- Astro 129: Chapter 1a Theoretical Model of the Sun
Proton-Proton Chain We expect fusion to occur mostly near the
center of the sun where the temperature is high enough for fusion
reactions to occur (T > 10 million K) We need to address the
issue of the propagation of energy produced in the center to its
surface. To learn about the interior of the sun scientist have
followed the following approach: Create a model of the sun which
contains as much of the physics as we know and fit this model to
the observations.
- Slide 11
- Astro 129: Chapter 1a Theoretical Model of the Sun
Proton-Proton Chain Model Assumption: 1.Hydrostatic Equilibrium (A
balance between the weight of a layer in a star and the force from
pressure that supports it. 1.Thermal Equilibrium (A balance between
the input and outflow of heat in a system). As a consequence while
the temperature in the solar interior is different at different
depths, the temperature at each depth remains constant in time.
2.Energy Transport via radiative diffusion and convection) (F P =
P/A, where P is pressure, A is area and F P is the force from this
pressure)
- Slide 12
- Theoretical Model of the Sun Proton-Proton Chain Convection is
the movement of molecules within fluids (i.e. liquids, gases). In
the suns convection zone hotter gas rises outward and cooler gas
descends inwards thus facilitating the bulk transfer of energy
outward. Radiative Diffusion is the process by which the gamma rays
released in fusion reactions are absorbed in only a few millimeters
of solar plasma and then re-emitted again in random direction and
at slightly lower energy. It takes a long time for radiation to
reach the Sun's surface. Estimates of the photon travel time range
between 10,000 and 170,000 years.
- Slide 13
- Astro 129: Chapter 1a Fitting Models of the Sun to Observations
Proton-Proton Chain Observations: 1.Surface Temperature (T = 5,800
K) 2.Solar Luminosity (L = 3.910 26 W) 3.Gas pressure and density
at the Suns surface are almost zero. The result is a model of how,
T ,L , pressure, mass and density vary as a function of distance
from the Suns center.
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- Astro 129: Chapter 1a The Suns Oscillations Proton-Proton Chain
The sun vibrates (oscillates) at certain frequencies. We can learn
about the interior of the sun by studying oscillations on the
surface of the sun. The science studying wave oscillations in the
Sun is called helioseismology. The solar oscillations are thought
to be produced by the motion of fluid (turbulence) in the
convection zone. A simulated sound wave resonating in the Sun. The
regions that are moving outward are colored blue, those moving
inward, red. As the cutaway shows, these oscillations are thought
to extend into the Suns radiative zone.
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- Astro 129: Chapter 1a The Suns Oscillations Proton-Proton Chain
Waves in the sun are divided into three different types: 1.Acoustic
or pressure (p) modes, driven by internal pressure fluctuations
within the sun; their dynamics being determined by the local speed
of sound. 2.Gravity (g) modes, driven by buoyancy, 3.Surface
gravity (f) modes, akin to ocean waves along the stellar surface.
Different oscillation modes penetrate to different depths inside a
star.
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- Astro 129: Chapter 1a The Suns Oscillations Proton-Proton Chain
Discoveries made by helioseismology 1. Set limits on the amount of
helium in the Suns core and convective zone. 2. constrained the
thickness of the transition region between the radiative zone and
convective zone. 3. The convective zone and the radiative zone
rotate at different speeds, which is thought to generate the main
magnetic field of the Sun by a dynamo effect. A simulated sound
wave resonating in the Sun. The regions that are moving outward are
colored blue, those moving inward, red. As the cutaway shows, these
oscillations are thought to extend into the Suns radiative
zone.
- Slide 17
- Photon Transport from Suns Core Proton-Proton Chain Energy
radiated by photons from the surface of the sun originates from the
suns core. To get to the surface, however, energy is transported
via various mechanisms thus making it difficult to tell how it was
actually produced in the core. Photon transport from the core to
the surface: 1.Photons created in the Suns core from hydrogen
fusion diffuse towards the Suns surface (~ 170,000 years to get
out). 2.Beyond the radiative zone energy is transported to the
surface through convection (bulk transfer of plasma). 3.At the
surface photons are mostly radiated out through blackbody
radiation. How do we obtain direct evidence of hydrogen fusion in
the Suns core?
- Slide 18
- Solar Neutrinos Proton-Proton Chain A direct method of proving
that hydrogen fusion is occurring in the Suns core relies on
detecting neutrinos that are released during nuclear fusion. 4 1 H
4 He + + A neutrino is a subatomic particle with no charge and very
little mass. It can travel through ordinary matter almost
undisturbed. More than 50 trillion solar electron neutrinos pass
through the human body every second. The Sudbury Neutrino
Observatory
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- Solar Neutrino Detections Proton-Proton Chain On a rare
occasion a neutrino will strike a neutron and convert it to a
proton. Using this process Raymond Davis designed and built a
neutrino detector that used 100,000 gallons of perchloroethylene (C
2 Cl 4 ). Detection Method: Occasionally a neutrino will strike the
nucleus of one of the chlorine atoms ( 37 Cl) in the cleaning fluid
(C 2 Cl 4 ) and convert one of its neutrons into a proton, creating
a radioactive atom of argon ( 37 Ar). Atomic number of Cl = 17.
Atomic number of Ar = ?
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- Solar Neutrino Detections Proton-Proton Chain John Bahcall
calculated the expected detection rate of neutrinos but Davis's
experiment turned up only one third of this figure. The experiment
was the first to successfully detect and count solar neutrinos, and
the discrepancy in results essentially created the solar neutrino
problem. The Davis Experiment
- Slide 21
- Solving the Solar Neutrino Problem Proton-Proton Chain Where
were the neutrinos detected in the Davis experiment coming from?
The Kamiokande experiment in Japan led by physicist Masatoshi
Koshiba measured the direction of the incoming neutrinos and
confirmed that they emanated from the Sun. Kamiokande
Observatory
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- Solving the Solar Neutrino Problem Proton-Proton Chain The
Kamiokande experiment consisted of a large underground tank
containing 3000 tons of water surrounded by 1100 light detectors. A
solar neutrino would from time to time interact with an electron in
the water molecule. The electron recoils in roughly the direction
that the neutrino was travelling, so the electrons "point back" to
the sun. About half of the predicted neutrinos were detected in the
Kamiokande experiment. Kamiokande Observatory
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- Solution to the Solar Neutrino Problem Proton-Proton Chain
There are actually three types of neutrinos ( e, , ) but only e is
produced in the sun. The Davis and Kamiokande experiments could
only detect e. It turns out that the e can change into , during its
flight from the Sun to the Earth. The change between neutrinos is
called neutrino oscillations. The Sudbury Neutrino Observatory
- Slide 24
- Solution to the Solar Neutrino Problem Proton-Proton Chain The
solution to the solar neutrino problem came from the Sudbury
Neutrino Observatory (SNO) in Canada. SNO could measure all three
types of neutrinos. Instead of regular water SNO used heavy water.
In heavy water each of the hydrogen nuclei contain a proton and a
neutron. Process: neutrinos may knock a neutron out of one of the 2
H nuclei of the heavy water molecule. When the neutron is
recaptured by another nucleus it creates a flash of light that is
recorded. The Sudbury Neutrino Observatory
- Slide 25
- The Suns Structure Proton-Proton Chain 1. Core 2. Radiative
zone 3. Convective zone 4. Photosphere 5. Chromosphere 6. Corona 7.
Sunspot 8. Granules 9. Prominence
- Slide 26
- Photosphere Proton-Proton Chain The photosphere is the layer in
the solar atmosphere from which the Suns visible light is emitted.
The Sun is gaseous throughout its volume. Below the photosphere the
Sun is opaque to visible light. The photosphere is heated from
below by energy streaming outward from the solar interior.
- Slide 27
- Photosphere Proton-Proton Chain Question: Why is the thin
photosphere opaque? The photosphere is made primarily of hydrogen
and helium, and has a density of about 10 -4 kg/m 3. Despite its
low density the photosphere is opaque and we can only see through
~400 km of gas in the photosphere. The main reason for the
photosphere being very opaque is the presence of negative H ions
which are H atoms with an additionally bound electron. Answer:
Negative H ions in the photosphere are efficient light
absorbers.
- Slide 28
- Photosphere Proton-Proton Chain The edge or limb of the Sun
looks darker because we are seeing the upper photosphere which is
cooler. This effect is called limb darkening. The high-altitude gas
we observe at the Suns limb is not as hot (about 4,000 K) and thus
does not glow as brightly as the deeper, hotter gas (5,800 K) seen
near the disk center. The Suns spectrum is close to a blackbody
with a temperature of 5,800 K. Superimposed on this spectrum are
absorption lines produced by the cooler gas (4,000 K) in the outer
photosphere.
- Slide 29
- Photosphere Proton-Proton Chain The Photosphere has a blotchy
pattern called granulation. A granule is about 1,000 km wide.
Granulation is caused by convection currents. Where gas rises it is
hotter and looks brighter and when gas sinks into the photosphere
it is cooler and looks darker.
- Slide 30
- Photosphere Proton-Proton Chain Superimposed on the pattern of
granulation are even larger convection cells called supergranules.
A supergranule is about 35,000 km wide. Gases rise upward in the
middle of a supergranule, move horizontally outward toward its
edge, and descend back into the Sun. Supergranules detected in a
Doppler image.
- Slide 31
- Chromosphere Proton-Proton Chain The chromosphere is a thin
layer of the Sun's atmosphere above the photosphere, ~ 2,000 km
deep. The chromospheres spectrum is dominated by the red H emission
line at 656 nm and also contains emission lines of highly ionized
He, Ca, and ionized metals. T photosphere : 6,000K(bottom)
4,000K(top), over 400 km T chromosphere : 4,000K(bottom)
25,000K(top), over 2000 km During a total solar eclipse, the Suns
glowing chromosphere can be seen around the edge of the Moon. The
expanded area above shows spicules, jets of chromospheric gas.
- Slide 32
- Spicules Proton-Proton Chain Spicules are jets of gas that rise
from the top of the chromosphere and are located near the edges of
the supergranules. Each spicule lasts for about 15 min and together
all spicules cover about 1% of the sun's surface. A spicule rising
from the chromosphere.
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- Corona Proton-Proton Chain The corona is the outermost region
of the Sun's atmosphere and extends from the chromosphere to
several million kilometers. The corona is very faint compared to
the photosphere but can be viewed during a solar eclipse. An image
of the corona taken during an eclipse shows a number of streamers
extending millions of km above the sun.
- Slide 34
- Corona Proton-Proton Chain Properties of the solar corona:
Temperature 1 210 6 K Spectrum: emission-line spectrum of highly
ionized gases (G corona). The corona is 10 12 times as dense as the
photosphere. Example: One of the prominent lines found in the sun
is at 530 nm produced by Fe XIV (has lost 13 e of its 26 e ).
Because there are so few atoms in the corona, despite its high
temperature, the total amount of energy in these moving atoms of
the corona is rather low.
- Slide 35
- Corona Proton-Proton Chain
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- Corona Proton-Proton Chain Because of the large temperature of
the corona many ions and electrons escape the suns gravitational
pull and form the solar wind. The coronas high temperature results
in intense UV and X-ray emission. One of the major surprises was
the discovery that the temperature of the chromosphere and corona
increase with increasing distance from the sun. UV observations of
the corona made by SOHO show coronal holes that emit much less UV
and have lower temperature and density than their surrounding
areas. The solar wind is stronger near these coronal holes.
- Slide 37
- Sunspots Proton-Proton Chain (a)An isolated sunspot, (b) a
group of sunspots Sunspots are temporary cool regions in the solar
photosphere. Because of their relatively low temperature they
appear dark. They have typical sizes of ~ 10,000 km across and have
a lifetime of hours to years. T Umbra ~ 4,300 K and T Penumbra ~
5,000 K. Estimate the ratio of the flux from the umbra to that from
the photosphere assuming a similar emitting area.
- Slide 38
- Astro 129: Chapter 1a
- Slide 39
- Proton-Proton Chain The sun's rotation can be tracked by
observing the rotation of sunspots. Observations show that a
sunspot near the equator takes ~ 25 days to go around once whereas
sunspots at larger latitudes take longer. The sun does not rotate
as a rigid body. Different parts of the sun rotate at different
velocities. This is referred to as differential rotation.
Sunspotting
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- Astro 129: Chapter 1a
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- Sunspot Cycle Proton-Proton Chain The average number of
sunspots varies with a period of ~ 11 years. The periodic change in
the number of sunspots is called the sunspot cycle.
- Slide 42
- Sunspot Latitudes Proton-Proton Chain At the beginning of each
sunspot cycle, most sunspots are found near latitudes 30 north or
south. As the cycle goes on, sunspots typically form closer to the
equator.
- Slide 43
- Zeeman Effect Proton-Proton Chain When an atom is placed in a
magnetic field, the interaction between the field and the electron
current shifts the atoms energy. According to the principles of
quantum mechanics, only certain orientations are allowed. An energy
level is split into different energy levels, corresponding to the
different orientations of a particular electron orbit. Zeeman
showed that a spectral line splits when the atoms are subjected to
an intense magnetic field. The more intense the magnetic field, the
wider the separation of the split lines.
- Slide 44
- Sunspots have strong magnetic fields Proton-Proton Chain When
Hale focused a spectroscope on sunlight coming from a sunspot, he
found that many spectral lines appear to be split into several
closely spaced lines. Hales discovery showed that sunspots are
places in the photosphere that have strong magnetic fields.
- Slide 45
- Magnetograms Proton-Proton Chain A magnetogram is an image of
the sun showing variations in strength of a magnetic field. It is
based on the Zeeman effect. Hale also discovered that the Suns
polarity pattern completely reverses itself every 11 years. The
Suns magnetic pattern repeats itself only after two sunspot cycles,
which is why astronomers speak of a 22-year solar cycle.
- Slide 46
- Preceding and Following Members of Sunspot Groups Proton-Proton
Chain As a given sunspot group moves with the Suns rotation, the
sunspots in front are called the preceding members of the group.
The spots that follow behind are referred to as the following
members. Preceding members in one solar hemisphere all have the
same magnetic polarity, while the preceding members in the other
hemisphere have the opposite polarity.
- Slide 47
- The Magnetic-Dynamo Model Proton-Proton Chain Magnetic field
lines tend to move along with the plasma in the Suns outer layers.
Because the Sun rotates faster at the equator than near the poles,
a field line that starts off running from the Suns north magnetic
pole (N) to its south magnetic pole (S) ends up wrapped around the
Sun. Sunspots appear where the magnetic field protrudes through the
photosphere.
- Slide 48
- The Magnetic-Dynamo Model Proton-Proton Chain Preceding members
of sunspots move towards the equator and following members of
sunspots move towards the poles. When preceding members from the
two hemispheres meet at the equator they cancel and when following
members of sunspots reach the pole they initially cancel and then
reverse the polarity of the pole. This explains the magnetic field
reversal.
- Slide 49
- Rotation of the Solar Interior Proton-Proton Chain The solar
rotation period (shown by different colors) varies with depth and
latitude. The surface and the convective zone have differential
rotation. Deeper within the Sun, the radiative zone seems to rotate
like a rigid sphere. Rotation of the Solar interior
- Slide 50
- Magnetic Reconnection Proton-Proton Chain Plasma can easily
flow along magnetic field lines but takes more time to diffuse
across field lines. Occasionally the magnetic field lines protrude
through the suns surface (near the edges of supergranules) forming
giant coronal loops. Plasma flows along these loops. Whenever
magnetic field lines reconnect a large amount of energy is released
that heats up the surrounding plasma to high temperatures.
- Slide 51
- Plages and Prominences Proton-Proton Chain Plages are areas of
the chromosphere that are much brighter (especially in H ) than
their surroundings. Plages tend to form just before sunspots appear
and are thought to be produced by magnetic fields that protrude
from the sunspot regions below. These magnetic fields may compress
and heat the gas in the chromosphere. Prominences are columns of
gas from the chromosphere that follow magnetic field lines anchored
at sunspots. They can extend tens of thousands of meters above the
photosphere.
- Slide 52
- Solar Flares Proton-Proton Chain Solar flares are eruptions on
the sun that occur near sunspot groups. They can release near 10 26
joules of energy within a few hours. They heat nearby regions of
the suns atmosphere to temperatures of up to tens of millions of K.
Energy released in a flare is thought to come from magnetic energy
stored near the group of sunspots.
- Slide 53
- Coronal Mass Ejection Proton-Proton Chain In a coronal mass
ejection, more than 10 12 kilograms of high- temperature coronal
gas is blasted into space at speeds of hundreds of kilometers per
second. A typical coronal mass ejection lasts a few hours.
- Slide 54
- Astro 129: Chapter 1a
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