Transcript File - Mr. Catt`s Class

The Sun
Courtesy of SOHO - EIT Consortium/ESA/NASA
Chapter 11
11-1 Solar Properties
1. As viewed from the Earth,
– the Sun has an average angular
diameter of 31´59° and
– is at an average distance from the
Earth of 1.50  108 km.
The diameter of the Sun can
therefore be calculated as 1.39  106
km
– about 110 times Earth’s diameter
and about 10 times Jupiter’s.
2. Using Kepler’s 3rd law we find the mass of the Sun is
1.99  1030 kg,
– about 333,000 times Earth’s mass.
The Sun’s average density is 1.41 g/cm3
– about the same as the density of Jupiter.
3. By observing sunspots, Galileo concluded the Sun’s
rotation period is more than a month.
Today we know the Sun exhibits differential rotation
– 25 days at its equator and nearly 35 days near its
poles.
4. Sunspots are regions of the photosphere that are
temporarily cool and dark compared to surrounding
regions.
Figure 11.03a: Sunspots on the Sun’s surface
(a) Courtesy of William P. Sterne, Jr.
11-2 Solar Energy
1. The Sun emits energy in all parts of the
electromagnetic spectrum.
2. Luminosity is the rate at which electromagnetic
energy is emitted; the solar luminosity is about 3.85
 1026 watts.
3. Solar energy strikes the Earth at the rate of 1,370
watts/m2.
The Source of the Sun’s Energy
1. In the mid-1800s, von Helmholtz and Kelvin proposed that a slow
gravitational contraction is the source of the Sun’s energy.
– Such a mechanism would power the Sun for only a
few hundred million years.
2. By the early 20th century geologists showed the Earth was
billions of years old—a period 10 times longer than necessary for
gravitational contraction to produce the Sun’s energy.
– The theory had to be abandoned.
3. Einstein proposed in 1905 that mass and energy are
interconvertible.
During the 1930s, physicists worked out the theory of nuclear
reactions.
Solar Nuclear Reactions
1. An atom’s nucleus is composed of protons
(massive, positively charged particles) and
neutrons (massive nuclear particles with no electric
charge).
2. Nuclear reactions involve forces between nuclear
particles.
3. In nuclear fusion, two nuclei combine to form a
larger nucleus, releasing energy in the process.
In the Sun, 4 hydrogen nuclei are fused to form 1
helium nucleus and energy.
4. The fusion process in the Sun converts only 0.7% of
the original mass into energy.
5. To produce the Sun’s energy output, nearly 5 million
metric tons of matter must be converted into energy
each second.
This in turn, requires 626 billion kg of hydrogen be
transformed into 622 billion kg of helium.
6. The proton-proton chain is the series of nuclear
reactions that begins with 4 protons and ends with a
helium nucleus.
This chain is the main fusion process in the Sun,
responsible for 98.5% of the energy production.
Figure 11.07
7. Deuterium is a hydrogen nucleus that contains one
neutron and one proton.
8. A positron is a positively charged electron emitted
from the nucleus in some nuclear reactions.
9. A neutrino is an elementary particle that has little
mass and no charge but carries energy from a
nuclear reaction.
11-3 The Sun’s Interior
1. Because of the high temperatures on and in the Sun,
most electrons are stripped away from their nuclei.
Most of the material of the Sun’s interior consists of
free nuclei and free electrons.
2. The behavior of this solar material is similar to that
of a simple gas. The most important properties in
describing gases are: temperature, pressure, and
particle density.
3. Particle density is the number of separate atomic
and/or nuclear particles per unit of volume.
Pressure, Temperature, and Density
1. Pressure is the force per unit area.
The two factors that determine the pressure of a gas
are its density and the speed of its constituent
particles.
2. Since the temperature of a gas is a measure of the
speed of its constituent particles, pressure is related
to temperature.
3. The greater the density of a gas in a container, the
higher the number of collisions with the walls of
the container.
Thus, pressure is related to density.
4. Under ideal conditions, gas pressure is
proportional to its density times its temperature.
Hydrostatic Equilibrium
1. In a star or a planet, hydrostatic equilibrium is the
balance between pressure caused by the weight of
material above a thin layer and the upward pressure
exerted by material below that layer.
Figure 11.11
2. Pressure at the Sun’s center is calculated to be
2.5  1011 times that on the surface of the Earth.
3. The high pressure and temperature at the Sun’s
core results in fusion reactions; the solar core
extends out to about 25% of the Sun’s radius.
Energy Transport
1. Three possible methods by which energy can be
transferred from the center of the Sun outward:
Conduction, Convection, Radiation
(a) Conduction: The transfer of energy in a solid by
collisions between atoms and/or molecules.
This is not a significant factor in transporting
energy within the Sun.
(b) Convection: the transfer of energy in a gas or
liquid by means of the motion of the material.
•
In a star, convection between adjacent layers is
significant only when the temperature difference
is great compared to the pressure difference.
•
Convection occurs inside the Sun within about
200,000 km of the surface.
•
Convection is not a factor in the Sun’s core.
(c) Radiation: the
transfer of energy by
electromagnetic
waves.
Inside the Sun and
most stars, radiation
is the principal means
of energy transport.
Fig. 11-12
2. The Sun’s material is nearly
opaque, so emitted radiation
travels only a short distance
(about 1 cm) before being
absorbed and then reemitted.
It takes a very long time for a
gamma-ray photon produced by a
fusion reaction to reach the Sun’s
surface.
Thus any information carried by
such photons about the Sun’s
core is lost.
Figure 11.13
3. Neutrinos are produced in the solar core and
they rarely interact with matter.
Their very low mass implies a speed very
close to the speed of light.
Thus they can tell us about the current
conditions in the Sun’s core.
Solar Neutrinos and the Standard Solar Model
1. The standard solar model is the generally accepted
theory of solar energy production.
2. The model predicts that so many neutrinos flow
from the Sun that 65 billion neutrinos pass through
every cm2 of your body per second.
3. Neutrinos react very little with ordinary matter so
detecting them is difficult.
Astronomers want to measure the number of
neutrinos reaching the Earth in order to check the
standard solar model.
4. Large detectors have been built
that confirm the solar origin of
neutrinos but report finding only
50 to 60% of the neutrinos
predicted by theory.
– The discrepancy between
theory and experiment is
referred to as “the solar
neutrino problem.”
– The neutrino deficit depends
on the energy of the
neutrinos captured by the
experiment.
Courtesy of ICRR (Institute for Cosmic Ray Research), The University of Tokyo
5. Neutrinos come in three types, related to the
electron and (its lesser known relatives) the
muon and tau particles.
But most experiments were designed to detect
electron neutrinos.
According to the MSW theory, the observed
neutrino deficit could be the result of neutrino
oscillations (neutrinos changing from one type
to another during their flight from the Sun to
Earth).
6. The SNO experiment uses the properties of heavy
water to detect all three types of neutrinos.
– In 2002, it found that the number of electron
neutrinos is about a third of the total number
of neutrinos reaching the detector, showing
that neutrinos oscillate.
– The SNO results are in excellent agreement
with the predictions of the standard solar
model.
7. Support for the SNO results came in 2002 from
the KamLAND project, which measures antineutrinos from all nuclear plants in Japan.
An antineutrino is a mirror image of a neutrino.
8. Neutrino oscillations imply that neutrinos have
mass, albeit very small.
Since they are the most numerous particles in
the universe (other than photons), they
contribute to a small degree to its overall mass
and influence its evolution.
9. So far we have only measured about 0.005% of
the total number of neutrinos emitted by the Sun.
Future experiments will attempt to measure the
remaining neutrinos and will provide a further
test for the standard solar model.
Helioseismology
1. In 1962 it was discovered that the Sun vibrates.
The pulsations are caused by waves, similar to
sound waves, produced by pressure fluctuations in
the turbulent convective motions inside the Sun.
2. Helioseismology is the study of the propagation of
these waves.
It allows us to see “through” the Sun and helps us
predict solar activity.
Figure 11.14a: A computer
model of solar resonance that
produces the observed
vibrations of the photosphere.
Figure 11.14b: The solar surface showing
oscillations: blue areas are moving toward us,
and red areas are moving away from us.
Courtesy of AURA/NOAO/NSF
Courtesy of NASA/JPL-Caltech
Figure 11.14c: SOHO sees "through" the Sun by observing disturbances at
its near side caused by pressure waves generated at its far side.
Courtesy of NASA/ESA
11-5 The Solar Atmosphere
The Photosphere
1. The solar atmosphere is conveniently divided into
three regions;
– the photosphere,
– the chromosphere,
– and the corona.
2. Photosphere is the visible “surface” of the Sun. It is
the part of the solar atmosphere from which light is
emitted into space. The photosphere is a very thin
layer—400 km thick.
Figure 11.16a: The solar disk, the photosphere, in visible light.
Courtesy of NASA/JPL-Caltech
3. When observing the limb of the Sun (the
apparent edge of the Sun as seen in the sky), it
appears darker than the center of the solar disk.
That’s because when we observe the limb of the
Sun we see to a lesser depth because the line of
sight is at a grazing angle.
4. The photosphere varies in temperature from
about 6,500 K at its deepest to 4,400 K near its
outer edge.
Overall, the light received from the photosphere
is representative of an object with temperature of
about 5,800 K.
5. The pressure of the outer photosphere is 0.01
the pressure of the Earth’s surface.
Knowing the pressure and temperature we can
calculate the density of particles in the outer
photosphere:
– it is about 0.0005 of the density of air at sea
level on Earth.
6. The base of the
photosphere shows
granulation; the
Sun’s surface is
divided into small
convection cells.
Granules are areas
where hot material
(light areas) is
rising from below
and then
descending (dark
surroundings).
Figure 11.18a: Granules and sunspots near
the solar eastern limb
The image taken at the Swedish 1-meter Solar Telescope on the island of La Palma, Spain, is courtesy of the Lockheed Martin Solar &
Astrophysics Laboratory, the High Altitude Observatory, and the Institute for Solar Physics of the Royal Swedish Academy of
7. The chemical composition of the photosphere is
(by mass) 78% hydrogen and 20% helium; the
remaining 2% consisting of 60 elements found on
Earth.
– The similarity in composition between Earth
and Sun points to their common origins.
8. From our knowledge of nuclear fusion, we know
the Sun’s core must hold more helium.
Overall, the Sun (by mass) is theorized to be
about 73% hydrogen, 25% helium, and 2% for all
other elements.
The Chromosphere and Corona
1. The chromosphere is the region of the solar
atmosphere (about 2,000 km thick) that lies
between the photosphere and the corona.
– It is not usually observable from Earth except
during a total solar eclipse.
– It has a bright-line (emission) spectrum.
2. Corona is the outermost portion of the Sun’s
atmosphere that can only be seen during a total
solar eclipse.
Figure 11.19b: H-alpha emissions scan be seen in prominences above the
limb of the Sun during total solar eclipses.
Courtesy of UCAR/NCAR/High Altitude Observatory/NASA
3. Narrow jets of gas (spicules) shoot upward from
the chromosphere into the corona, reaching a
height of 6,000 to 10,000 km and lasting from 10
to 20 minutes.
4. The temperature increases as we move outward
from the photosphere; it is as high as 30,000 K in
the outer portions of the chromosphere and 2
million K in the corona (increasing rapidly in a
transition region of about 300 km between the
chromosphere and corona).
Figure 11.20: Spicules
This image, taken at the Swedish 1-meter Solar Telescope (SST) on the island of La
Palma, Spain, is courtesy of Dr. Bart De Pontieu of the Lockheed Martin Solar &
Astrophysics Laboratory
– The leading hypothesis is
that the heating results
from the interaction
between the Sun’s
magnetic field and its
differential rotation.
– The presence of “solar
moss” shows that the
field is highly organized in
the transition region.
Courtesy of NASA/JPL-Caltech
5. The high temperatures in the
chromosphere and
especially the corona are not
well understood.
Figure 11.21: The "solar
moss" observed in the
extreme UV.
Fig. 11-23a
Courtesy of SOHO (ESA & NASA)
6. A prominence is an
eruption of solar
material beyond the
disk of the Sun.
Prominences may
reach as high as a
million kilometers
above the
photosphere. Slowmoving ones can last
for several days, while
explosive ones can
reach speeds of 1,500
km/s.
The Solar Wind
1. The solar wind is the continuous flow of nuclear
particles (mostly protons and electrons) from the
Sun.
2. Coronal holes, dark areas in an X-ray image of the
Sun, correspond to regions of low density where the
magnetic field lines are open; they provide a
corridor for charged particles to escape into space,
generating the solar wind.
3. Due to the solar wind, the Sun loses about 6  1016
kg every year, a tiny fraction of its total mass.
4. Near the Earth the solar wind travels at about 400
km/s and has a density of 2–10 particles/cm3.
5. Auroras form when the charged particles from the
solar wind that have been trapped by Earth’s
magnetic field strike upper atmosphere molecules
causing them to glow.
11-6 Sunspots and the Solar Activity Cycle
1. Dark spots on the Sun were first reported by the
Chinese in the 5th century B.C. Galileo and
Thomas Harriott were the first Europeans to
report sunspots in the early 17th century.
– Sunspots are temporary phenomena lasting
from a few hours to a few months.
2. Sunspots are about 1,500 K cooler than the
surrounding photosphere. Thus they are about 3
times less bright than their surrounding region.
NSO/AURA/NSF
3. The explanation for sunspots involves the Sun’s
magnetic field.
The strength of this field can be measured using
the Zeeman effect (the splitting of spectral lines
by a strong magnetic field).
4. Sunspots often appear in pairs, aligned in an
east-west direction, and have opposite magnetic
polarities, one being north and the other south.
5. The magnetic field in a sunspot is about
1,000 times that of the surrounding
photosphere.
6. In 1851, Schwabe discovered the sunspot
cycle, which lasts about 11 years.
7. The Sun went through a period of inactivity
during 16451715. This period corresponds
to the “Little Ice Age” climatic period on
Earth.
Modeling the Sunspot Cycle and the Sunspots
1. At a sunspot maximum, most spots occur about 35° north
or south of the equator.
As the cycle progresses, the spots are seen closer and
closer to the Sun’s equator.
When the spots reach the equator, the cycle is at a
minimum and begins again.
2. The plot that shows the location and relative number of
sunspots is called the butterfly diagram.
Figure 11.26: Butterfly graphs of sunspot activity
Courtesy of NASA/NSSTC/HATHWAY 2006/07
3. The leading model for the sunspot cycle involves
patterns of magnetic field lines, generated by the
flow of the hot, ionized gases within the Sun’s
interior, close to the boundary between the radiative
and convective zones.
– At this boundary, the shear in the Sun’s rotational
velocity drives the formation of the field.
4. The lines form tubes that gradually become twisted
due to the Sun’s differential rotation.
– Forced to the surface due to convection, these
magnetic tubes become visible as sunspots.
– Breaking the surface weakens the field lines and
the sunspots die out.
Figure 11.27: Magnetic field lines in Sun
5. The Sun’s overall magnetic field reverses from
one sunspot cycle to the other.
– The entire magnetic cycle of the Sun is 22
years, twice the sunspot cycle of 11 years.
– During each sunspot cycle, the polarity of the
leading sunspot of a pair in a given
hemisphere is the same as the polarity of the
Sun’s magnetic pole for that hemisphere
6. Observations show that the Sun’s magnetic
poles are connected to sunspots of opposite
polarity (trailing sunspots) near its equator by
giant loops of hot plasma that extend into the
corona.
The field steadily weakens due to the transfer of
opposite magnetic flux to the poles.
7. At the height of sunspot maximum, the field
reverses and begins to grow in a new direction.
During the flip the field is weak and uneven.
8. SOHO observations give us a better
understanding of a sunspot region.
– The strong magnetic field there behaves like a
“plug” that stops the normal upward
convective flow.
– This cools the region above the plug, which
then becomes denser and plunges downward
at high speeds, drawing the surrounding
plasma and field toward the center of the
sunspot.
– This increases the strength of the sunspot and
this cycle repeats for as long as the field is
strong enough to behave like a plug.
Courtesy of ESA/NASA/Office of Space Science/SOHO
Figure 11.29: An artist’s
version of the region below
a sunspot.
Solar Flares and Coronal Mass Ejections
1. The turbulent magnetic field of the Sun causes
explosive flare-ups called solar flares, which
occur mostly during sunspot maxima.
2. According to the sunspot model, flares occur
when a great number of twisted tubes of magnetic
field lines release their energy at once through the
photosphere.
3. Flares blast out large numbers of very energetic
charged particles. In the case of the largest
flares, these particles can reach Earth in less
than one hour.
4. Large solar flares cause spectacular auroras and
can affect earthly radio transmissions if the
ionosphere is disrupted by the high-energy
particles from the flare.
5. Coronal mass ejections
(CMEs) are events in which
hot coronal gas is suddenly
ejected into space at speeds
of hundreds of km/s.
6. CMEs, flares and prominences
are related phenomena.
Figure 11.32: A coronal mass ejection
showing twisted magnetic field lines
7. Twisted field lines are seen in
CMEs. Just like twisted coils
of spring metal, they contain
energy that is used to blast
the material in space.
The SOHO/LASCO data used here are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fuer Aeronomie
(Germany), Laboratoire d'Astronomie (France), and the University of Birmingham (UK). SOHO is a project of international c
Figure 11.31: A coronal mass ejection
Courtesy of SOHO - EIT Consortium/ESA/NASA