Download The Sun: A Model Star

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Astrophysics
Dr Dirk Froebrich
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The Sun: A Model Star
• Our Sun is the nearest star. The fascinating properties and phenomena
of the solar surface layers are easily observed and have been studied
intensely. Unfortunately, models for understanding solar phenomena
have not kept pace with such detailed data. Because the Sun is a fairly
typical star and because it is the only star that spans a large angular
diameter as seen from the Earth, the discussion here serves as the
physical basis to investigate the other stars.
Sun
Mass (1024 kg)
1,989,100.
6
3 2
GM (x 10 km /s )
132,712.
12
3
Volume (10 km )
1,412,000.
Volumetric mean radius (km) 696,000.
Mean density (kg/m3)
1408.
2
Surface gravity (eq.) (m/s )
274.
Escape velocity (km/s)
617.7
Ellipticity
0.00005
2
Moment of inertia (I/MR )
0.059
Visual magnitude V
-26.74
Absolute magnitude
+4.83
24
Luminosity (10 J/s)
384.6
6
Mass conversion rate (10 kg/s) 4300.
Mean energy production (10-3 J/kg) 0.1937
Surface emission (106 J/m2s)
63.29
Spectral type
G2 V
Earth
5.9736
0.3986
1.083
6371.
5515.
9.78
11.2
0.0034
0.3308
-3.86
(Sun/Earth)
333,000.
333,000.
1,304,000.
109.2
0.255
28.0
55.2
0.015
0.178
PH507
Astrophysics
Dr Dirk Froebrich
2
Model values at center of Sun:
Central pressure:
2.477 x 1011 bar
Central temperature:
1.571 x 107 K
Central density:
1.622 x 105 kg/m3
The Structure of the Sun
• The average density of the Sun is only 1400 kg/m3 - consistent with a
composition of mostly gaseous hydrogen and helium.
• From its angular size of about 0.5° and its distance of almost 150
million kilometres, we determine that its diameter is 1,392,000
kilometres (109 Earth diameters and almost 10 times the size of the
largest planet, Jupiter).
• All of the planets orbit the Sun because of its enormous gravity. It has
about 333,000 times the Earth's mass and is over 1,000 times as massive
as Jupiter.
• The Sun is made of 94% Hydrogen, 6% Helium, - the other elements
make up just 0.13% (the three most abundant ‘metals’ Oxygen, Carbon,
and Nitrogen make up 0.11%).
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The Sun’s atmosphere has the following layers (from innermost to
outermost):
o The photosphere is about 300 km thick. Most of the Sun's
visible light that we see originates from this region.
o The chromosphere is about 2000 km thick. We only see this
layer and the other outer layers during an eclipse.
o The corona extends outwards for more than a solar radius.
The Photosphere
An image of the Sun's Photosphere shows:
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Limb Darkening. Limb darkening is evidence that the temperature of
the Sun's photosphere decreases outwards.
Sunspots
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Astrophysics
Dr Dirk Froebrich
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The Sun's Spectrum is an Absorption Spectrum
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Since the photosphere is cooler and less dense than the interior
region it allows the continuous blackbody spectrum to flow through
it.
Only at the wavelengths at which atoms in the photosphere can
absorb light will photons be impeded in their outward travel.
Sunspots:
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Sunspots are regions with high magnetic fields (1000 x higher
magnetic field than average)
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Typical size of spots is similar to the size of the Earth.
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These regions are cooler (redder) than average, so they look darker
than the surrounding hotter region.
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Sunspots are related to X-ray flares, mass ejections and the aurora
seen on earth
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Astrophysics
Dr Dirk Froebrich
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Close-up Picture of a group of Sunspots
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The darkest regions (umbra) have the largest magnetic fields and the
coolest temperatures. The outer brighter region is the penumbra.
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Sunspots come in pairs: each member of the pair has opposite
polarity. (I.e. one is a north magnetic pole, the other is south.)
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Each sunspot region lasts for a few days to a few weeks.
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The filaments in the penumbra are due to the magnetic lines of force.
Movement of Sunspots
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Movements of spots reveal that the Sun rotates with a period close
to one month.
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Equator rotates faster than the higher lattitudes. Differential
Rotation
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You can find photos of the Sun in many different wavelengths
(updated daily) at the website:
http://umbra.nascom.nasa.gov/images/latest.html
http://science.nasa.gov/ssl/pad/solar/surface.htm
Granules
Close-up Picture of the Photosphere
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Granules are the cell-like features seen on the Sun's photosphere that
cover the entire solar surface, except for the sunspot regions.
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Astrophysics
Dr Dirk Froebrich
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The granules are the tops of convective cells which lie in the
convective zone just below the photosphere.
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Each cell ranges in size from 100 km to 1000 km across and may last
up to half an hour.
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The bright regions are zones where hot gas rises. They are the tops of
deep gas columns where energy is transported by convection.
Spectra of the centers of the granules shows these regions to be a few
hundred Kelvin hotter than the surrounding darker lanes.
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The dark borders are the places where the cool gas sinks.
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The gas moves outwards or inwards at speeds up to 7 km/s.
(Measured through Doppler shifts.).
The Sun's Chromosphere
A Solar Eclipse
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The photosphere is much brighter than the outer parts of the Sun's
atmosphere (the chromosphere and the corona), so regular photos of
the Sun do not show the outer atmosphere.
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During a solar eclipse the Moon blocks out the light from the
photosphere and we can only see the light coming from the
chromosphere and corona.
The Chromosphere with a close-up of the spicules.
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The Chromosphere is not exactly a sphere: there are many spicules
and prominences which jut outwards.
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Astrophysics
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Magnetic fields help support the spicules and the prominences.
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The red colour results from the emission of Balmer-alpha photons:
electrons jumping from the n=3 level to the n=2 level.
The emission lines can only occur if the gas in the chromosphere is very
hot and the density is very low. The chromosphere is hotter (but less dense)
than the photosphere.
In the spicules, which are best observed in Hα, gas is rising at about 20 to
25 km/s. Although spicules occupy less than 1% of the Sun’s surface area
and have lifetimes of 15 minutes or less, they probably play a significant
role in the mass balance of the chromosphere, corona, and solar wind, and
occur in regions of enhanced magnetic fields
Solar spicules, short-lived narrow jets of gas that typically last mere
minutes, can be seen sprouting up from the solar chromosphere in this Hα
image of the Sun. The spicules are the thin, dark, spikelike regions. They
appear dark against the face of the Sun because they are cooler than the
solar photosphere
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Astrophysics
Dr Dirk Froebrich
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Prominences
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Close-up picture of the chromosphere showing a prominence.
The prominences are loops of gas which arch over sunspot regions.
The quiescent prominences are very stable and can last weeks or months.
Eruptive Prominences
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Some of the prominences will erupt, causing gas to be flung
outwards.
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The gas travels outwards about 70,000 km in the course of a few
hours.
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Prominences are more likely to erupt when the magnetic fields near
the sunspots are changing.
Variation of Temperature in the Sun's Atmosphere:
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Astrophysics
Dr Dirk Froebrich
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Photosphere: Temperature decreases outwards.
o At bottom: T = 6400 K
o At top: T = 4000 K
Chromosphere: Temperature increases outwards.
o At top: T = 10,000 K
Transition Zone: Temperature shoots up to near 1 million K
Corona: Temperatures increase to about 2 million K
The source of this heat is not well understood. Current theories
suggest that magnetic waves might transport energy from the
convective zone to the corona.
The Transition Zone
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The next picture shows the transition zone as seen through a filter
which only sees the light coming from an electronic transition of
Sulfur VI at temperatures of about 200,000ºC. Instead of hydrogen,
the light emitted by the transition region is dominated by such ions
as C IV, O IV, and Si IV (carbon, oxygen, and silicon each with
three electrons stripped off). These ions emit light in the ultraviolet
region of the solar spectrum that is only accessible from space.
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Astrophysics
Dr Dirk Froebrich
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These emission lines are Ultra-violet. which are only possible when
the gas is very hot, near 100,000 K.
The structures seen here are similar to those seen in the
chromosphere.
The Corona
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A visible light photograph of the Corona during a solar eclipse.
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Astrophysics
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Photograph of the solar corona during the July, 1991 eclipse, at the peak of
the sunspot cycle. At these times, the corona is much less regular and much
more extended than at sunspot minimum. Astronomers believe that coronal
heating is caused by surface activity on the Sun. The changing shape and
size of the corona are the direct result of variations in prominence and flare
activity over the course of the solar cycle.
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The Corona emits X-rays.
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This image corresponds to an electronic transition of highly ionized
iron. (Iron stripped of 11 of its electrons.)
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Astrophysics
Dr Dirk Froebrich
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Iron can only lose 11 electrons and emit this X-ray light if the
temperature is more than one million K.
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The dark regions are coronal holes which are lower density than
average.
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The solar wind originates from the coronal holes.
Coronal Loops
Huge numbers of small, closely intertwined magnetic loops continuously
emerge from the Sun's visible surface, clash with one another and dissolve
within 40 hours.
The loops seem to form a tight pattern that form a magnetic carpet. Their
interaction generates electrical and magnetic short-circuits (magnetic
reconnection) and releases enough energy to heat the corona to
temperatures hundreds of times higher than those of the solar surface.
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Astrophysics
Dr Dirk Froebrich
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Coronal loops come in a variety of shapes and sizes, but most are
enormous, capable of spanning several Earth's. (Photo: NASA and the
TRACE team)
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Astrophysics
Dr Dirk Froebrich
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Solar Flares
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Solar flares are large outbursts similar to eruptive prominences, but
larger and more energetic.
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Solar flares increase the amount of particles which escape into the
solar wind.
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If the particles ejected from the flare hit the Earth, then we get
intense auroral displays.
A negative effect is that the solar wind particles can disrupt radio
transmissions.
Coronal Mass Ejection
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When an eruptive prominence or a solar flare occurs, a coronal mass
ejection (CME) can also take place.
A CME is a stream of plasma (charged particles) ejected from the corona.
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Astrophysics
Dr Dirk Froebrich
14
The Solar Wind
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UV images show the flow of gas from the Sun.
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The solar wind is a stream of charged particles (protons and
electrons) which flow outwards from the coronal holes.
The wind speed is high (800 km/s) over coronal holes and low (300
km/s) over streamers. These high and low speed streams interact
with each other and alternately pass by the Earth as the Sun rotates.
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The solar wind particles flow throughout the solar system. The
variations buffet the Earth's magnetic field and can produce storms in
the Earth's magnetosphere
Further Evidence for the Solar Wind
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A comet's tail always points away from the Sun, no matter in what
direction it moves.
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The particles in the solar wind push outwards on the gas sublimating
from the comet so that the tail points away from the Sun.
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Astrophysics
Dr Dirk Froebrich
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Aurora
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When solar wind particles hit atoms in the Earth's atmosphere, they
cause atoms excitation.
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The electrons in the excited atoms then jump down to a lower state
and give off radiation.
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The green and red aurora are usually due to electronic transitions in
Oxygen.
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There is almost always an auroral oval over the Earth's north and
south magnetic poles.
The size of the auroral oval and the intensity of the emissions depend
on the strength of the solar wind.
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The Solar Cycle
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The number and location of sunspots and the severity and number of
flares, eruptive prominences and coronal mass ejections are not
constant in time.
The number of sunspots (R=f+10*g) on the Sun varies on an 11-year
cycle. The number increases from zero at solar minimum, to over
100 at solar maximum, 5 1/2 years later.
Every 11 years the polarity of the Sun's magnetic field flips.
Therefore, the Sun's magnetic field varies on a 22-year cycle.
As the number of sunspots increases, the number of flares and other
forms of activity increase.
The luminosity of the Sun also increases when there are lots of spots!
(Bright plages also increase when sunspots increase.)
The time at which there is a maximum number of spots is called
Solar Maximum.
The most recent Solar Maximum occurred late in 2000.
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Astrophysics
Dr Dirk Froebrich
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The period of time from 1645 to 1715, known as the Maunder
minimum, was a time with a very low number of sunspots.
During the Maunder minimum Europe had colder than usual
weather.
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Astrophysics
Dr Dirk Froebrich
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Predictions about the present sunspot cycle from
http://science.nasa.gov/ssl/PAD/SOLAR/sunspots.htm.
Polarity of Sunspots
• The most important characteristic of a sunspot is its magnetic field. The
magnetic field in a typical sunspot is about 1000 times greater than the
field in neighbouring, undisturbed photospheric regions. Typical field
strengths are near 0.1 T, but fields as strong as 0.4 T have been
measured.
• Related to the magnetic field is a horizontal flow of gas in the sunspot
penumbra: gas moves out along the lower filaments and inward along
the higher filaments (at speeds up to 6 km/s).
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In each sunspot pair, one sunspot is a north pole and the other is a
south pole.
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Note that in the northern hemisphere of the Sun, the north poles are
always to the right of the south poles.
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In the southern hemisphere, the south poles are always to the right of
the north poles.
Location of Sunspots
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At the beginning of each 11-year cycle the sunspots appear at middle
latitudes above and below the Sun's equator. Very few sunspots are
ever found at latitudes greater than ±40°.
As the cycle progresses, the sunspots appear closer to the equator .
Most spots are near ±15° at maximum, and the few spots at the end
of the cycle cluster near ±8°.
The cycles overlap at the time of sunspot cycle minimum with old
cycle spots near the equator and new cycle spots at high latitudes.
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Astrophysics
Dr Dirk Froebrich
A sunspot dies at the same latitude where it was born
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Astrophysics
Dr Dirk Froebrich
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The sunspots follow active latitude belts during the course of a cycle.
Less clear, but just as tantalising, large active regions and spot
groups seem to fall into preferred longitudes during a cycle - active
longitude belts. Concentrations of magnetic fields appear to persist
below the photosphere, so that new spot groups arise from about the
same locations as previous ones. The active longitude belts
sometimes appear about 180° apart
This plot is known as a "butterfly diagram". It shows the number and
position of sunspots over time.
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Astrophysics
Dr Dirk Froebrich
20
At the end of the 11 year cycle the opposite polarity sunspots at the equator
"cancel" each other out and the Sun's polarity flips.
Flux Tube Model for the Sun's Magnetic Activity
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The magnetic field lines get "frozen" into the gas of the Sun and get
dragged around when the gas moves.
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The differential rotation of the Sun drags the magnetic field lines
into "horizontal" tubes which break out of the surface, causing
sunspots with the observed polarity.