The Sun - Our Star
• Sun’s diameter 100 times
the Earth’s
• Sun’s mass is 700 times the
mass of all the planets put
together
• The energy source of the
Sun is the conversion of
hydrogen atoms into helium
atoms through nuclear
fusion in the core of the
Sun. Without this energy
source the Sun would
collapse.
• Composition of the Sun is
71% Hydrogen, 27%
Helium, 2% heavier
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elements
How do we know what we know?
• Our understanding of the
Sun is a combination of
measurements of observed
quantities (diameter,
surface temperature,
spectra) and computer
models based on physical
laws (internal temperature
and density).
• Models are correct so far
as they can predict the
observed properties of the
Sun.
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Solar Seismology
• Solar seismology is the study of the Sun’s interior by
analyzing wave motions on the Sun’s surface and
atmosphere
• P-mode (compression) waves only
• The wave motion can be detected by the Doppler shift
of the moving material
– absorption line shifts in the photosphere are driven by
processes in the convective region below the surface
• The detected wave motion gives temperature and
density profiles deep in the Sun’s interior
• These profiles agree very well with current models
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Solar Seismology
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Solar Structure
• The Sun is completely gaseous.
• The atoms in this gas strongly absorb
light.
• Because of this absorption energy
travels very slowly through the Sun.
It takes about 100,000 years for
energy to travel from the core to the
solar surface.
• Above a certain height however the
gas is too thin to absorb light
effectively. The point at which the
gas becomes transparent is called the
photosphere.
• The photosphere is considered the
“surface” of the Sun. Surface
temperature about 6000 Kelvin. 5
Solar Structure
• At the core of the Sun, where nuclear
fusion occurs, temperatures are about
15 million degrees Kelvin.
• As this energy propagates outwards it
initially is carried by photons of light.
This region of the Sun is called the
radiative zone.
• Near the photosphere this process of
energy transfer becomes less efficient
and energy begins moving outward
through convective motions of the
gas. As gas heats it rises to the
surface. Once it reaches the surface it
releases heat and sinks. This region is
called the convective zone.
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Solar Granules
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Solar Granules
•Caused by convection
•Red regions show rising gas
•Black regions show falling gas
•White specks show regions of concentrated magnetic fields
•White bar is 5000km long
•Astronomy Picture of the day from 16 April 2010
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Hydrostatic Equilibrium
• Requires that the pressure
generated by the fusion
reactions in the core of the
Sun must be in exact
balance with the weight of
material falling inwards
due to the Sun’s gravity.
• Without this balance the
Sun would either collapse
(gravity wins) or explode
(pressure wins).
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The Sun’s Energy Source
• The Sun is not burning. If it
were burning fuel like coal it
would have exhausted its fuel
long ago.
• The slow collapse of the Sun was
once thought to be the energy
source but that wouldn’t have
lasted more than a few million
years.
• It wasn’t until the 20th century
that physicists understood the
process behind what powers the
Sun.
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Einstein’s Mass-Energy Relation
• In 1905 Albert Einstein
recognized that mass and
energy were related
through the formula:
E=mc2
(m =mass, E=energy, c=speed of light)
• What this means is that a
small amount of mass
could be converted into an
enormous amount of
energy.
• The means by which the
Sun generates this energy
is through nuclear fusion.
Albert Einstein (1879-1955)
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Nuclear Fusion
• Nuclear fusion involves
combining two atomic nuclei
together to form one, larger
nucleus.
• This can not occur under
normal conditions because
the electrical repulsion
between protons prevents the
nuclei from fusing.
• At extremely high
temperatures however this
electrical repulsion is
overcome and the nuclei fuse.
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Nuclear Fusion
•
•
•
The specific steps of nuclear
fusion follow a process
called the proton-proton
chain
Through this process 2
neutrinos, 2 positrons, 2 1H
and a 4He is created by the
fusion of 6 1H.
The mass of all the particles
created is less than the sum
of the masses of colliding
particles. This difference in
mass was converted into
energy through E=mc2.
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Studying the Solar Interior
• The neutrinos generated in
the fusion process escape
rapidly through the Sun.
• These neutrinos are very
difficult to detect but they can
be detected.
• This type of neutrino
“telescope” can check
whether our understanding of
the solar interior is correct.
• It seems that the number of
A neutrino detector in Japan.
neutrinos measured match
This is a giant tank of water buried deep
computer models but only if
underground. Tiny bursts of light due to
neutrinos from the Sun are detected by
light sensitive detectors on the walls of
the tank.
– neutrinos have mass
– neutrinos can oscillate among
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three different states or types
Sunspots
• Sunspots are dark
splotches on the
photosphere of the Sun.
• Sunspots are dark because
they are cooler than their
surroundings.
• The reason they are cooler
is because intense
magnetic fields prevent
hot, ionized gases from
entering so the region
This sunspot is twice the size of the Earth! cools forming a spot
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Solar Magnetic Activity
• Like several of the
planets the Sun
generates a
magnetic field.
• Unlike the planets
the strength of the
field strongly
varies with time
and with location
on the Sun
• The most easily
detectable
evidence for solar
magnetic activity
are the Sunspots
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Prominences & Flares
• Prominences are associated with pairs of sunspots are are arcs
of magnetic field with glowing gas trapped inside.
• Flares are explosive events where strong bursts of X-rays are
emitted. These are due to rapid changes in the magnetic field.
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The Sun’s Atmosphere
• The extremely low-density gases that lie above
the photosphere make up the Sun’s atmosphere
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The Chromosphere
• The lower part of the
atmosphere is referred to as
the chromosphere
– The chromosphere appears
as a thin red zone around the
dark disk of a totally
eclipsed Sun
– The red is caused by the
strong red emission line of
hydrogen H
– The chromosphere contains
millions of thin columns
called spicules, each a jet of
hot gas
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The Corona and Solar Wind
The corona is only
visible from Earth during
a Solar Eclipse.
• The outer atmosphere of the Sun is
called the corona. The temperature
of the corona can be several
million degrees.
• Magnetic fields are thought to be
responsible for these very high
temperatures.
• The corona is so hot it expands
outward along with the Sun’s
magnetic field and streams through
the Solar System. This is known as
the Solar Wind.
• Depending on the Sun’s magnetic
activity level, the Solar Wind may
change in velocity and density.
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Heating of the Chromosphere and Corona
• While the Sun’s magnetic field cools sunspots and
prominences, it heats the chromosphere and corona
• Heating is caused by magnetic waves generated in the
relatively dense photosphere
– These waves move up into the thinning atmospheric gases,
grow in magnitude, and “whip” the charged particles found
there to higher speeds and hence higher temperatures
– Origin of waves may be from rising bubbles in convection
zone
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Heating of the Chromosphere and Corona
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The Solar Cycle
A plot of the number of sunspots
with time from 1860 - 2000
A solar cycle is the time between
two minima (low sunspot counts).
• Sunspots, prominences,
flares and the strength of
the solar wind all vary
over an approximately 11year cycle
• The magnetic field of the
Sun will strengthen and
weaken over 11 years.
• The polarity of the Sun’s
field will also flip. The
North pole becomes the
South pole and vice versa.
It takes 22 years to return
to its original polarity.
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Maunder's Sunspot Diagram
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Changes in the Solar Cycle
• The cycle may vary from 6 to 16 years
• Considering the polarity direction of the sunspots, the
cycle is 22 years, because the Sun’s field reverses at the
end of each 11-year cycle
• Leading spots in one hemisphere have the same polarity,
while in the other hemisphere, the opposite polarity leads25
Solar Seismology Reveals
• Difference in rotational speeds of convective
zone and radiative zone
– Contributes to magnetic dynamo
• Jet streams in convective zone deep below solar
surface
• Map far side of solar surface
– Sunspots on far side affect doppler shifts of
absorption lines on near side
• Cyclic time variation in differential rotation
(torsional oscillation) is similar to solar cycle
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Solar Cycle and Climate
Midwestern United States and Canada experience a 22year drought cycle
Few sunspots existed from 1645-1715, the Maunder
Minimum, the same time of the “little ice age in
Europe and North America
Solar Cycle and Climate
Number of sunspots correlates with change in ocean
temperatures
Recent Solar Observatories
• SOHO - Solar & Heliospheric Observatory
– Hurricane-sized whirlpools spotted on the Sun
(YouTube)
• Stereo – two satellites
– direct simultaneous far and near side observations
starting in 2011
– Mercury sodium (?) tail movie
• SDO – Solar Dynamics Observatory
– YouTube Channel: spicules, prominences, flares,
internal solar dynamic model animations
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Solar System by Voyager 1
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Extent of Solar Wind
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Bow Shock Around Star R
Hydrae
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