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Chapter 15
The Sun
UVU Survey of Astronomy
Radius: 700,000 km
Mass: 2.0 × 1030 kg
Density: 1400 kg/m3 or 1.4 xH2O
Rotation: Differential
Period: about a month
Surface temperature: 5800 K
The apparent surface of the Sun is the
Photosphere
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Chapter 15
The Sun
UVU Survey of Astronomy
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Chapter 15
The Sun
UVU Survey of Astronomy
Luminosity – total energy
radiated by the Sun – can
be calculated from the
fraction of that energy that
reaches Earth.
This diagram illustrates
how one can extrapolate
from the radiation hitting
Earth to the entire output
of the Sun.
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Chapter 15
The Sun
UVU Survey of Astronomy
Mathematical models,
consistent with
observation and
physical principles,
provide information
about the Sun’s interior.
In equilibrium, inward
gravitational force must
be balanced by outward
pressure
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Chapter 15
The Sun
UVU Survey of Astronomy
Although the average density of
the Sun is only slightly more dense
than water the density at the core
is extremely high.
The temperature is relatively low
at the surface but is near 15 million
K in the core.
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Chapter 15
The Sun
UVU Survey of Astronomy
The Solar interior has been understood theoretically
for years. In recent years we have been able to
investigate it by observation. Doppler shifts of solar
spectral lines indicate a complex pattern of vibrations
that can be used much like seismic waves on the
Earth to
determine
the interior
construction
of the Sun.
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Chapter 15
The Sun
UVU Survey of Astronomy
Energy transport in gases is limited to convection and
radiation. In the radiation zone the particles are so tightly
packed that the energy can only pass by radiation.
The radiation zone is relatively transparent; the cooler
convection zone is opaque.
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Chapter 15
The Sun
UVU Survey of Astronomy
Before we can discuss how the heat of the Sun is
generated we have to consider the following:
Four Forces of Nature
Force
Strength
Strong Nuclear (pull)
1040
Electro-Magnetic
1028
Weak Nuclear (push)
1026
Gravitational
1
Einstein’s famous
equation for the
equivalence of
matter and
energy
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The Heart of the Sun
UVU Survey of Astronomy
This is the first step in
the three-step fusion
process that powers
most stars.
Proton –Proton Chain
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The Sun
UVU Survey of Astronomy
The ultimate result of the process is:
The helium stays in the core;
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Chapter 15
The Sun
The energy is in the form of gamma rays, which
gradually share their energy with the body of the
Sun as they travel out from the core, emerging in
all of the wavelengths of the electromagnetic
spectrum;
The neutrinos escape without interacting.
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The Solar Atmosphere
The cooler chromosphere, is the pink
layer above the photosphere. It is hard
to see directly as the sun is too bright,
unless Moon completely covers the
photosphere.
The much less dense corona can also
be seen during a total eclipse.
Temperatures here range from 1 to 4
million K.
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Chapter 15
The Sun
Solar corona
changes along
with sunspot
cycle; it is
much larger
and more
irregular at
sunspot peak
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Chapter 15
The Sun
Features of the the Sun’s Surface
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The visible top layer of
the Sun, the Photosphere, is the top
convection zone and is
granulated. The areas
of upwelling hot gas are
light, surrounded by
areas of sinking cooler
gas.
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Chapter 15
The Sun
Sunspots are the most
fascinating of all the solar
features. Galileo saw them in
1610 and they have been
studied actively ever since.
They appear dark because they
are slightly cooler than the
surroundings. The dark central
part is the umbra and lighter
surrounding part is the
penumbra
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Chapter 15
The Sun
UVU Survey of Astronomy
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Chapter 15
The Sun
The Sun reverses magnetic
polarity every 11 years. Sun
spots cycle through a maximum
every polarity cycle . It takes two
11 year half-cycles to make one
full 22 year cycle
Schematic Formation of
Sunspots : Notice the
interaction between
magnetic field lines and
the differential rotation.
Magnetic lines trapped in
the plasma stretch and
break to form Sun Spots
They are like the ends of bar
magnets with lines of force
looping between them.
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Chapter 15
The Solar Surface
At the beginning of a half
cycle most of the Sun
spots are near 30 degrees
north and south. By the
end most are nearer the
equator
The number of
Sunspots go through a
maximum each half
cycle. Some half cycles
show more activity than
others.
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Chapter 15
The Sun
Over the years efforts have been made, with few
successes, to correlate sun spot activity with
events on the Earth. The coincidence of the
Maunder minimum with the mini-ice-age in
Europe is often designated as a cause-and-effect.
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Chapter 15
The Sun
A Solar flare is a large explosion
on Sun’s surface, driving matter
into space in seconds or
minutes. This matter adds to
the Solar Wind,
Hot matter escapes Sun often
through coronal holes in Coronal
Mass Ejections, which can be
seen in X-ray images as bright
white spots
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Chapter 15
The Sun
UVU Survey of Astronomy
Neutrinos are emitted directly from the core
of the Sun, and escape, interacting with
virtually nothing. Being able to observe these
neutrinos would give us a direct picture of
what is happening in the core. Unfortunately,
they are no more likely to interact with Earthbased detectors than they are with the Sun;
the only way to spot them is to have a huge
detector volume and to be able to observe
single interaction events.
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Chapter 15
The Sun
This is a large solar
neutrino detector.
Interactions (dim
flashes) take place in
the liquid, which
reaches the top of the
dome when full,
Detection is by the
glass photomultiplier
tubes.
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Chapter 15
The Solar Surface
Detection of solar neutrinos has been going on for
more than 30 years now; despite very different
detection methods and energy sensitivities, all
experiments agree that they are seeing about 30–
50% of the expected number of neutrinos. Could be:
• Problem with solar model
• Problem with our understanding and detection of
neutrinos
The second option seems more likely today.
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Chapter 15
The Solar Surface
In-Class Quiz
The Sun ___________ magnetic
polarity every 11 years.
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End of Chapter 15
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