Transcript The Sun – A Typical Star

The Sun – A Typical Star
The Sun
• The Sun is the largest object in the
solar system both in size and in mass
• Once worshiped as a god, it is now
studied intently as the single, closest
representative of an average star
• While just a 'simple collection of hot
gas', it has amazing complexity and,
of course, without it there would be
no life on Earth
The Sun
We will NOT be looking at the Sun thru the 16"
Solar and Heliospheric Observatory
The Sun
Physical Properties:
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Radius:
Mass:
Luminosity:
Average Density:
Rotation at equator:
Surface Temperature:
Absolute Magnitude:
Spectral Class:
Luminosity Class:
6.960 x 108 meter
1.989 x 1030 kilogram
3.847 x 1026 Watt
1.4 (Water = 1.0)
24.7 days
5800 ºK
4.8
G2
V
Solar Structure
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Interior
Photosphere
Chromosphere
Transition Region
Corona
Solar Wind
Heliosphere
Solar Spectra
Solar Spectra
Solar Structure: Interior
• Core
– Inner 25%
– Energy Production
• Radiation Zone
– Inner 53%
• Convection Zone
– Outer 22 %
Solar Structure: Photosphere
The photosphere is
the 'surface' of
the Sun – where all
the light appears
to come from.
Photosphere
The two most obvious things to note are
1. The 'sharp' edge of the Sun
2. The dimming of the Sun's brightness as
you look from the center to the edge
The seemingly sharp edge
is caused because most of
the light comes from the
upper 200 Km (0.03%) of
the solar disk
Limb Darkening
Limb darkening is caused by
the opacity:
We can see only into a given
distance; if we look at the
center, then this relates to a
greater depth (and therefore
a higher temperature and
brighter light)
Looking near the limb, we see
a shallower depth, cooler
temperatures and diminished
light
Photospheric Features
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Sunspots
Faculae
Granulation
Supergranulation
Sunspots
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They are dark markings on the
photosphere that can be easily seen.
They measure a few tens of
thousands of kilometers across.
They are usually found in groups of
several to a hundred spots, most of
them small but often dominated by
one or a couple large spots.
They consist of two parts, a dark
umbra and a lighter penumbra.
Sunspots are areas where a
concentrated magnetic field
protrudes through the hot gases of
the photosphere.
The field inhibits convection from
below, making sunspots about 2500
degrees K cooler than the
surrounding area
They are found in pairs, one with N
magnetic polarity, the other with S
polarity
Sunspots
Like the sunspot, squares A and B are exactly
the same color of gray
Sunspot Group
A Bit Closer
Sunspot Cycle
• The sunspot cycle is a periodic event where
the number of sunspots climbs from a
minimum to a maximum in 11 years
• Magnetic Polarity flips and another 11 year
cycle ensues
• Sometimes the minimum is little or none
– Maunder minimum (1640 –1700 AD)
• “Little Ice Age”
Max
Min
It seems that the Sun is not being
very well behaved lately.
The current Solar maximum is
very small - launching a debate
between the Global Warming
advocates and the ‘perhaps a new
little ice age’ advocates.
Sunspots
Spots start a cycle at about 30 degree latitude and move toward the equator as
they form later in the cycle - giving rise to the Butterfly Diagram
This magnetogram of the Sun's disk
was obtained by the MDI instrument
on board SOHO on 27 August 2006.
The magnetogram shows the strength
and sign of the magnetic field in the
Sun's photosphere in greyscale:
White
•strongly positive
•outward directed magnetic field
•north (N)
Black
•strongly negative
•inward directed magnetic field
•south (S)
The sunspots in the active region just below the centre of the Sun's disk have
the reverse orientation compared to the sunspots of the current solar cycle. This
region - no. 905, as assigned by the Space Environment Center - is located in the
southern hemisphere with the white (N) spots leading and the dark (S) spots trailing
with respect to the Sun's rotation.
May be the start of the reversal process.
Solar Rotation
Babcock’s Magnetic Dynamo Theory
Faculae
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Faculae are irregular bright
patches on the solar disk.
Often, sunspots will be seen
to be embedded in faculae,
though free-standing faculae
are also commonly seen as
well.
Faculae are most easily seen
near the solar limb,
They are about ten percent
brighter than the bare
photosphere.
Granulation
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Granulation is the fine-grain
structure of the photosphere.
Individual 'grains' are about
1000 km across.
The granulation is constantly
changing, usually over time
scales of minutes or less.
Each 'grain' is a convective
cell which consists of a bright,
roughly polygonal area of hot
rising gas, and a cooler edge
channel of descending gas.
Supergranulation is a larger scale, about 30000 Km across,
version of the same effect.
Granulation
Solar Structure: Chromosphere
•Plages
•Spicules
•Flares
•Promenences and Filaments
•Coronal Mass Ejections (CMEs)
Plages
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Plages are bright emission
regions in the chromosphere
which surround photospheric
sunspots.
They coincide with faculae in
the photosphere beneath
them.
Plages are irregular in shape
and variable in brightness,
marking areas of nearly
vertical emerging or
reconnecting magnetic field
lines.
Spicules
• Spicules are small jet-like
eruptions.
• They are usually seen as
dark streaks in hydrogenalpha light except at the
limb, where they are seen
as emission features.
• They last only a few
minutes and eject
material into the corona
at speeds up to 30
km/second.
Flares
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Flares are the most violent
eruptions on the Sun. They
occur in the chromosphere and
corona above complex sunspot
groups.
During a flare temperature in
the region rises to 5 million
degrees Kelvin.
Vast quantities of particles and
radiation are released into
space.
The flare lasts no more than
20 minutes.
Flares are the result of
magnetic field stress and are
seen most often where lines of
opposite polarity conflict.
An X Flare – as seen by SOHO
Medium X-Ray Flare
Promenences and Filaments
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Prominences are clouds of
material suspended above the
chromosphere by magnetic field
loops.
When seen against the
chromosphere, they are visible by
absorption and are called
filaments.
When seen near the limb, they can
be seen against the sky as
emission features.
They generally come in two broad
classes: active and quiescent.
– Quiescent prominences occur away
from active regions and last for
many months.
– Active prominences are associated
with sunspots and flares. They
have violent motions, change fast
and last for only a few hours.
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Coronal Mass Ejections occur when the
tops of the prominences snap and eject
material into space
The Solar Limb
The Solar Limb
Solar Structure: Corona
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Scattering of
light from the
photosphere by
electrons
The ionized gas is
due to the
temperature of
2,000,000 K
Most of the light
is in the X-ray
range
Corona
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Coronal holes are large
regions
of exceptionally low
density and
temperature in the
corona.
They last for several
rotations of the Sun
They are the source of
the strongest solar
winds
The corona in X-ray
Solar Structure: Solar Wind
• The flow of coronal gas into space is
called the Solar Wind.
• It moves at a speed of 450 Km/s
which means it moves past the Earth
in about 4 days.
• A denser stream (at 700 Km/s) is
emitted from a coronal hole
Coronal Mass Ejections
Solar Structure: Heliosphere
• The boundary between the region of
space dominated by the Sun’s
magnetic field and interstellar space
is called the Heliopause
– This seems to occur at about 100 AU
• The region within is known as the
heliosphere
Where does the energy come from?
The intensity of sunlight on the Earth is the Solar
Constant = 1400 Watts/meter2
Therefore the Solar Luminosity is 1400 Watts/meter2
times the area of a sphere at 1 AU = 4 x 1026 Watts
Source of Energy
Ideas in the past have included:
– Combustion
• Burning Hydrogen in oxygen produces about 107 Joules/Kg; at the
Sun’s energy output this would last about 50 billion seconds to
consume all the mass of the Sun, or about 2000 years
– Meteor Impact
• Objects falling into the sun impact at about 600 Km/sec
• To match the energy output, about 1/10 of the Earth’s mass would have to
fall in each year (We don’t see this much!)
• The additional mass would increase the gravitational force and cause the
Earth’s year to shorten by about 1 minute/century. (This is easily measured,
but doesn’t happen)
– Gravitational Collapse
• If the sun gets compressed under its own weight, the energy output would
imply that each 1 meter reduction would be about 10 days worth of energy
• This 1 Kilometer reduction every 50 years would be unnoticable
• If the sun began as a large gas cloud, it would have reached its present size
in about 20 million years
• Geological records indicate 3-4 billion years
Source of Energy
Nuclear Fusion
Conversion of Hydrogen to Helium is a
much more efficient process than
combustion
There are two main processes:
– The Proton-Proton Reaction
– The Carbon-Nitrogen-Oxygen Reaction
T < 10,000,000 K
At less than 10 million degrees, there is not enough
energy to force the protons together against the
repulsive electromagentic forces
Proton-Proton Reaction
T > 10,000,000 K
Proton-Proton Reaction
T > 10,000,000 K
Carbon-Nitrogen-Oxygen Reaction
T1/2 =124 s
Carbon-Nitrogen-Oxygen Cycle
T > 16,000,000 K
Diffusion of Energy
Prof. Gerhard H. Jirka
Institute for Hydromechanics
University of Karlsruhe
Diffusion of Energy
Energy produced in 1 sec in the Sun’s
core
Energy reaches the Sun’s surface in
170,000 yrs but is spread out over
100,000 yrs
120,000
Brightness is insensitive to change
170,000
220,000
Solar Neutrinos
• Produced in nuclear reactions, these
particles are so weakly interacting
that the Sun is transparent
• Every second 1016 neutrinos pass
through your body
• Problem: Where’s the neutrinos?
Super Kamokande
Where’s the neutrinos?
Neutrino Oscillations
There are 3 mass neutrinos associated with 3 particles:
electrons, tau mesons and mu mesons
We’ve been looking for the electron neutrinos
Let’s think of the 3 types as colors
electron
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red
tau
blue
mu
green
There is evidence that they can ‘oscillate’ between the 3 colors over
time. When they leave the Sun they are ‘red’, but by the time they
reach Earth they are another color
The penalty? The neutrino must have some mass (just a little)