Download Garden-Variety Star - Wayne State University Physics and Astronomy

The Sun:
A Garden -Variety Star
1 March 2005
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The Sun
Biggest object in the
solar system
diameter 1,392,000 km
109 x Earth’s diameter
10 x Jupiter’s diameter
Most Massive
333,000 x Earth’s mass
1,000 x Jupiter’s mass
so heavy, everything else orbits around it!
so heavy, it makes its own heat and light
Temperature of 15,000,000 K in its core
nuclear power!
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The Sun’s Profile
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The Sun’s Composition
The Sun contains the
same elements as the
Earth, but not in the
same proportions
About 73% of the Sun’s
mass is comes from
hydrogen, and another
25% from helium
Other chemical elements
make up the rest 2%
The fact that the Sun are
mostly made up of H and
He was first shown by
Cecilia Payne-Gaposchkin
1st
The
woman to get a PhD in
astronomy in the U.S.
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Elements
Fraction
Hydrogen
92.1%
Helium
7.8%
Oxygen
0.061%
Carbon
0.030%
Nitrogen
0.0084%
Neon
0.0076%
Iron
0.0037%
Silicon
0.0031%
Magnesium
0.0024%
Sulfur
0.0015%
All others
0.0015%
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The parts of the Sun
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The Sun's Interior
From inside
out:
Core
Radiative zone
Convection
zone
Photosphere
Chromosphere
Transition
region
Corona
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The Sun's Core
The core
is the innermost 10% of
the Sun's mass
generates energy from
nuclear fusion
has the highest
temperature and
density
temperature 10 million K
density = 160 x density of
water = 20 x density of
iron
at this temperature, the
core is a gas
no molten interior
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How does Heat from the Core Reach Us?
Three ways to transfer heat:
Conduction: direct contact
A spoon in a hot cup of coffee gets warm
Convection: moving currents in a
fluid
Hot air rises
Hot current in boiling water
Radiation: electromagnetic waves
emitted by a heat source and
absorbed by a cooler material
Electric stove
Heat from the Sun reaches us
through the EM waves it emits
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Radiative Zone
Radiation transfers heat from the interior of the Sun to its
"cooler" outer layers
The core & radiation zone make up 85% of the Sun
The temperature drops from 10 million K at the inner side
of the radiative zone to 2 million K at its edge
The energy generated in the core is carried by photons that
bounce from particle to particle through the radiative zone
The photons are too energetic to
be absorbed by atoms
Each photon bounces so many
times that it is estimated to take
one million years to reach the
outer edge of the region
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Convection Zone
Matter at the base of the convection zone is “cool” enough (2
million K) for the atoms to absorb energy and hold on to it
Convection occurs in this region
The hotter material near the top of the radiation zone (the
bottom of the convection zone) rises while the cooler material
sinks — heated below like a pot of boiling water
It takes a week for the hot material to carry its energy to the top
of the convection zone
cool
convection zone
hot
radiative zone
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Photosphere
This is the Sun’s deepest layer that one
can see from the outside
Photosphere means “light sphere”
It is the visible “surface” of the Sun
From this layer, photons can finally escape to
space
The surface is not something one could land
or float on
The photosphere is about 500 km thick
The gas is so dense that you could not see
through it
The gas emits a continuous spectrum of light
It features sunspots
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Temperature of Photosphere
The photosphere temperature is about 5,800 K
The sunspots appear darker because they are cooler
than their surroundings
The center of a typical sunspot has a temperature of 4,000 K
The spectrum and energy output of the radiation
emitted from the photosphere obey Wien’s Law and
Stefan-Boltzmann law
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Features of Photosphere
Sunspots
dark spots, 1500 K,
cooler than
surroundings
glow by themselves
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granules
tops of convection cells
700 to 1000 km diameter
last 10 minutes
centers ~ 100 K hotter than
edges
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Sunspots
Discovered by Galileo Galilei
Sun's surface sprinkled with small dark
regions - sunspots
Sunspots are darker because they are
cooler by 1000 to 1500 K than the rest of
the photosphere
Spots can last a few days or as long as a
few months
Galileo used the longer-lasting sunspots to
map the rotation patterns of the Sun
Sunspots number varies in a cycle with an
average period of 11 years
Cycle starts with minimum and most of
them are at around 35° from the solar equator
At solar maximum (number peaked), about 5.5
years later, most of the sunspots are within just
5° of the solar equator
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Sunspots and Magnetic Field
Sunspots = regions of strong magnetic
fields
Found by observation of Zeeman effect
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Sun Rotates
Galileo
discovered sunspots
sunspots moved  sun rotates
Rotation – speed depends on
latitude
equator once/25 days
30º N once/26.5 days
60º N once/30 days
Jupiter also does this
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Chromosphere
Visible during solar eclipses
as a thin pink layer at the
edge of the dark Moon
Colorful layer – “color sphere”
Color due to hydrogen bright
emission line
Also shows yellow emission due
to helium – discovered in 1868 –
new element previously not seen
on Earth
Helium was found on Earth in
1895
The chromosphere is only
2,000 to 3,000 km thick
Temperature rises outward
away from the photosphere –
from 4,500 K to 10,000 K
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Transition Region
It’s a thin region
(about 10 km thick)
in the Sun’s
atmosphere where
temperature changes
from 10,000 K to
nearly 1,000,000 K
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Solar Weather
The Sun has complex
and violent weather
patterns
Chromosphere contains
jet-like spikes of gas –
called spicules
Spicules rise vertically
through the
chromosphere
Last 10 minutes
Consist of gas jets, at
30 km/s
Rise to heights of 5000
to 20000 km
T ~ chromosphere
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Corona (1)
The outermost part of the Sun’s
atmosphere is called the corona
It is visible during total solar
eclipses as a pearly-white glow
around the dark Moon
Total solar eclipse in 1973
The corona has a very high
temperature of ~1-2 million K
It is known to be very hot because it contains multiply
ionized atoms
At very high temperatures, atoms like iron can have 9 to 13
electrons ejected (the atoms become ionized)
9-times ionized iron is only produced at a temperature of 1.3
million K
13-times ionized iron means the temperature gets up to 2.3
million K!
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Corona (2)
Most of the corona is trapped
close to Sun by loops of
magnetic field lines
In X-rays, those regions
appear bright
Some magnetic field lines
do not loop back to the Sun
and will appear dark in X-rays
These are called coronal holes
More details visible at
short wavelengths
A solar eclipse photographed in the extreme ultraviolet
taken by the SOHO spacecraft
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X-rays from the Corona
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Prominences
Bright clouds of gas
forming above the
sunspots
Quiet prominences
40,000 km above surface
Last days to several
weeks
Eruptive prominences
700 km/s
Rare
Surge prominences
Last up to a few hours
Shoot gas up to 300,000
km
Gas speed ~1300 km/s
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Prominences follow
magnetic-field loops
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Solar Flares
Solar flares are eruptions more powerful than surge
prominences
Flares last from a few minutes to a few hours
A lot of ionized material is ejected in a flare
Unlike the material in prominences, the solar-flare
material moves with enough energy to escape the
Sun's gravity
When such a burst of ions reaches the Earth, it
interferes with radio communication
Sometimes a solar flare will cause voltage pulses or
surges in power and telephone lines
Brownouts or blackouts may result
Humans traveling outside the protection of the Earth's
magnetic field will need to have shielding from the
powerful ions in a flare
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Solar Wind
Fast-moving charged particles (mostly protons and
electrons) can escape the Sun's gravitational
attraction
The stream of particles is called the solar wind
They move outward at a speed of about 400 km/s
They can reach the farthest reaches of the solar system
Solar-wind particles passing close to a planet with a
magnetic field are deflected around the planet
Some are deflected to the planet's magnetic poles
As the particles hit the planet's atmosphere, they cause
the molecules in the atmosphere to produce beautiful
curtains of light called the auroras
Aurora borealis in the northern hemisphere
Aurora australis in the southern hemisphere
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Is the Sun a Variable Star?
What is more certain than that the Sun will
rise tomorrow?
We’ve already seen that sunspots follow an
11-year cycle
On longer time scales, the Sun undergoes
changes in overall activity
Changes are only about 0.1%!
Yet this is enough to affect our climate
In the mid 1600’s the Sun’s output was
particularly low  the “Little Ice Age”
Other stars are seen to vary by 0.3%, up to
1%
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