Download The%Sun - Learn@Illinois

ASTR%100%(%Lecture%20
Topics%for%Today
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How can sunlight tell us the surface
temperature of the Sun?
How does the Sun generate energy?
How does the energy get out of the Sun?
How do we know what is happening inside
the Sun?
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Windows to the Universe Original
The%Sun
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The Sun is a huge, glowing ball at the center of our solar system. The sun provides light, heat, and other
energy to Earth. The sun is made up entirely of gas. Nine planets and their moons, tens of thousands of
asteroids, and trillions of comets revolve around the sun. The sun and all these objects are in the solar system.
Earth travels around the sun at an average distance of about 92,960,000 miles (149,600,000 kilometers) from
it.
The sun's diameter is about 864,000 miles (1.4 million kilometers), approximately 109 times Earth's radius.
The following example may help you picture the relative sizes of the sun and Earth and the distance between
them: Suppose the radius of Earth were the width of an ordinary paper clip. The radius of the sun would be
roughly the height of a desk, and the sun would be about 100 paces from Earth.
The%Sun%has%99.8%%of%the%mass%in%
the%en;re%solar%system!
NASA
The Sun's mass is roughly 2 X 1027 tons
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The sun's mass is roughly 2 X 1027 tons. This number would be written out as a 2 followed by 27 zeros. The
sun is 333,000 times as massive as Earth.
The sun's average density is about 90 pounds per cubic foot (1.4 grams per cubic centimeter). This is about
1.4 times the density of water and less than one-third of Earth's average density. Although at the surface, it is
really a very-low-density gas—3,000 times less dense than the air you breathe. And at the center of the Sun,
the density is 20 times that of solid iron, but it is still a gas!
The Nebraska Astronomy ClassAction Project
The%Sun%is%made%mostly%of%
hydrogen%and%helium%gas
Only 2% of the Sun’s mass is made of
elements other than hydrogen and helium
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For every 1 million atoms of hydrogen in the sun, there are 98,000 atoms of helium, 850 of oxygen, 360 of
carbon, 120 of neon, 110 of nitrogen, 40 of magnesium, 35 of iron, and 35 of silicon. But hydrogen is the
lightest of all elements, and so it accounts for only about 72 percent of the mass. Helium makes up around 26
percent. Everythng else on the period
The%Sun’s%Energy%Output
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Sun’s energy output = 3.8 x 1026 Watts
How much is that?
\<
NASA
Every square millimeter
of the Sun’s surface
radiates more energy
than a 60-watt light bulb!
In one second, the Sun
generates enough
energy to power the U.S.
for 3.5 million years!
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The Sun generates enough energy to power 6 trillion trillion 60-watt light bulbs.
Astronomers%use%the%Kelvin%
temperature%scale
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Same degree
size as Celsius
scale
All thermal
motion ceases
at 0 K absolute zero
Water freezes
at 273 K and
boils at 373 K
Windows to the Universe Original
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Astronomers and physicists express temperatures of the sun and other objects on the Kelvin temperature
scale. Zero degrees Kelvin (written 0 K) is absolute zero (–459.7°F). This is the temperature at which an
object contains no thermal energy that can be extracted.
The Kelvin temperature scale is useful in astronomy because it is based on absolute zero and, consequently,
is related directly to the motion of the particles in an object.
On this scale, the Sun’s surface temperature is 5,800 K. Temperatures in the sun's core reach over 15 million
K.
How%can%sunlight%tell%us%the%
surface%temperature%of%the%Sun?
NASA
The Sun’s photosphere (its “surface”) has a temperature
of about 5,800 K (over 10,000℉)!
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The Sun’s surface is called the “photosphere” - where the Sun’s gas becomes opaque. It is a layer of gas
about 500 km deep. The photosphere has a temperature of about 5,800 K (over 10,000℉), and it is the
source of most of the sunlight received by Earth
Although the photosphere appears to be substantial, it is really a very low-density gas—3,000 times less
dense than the air you breathe. To find gases as dense as the air at Earth’s surface, you would have to
descend about 70,000 km below the photosphere—roughly 10 percent of the way to the Sun’s center.
The Nebraska Astronomy Applet Project
Thermal%Radia;on
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Hot, opaque objects emit thermal radiation,
including stars, hot metal, you…
This light has a spectrum that depends only
on the object’s surface temperature
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A heated iron nail will begin to glow, emitting photons. This is different from a burning process because no
chemical change is involved.
When first heated, the nail glows dimly and is red. As its temperature rises, it gets brighter and glows orange.
At higher temperatures, it gets even brighter and glows yellow.
Note the spectral curve at right. This is a plot of the intensity of the light vs. the wavelength of the light. It
shows the relative brightness of the light at each wavelength. Notice that as the nail heats up, the intensity of
the light (the height of the curve) increases.
What%is%a%spectral%curve?
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We represent the
composition of the
light from an object
with a spectral
curve
Wavelength (or
color) of the light on
the horizontal axis
Intensity (or energy
output per second)
on the vertical axis
Intensity
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The spectral
curve of sunlight
400
500
600
700
Wavelength (nm)
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Two%Proper;es%of%Thermal%
More blue light
Radia;on
than red light,
object appears blue
1. Hotter objects
emit more light
per unit surface
area at all
wavelengths -the 12,000K
object dominates.
2. The hotter an
object is, the
shorter the
wavelength of its Spectral curves for temperatures
maximum output
3000 K, 6000 K, and 12000 K
The Nebraska Astronomy Applet Project
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A) For example, a 12,000 K star emits a lot more light per unit area at every wavelength than a 3000 K star.
B) For example, the peak wavelength of emission for the 12,000 K star is much shorter than that of the 3000 K
star.
Two%Proper;es%of%Thermal%
Radia;on
1. Hotter objects
emit more light
per unit surface
area at all
wavelengths -Zoom-in
the 12,000K
object dominates.
2. The hotter an
object is, the
shorter the
wavelength of its Spectral curves for temperatures
maximum output
3000 K, 6000 K, and 12000 K
The Nebraska Astronomy Applet Project
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A) For example, a 12,000 K star emits a lot more light per unit area at every wavelength than a 3000 K star.
B) For example, the peak wavelength of emission for the 15,000 K star is much shorter than that of the 3000 K
star.
Two%Proper;es%of%Thermal%
red light than
Radia;on More
blue light, object
appears red
1. Hotter objects
emit more light
per unit surface
area at all
wavelengths -Zoom-in
the 12,000K
object dominates.
2. The hotter an
object is, the
shorter the
wavelength of its Spectral curves for temperatures
maximum output
3000 K, 6000 K, and 12000 K
The Nebraska Astronomy Applet Project
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A) For example, a 12,000 K star emits a lot more light per unit area at every wavelength than a 3000 K star.
B) For example, the peak wavelength of emission for the 15,000 K star is much shorter than that of the 3000 K
star.
The%color%of%thermal%radia;on%can%
tell%us%an%object’s%temperature!
USGS
The temperature of a lava flow
can be estimated by observing
its color (about 1500 K)
David M. Jensen
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For thermal radiation, white-hot is hotter than red-hot, and blue-hot is hotter than white-hot!
The%Sun’s%Spectral%Curve
Sch
The Sun’s spectral curve reveals its
surface temperature to be 5,800 K!
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We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide
range of wavelengths.
Thought%Ques;on
Which star is hotter?
A. Capella (yellow)
B. Vega (blue)
C. Antares (red)
Stars, Jim Kaler
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Answer B
We can use an object’s thermal radiation to measure its temperature!
Thought%Ques;on
The graph below shows spectral curves for
stars at different surface temperatures.
Which has the highest surface temperature?
A
B
C
The Nebraska Astronomy Applet Project
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Answer A
How%does%the%Sun%generate%
energy?
NASA
The Sun generates energy from
nuclear reactions in its core
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Nuclear%Energy:%
Fission%vs.%Fusion
Oxford University Press.
Fission:
Fusion:
Big atomic nucleus splits into
smaller pieces
(like in nuclear power plants)
Small atomic nuclei combine
to make a bigger one (Sun,
stars)
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Nuclear power plants on Earth generate energy through nuclear fission reactions that split large
atomic nuclei (like uranium) into less massive fragments.
Stars make energy in nuclear fusion reactions that combine light nuclei (like hydrogen) into heavier
nuclei.
Hydrogen%Fusion
p
p
p
n
p
p
4 1H
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p
n
energy
1 4He
Where does the energy
come from?
4 hydrogens have more
mass than one helium!
‣ Excess mass becomes
energy by E=mc2!
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Nobel Media AB. http://www.nobelprize.org/
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The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. But, making one
helium nucleus releases only a small amount of energy. Atomic masses are very small (one
hydrogen atom is about 1.67x10-24 grams - it would take 270 trillion trillion hydrogen atoms to
weigh one pound at Earth’s surface), and only 0.7% of the mass becomes energy.
To generate 3.8x1026 watts of energy, the sun does ~1038 fusion reactions per second. This
transforms 600 million tons of hydrogen into 596 million tons of helium - and over 4 million tons of
mass into energy - every second!
Fusion%needs%high%temperatures%
and%densi;es
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Hydrogen nuclei are bare protons, which have positive charge. Therefore, they will repel each other
(opposites attract; like charges repel).
If the particles are moving fast enough, they can overcome this repulsion and get close enough for
the strong nuclear force to fuse them into one atomic nucleus. High temperatures are needed to
have such fast moving particles. Temperatures required - above 10 million K!
High densities mean more collisions, so more fusion reactions.
Fusion%needs%high%temperatures%
and%densi;es
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Hydrogen nuclei are bare protons, which have positive charge. Therefore, they will repel each other
(opposites attract; like charges repel).
If the particles are moving fast enough, they can overcome this repulsion and get close enough for
the strong nuclear force to fuse them into one atomic nucleus. High temperatures are needed to
have such fast moving particles. Temperatures required - above 10 million K!
High densities mean more collisions, so more fusion reactions.
Fusion%only%occurs%in%the%Sun’s%
core
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Only the Sun’s core
is hot and dense
enough
Temperature: 15
million K
‣ Density: 20 times
denser than iron!
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Enough hydrogen in
the core for 10 billion
years of fusion
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The Sun’s outer layers will never undergo fusion, but there’s enough hydrogen in the core to last 10 billion
years. Currently the Sun is about halfway through its fuel supply. We have about 5 billion years left!
How%does%the%energy%get%out%of%
the%Sun?
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Near the center, the
temperature is hot
Millions of degrees
‣ Gas is translucent
‣ Energy moves by
radiation (photons)
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Near the surface, the
gas becomes opaque
Energy moves by
convection
‣ Hot gas rises, cooler
gas falls
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Energy transport
inside the Sun
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Photons “bounce” their way out of the radiative zone, but they scatter from the dense particles of gas so often
that an individual photon may take 1,000,000 years to pass through the zone. On average, it takes about 10
trillion trillion bounces to get out of the radiative zone.
The highest level of the solar interior, the convection zone, extends from the radiative zone to the sun's
surface. It makes up about 66 percent of the sun's volume but only slightly more than 2 percent of its mass.
Sun’s%photosphere%shows%granules
NASA
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The photosphere has a mottled appearance. This is because it is made up of dark-edged regions called
granules. Each granule is about 1000 km across - approximately the size of Texas, lasting about 10-20
minutes.
Click for time lapse movie of granulation
Granules%result%from%convec;on
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Energy is flowing
outward from the
Sun’s interior
Convection brings
heat energy to the
Sun’s surface
Motion of the rising/
falling gas in the
photosphere causes
granulation
In convection, energy
is moved by hot
material rising and
cool material falling
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Heat energy flows outward from the Sun’s interior to keep the photosphere hot - 5,800 K!
Convection is the transfer of heat by the actual movement of the warmed matter
Convection brings heat energy to the Sun’s surface
Like a pot of boiling soup on a hot stove, the sun is in constant activity as the heat comes up from below.
Granules%result%from%convec,on
In convection, energy is moved by hot
material rising and cool material falling
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Natural convection occurs due to temperature differences which affect the density, and thus relative buoyancy,
of the fluid. Heavier (more dense) components will fall, while lighter (less dense) components rise, leading to
bulk fluid movement.
The Nebraska Astronomy ClassAction Project
Thought%Ques;on
Hint: Think thermal radiation - which emits more
light, a hotter object or a cooler object?
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Answer A
How%do%we%know%what%is%
happening%inside%the%Sun?
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Fusion produces
more than helium
and energy
It also produces
neutrinos
But, neutrinos are
hard to detect
Almost never interact
with matter
‣ Over 1 trillion flow
through your body
every second!
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Wondigoma
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Matter is almost transparent to neutrinos. On average, it would take a block of lead over a quarter of a lightyear long to stop one.
Neutrinos generated by the Sun’s fusion reactions escape the Sun immediately, not in hundreds of thousands
of years. And the Sun generates ~1038 of them per second! Roughly 100 billion neutrinos pass through every
square centimeter of you every second!
Detecting these neutrinos would be direct evidence of nuclear fusion!
A%neutrino%telescope!
Photo courtesy of SNO
Lawrence Berkeley National Laboratory
The Sudbury Neutrino Telescope in Canada
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The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located 6,800 feet (about 2 km)
underground in Ontario, Canada.
The SNO detector target consisted of 1,000 tonnes (1,102 short tons) of heavy water contained in a
6-metre (20 ft) radius acrylic vessel. The heavy water was viewed by approximately 9,600 detectors
mounted on a geodesic sphere. The cavity housing the detector is the largest man-made
underground cavity in the world.
Certain nuclear reactions can be triggered by a neutrino of the right energy. The SNO detects
neutrinos turning neutrons into protons.
The%Super(
Kamiokande%
neutrino%
telescope!
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Another neutrino detector is the Super-Kamiokande neutrino detector. A tank under a mountain in
Japan holds 50,000 tons of ultra-pure water. Once in a while a solar neutrino runs into a water
atom in the tank, which creates light. The tank is surrounded by over 11,000 photon detectors. This image shows technicians in a boat, inspecting some of the photon detectors. ‣
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The Super-K and
other neutrino
telescopes like it
have detected
neutrinos from solar
fusion
About ~10 neutrinos
per day - out of
trillions of neutrinos
passing through the
telescope
R.Svoboda, U.C. Davis [Super-Kamiokande Collaboration]
Direct%evidence%of%solar%fusion
Only(!) 500 days worth of data
was needed to produce this
"neutrino image" of the Sun
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It was taken not with light, but with neutrinos.
What%have%we%learned?
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The Sun
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Thermal Radiation
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The Sun’s size, composition, and temperature
The Sun generates energy through hydrogen fusion in the core
That energy first moves through the radiative zone of the Sun as light
Then it warms the gas in the convective zone of the Sun and that hot
gas bubbles up to the photosphere, producing granules
Hot objects naturally give off light
The color of that light depends on the temperature of the object
The Sun and other stars light comes from this thermal radiation
Neutrinos
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The fusion in the Sun’s core produces neutrinos
Neutrinos are VERY hard to detect, but we have telescopes that can
detect a few
Through neutrino observations, we can take a direct image of the
core of the Sun!
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