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Our Solar System: The Sun
The Sun is the closest star to Earth and is the main supply of energy for sustaining life on this planet. Research
on our closest star allows scientists to learn more about how solar system began, about Earth's own atmosphere,
and about other stars outside of our solar
system.
The Sun is a star and, much
can be categorized
one cannot go and take
scientists use the color (or
indication of the surface
The classes of stars are named O,B,A,F,G,K,
O stars are the hottest (>25000 Kelvin) and
coolest (<3500 Kelvin) and appear red. O and
bright and M stars are numerous but dim. The
middle of this temperature range and is
It is a medium sized yellow star with a surface
temperature of 5800 Kelvin.
and M. In this odd sequence
appear blue and M stars the
B stars are rare but very
Earth's sun sits right in the
designated as a G class star.
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Sun Facts and Fun Comparisons
like any other star in the Milky Way Galaxy,
according to its surface temperature. Of course
direct measurements from the star's surface so
spectral quality) of the light they give off as an
temperature.
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Out of the 100 billion stars in the Milky Way Galaxy, our Sun is but one. What
makes this star so special to the inhabitants of Earth is its proximity (nearness) to
our planet. The Sun is a mere 149,680,000 kilometers (93,026,724 miles) from
Earth. This distance is equivalent to 8 light minutes, where one light year is the
distance light can travel in one year or 9,500,000,000,000 kilometers. The next
closest star, Proxima Centauri, is 4.24 light years, much too far for its rays to
have any effect on Earth. In fact, it is even too dim to be viewed without a
telescope! For more information on stellar distances see NASA's StarChild Web
site
Here are some more Sun facts for you to know:
SUN FACTS
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Radius - 695,000 km or 108.97 that of Earth
Mass -1.989e30 kg or 332,830 times that of Earth
Temperature at surface - 5800 Kelvin
Temp in core - 15,600,000 Kelvin
EARTH FACTS
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Radius - 6378 km
Mass 5.98e24 kg
Temperature at surface - 281 K
Temp at core - 7000 K
More Sun Facts can be found at
The Stanford Solar Center Web site
The Structure of the Sun
The Sun is made mostly of a mixture of hydrogen and helium gases. Presently this mixture consists of ABOUT
75% hydrogen and 25% helium by mass. The immense gravity of the Sun holds these gases together. The Sun
exists in a state of matter called plasma. Plasma is considered the 4th state of matter beyond solid, liquid, and
gas. In this state, the gas is so hot that some or all of the atoms are separated into positively and negatively
charged ions. These charged particles are free to move and contribute to the formation of the Sun's magnetic
and electric fields.
Because the Sun is gaseous it does not have a single uniform rotation velocity. It rotates differentially, meaning
that it rotates faster at its equatorial regions (once per 24.5 days) than at the poles (once per 36 days). You can
watch an animation of the Sun rotating over a ten-day period at this NASA Science Education site.
Astronomers describe the structure of our sun by layers based on the differences in composition, activity and
temperature. From inner to outer most, these layers are the core, radiative zone, convective zone, photosphere,
chromosphere, and corona.
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For an image of the sun's layers for imageclick here. Core: The core of the Sun starts from the center and
extends to 25% of the Sun's radius. This region has a temperature of 15.6 million K and an atmospheric pressure
250 billion times greater than on the surface of the earth. The temperature and pressure are so intense that 2
atoms of hydrogen can be forced together to form 1 atom of helium in a reaction called nuclear fusion. The
energy of this fusion reaction is emitted as ultraviolet light, x-rays, visible light, infrared, microwaves, and radio
waves. In addition, energized particles are also emitted as solar wind. These forms of energy contribute to the
heat and light that sustain life on our own earth.
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Radiative Zone: The radiative zone extends beyond the Sun's core layer, for about another 55% of the Sun's
radius. Energy from the nuclear fusion reactions in the core is carried through the plasma of the radiative zone
by the sequential absorption and reemission of photons, or energy packets, by gas molecules. This long relay of
events transfers the energy to the upper layers of the Sun. The loss of energy in these transfers causes a gradient
of heat as one travels away from the core but the average temperature of this layer is 4 million Kelvin.
Convective Zone: The convective zone extends from the radiative zone through the final 20% of the Sun's
radius. As its name suggests this region carries the Sun's energy to the surface by means of convective currents,
currents that flow between two regions of differing temperature. As the hot gases rise from the center of the Sun
they loose heat, eventually cooling enough to cause falling currents of cool gas. Once the gas reaches the
radiative zone it takes on enough energy in the form of heat to cause it to rise again.
Photosphere: The surface of the Sun is marked by the photosphere. This layer is approximately 300-400
kilometers deep and is the layer that is most evident to observers on Earth. (Warning and Reminder! One should
never look directly at the Sun!) The photosphere has an average temperature of 5800 Kelvin and looks grainy
due to the convection currents bubbling up from the lower layers.
Chromosphere: Above the photosphere lies the chromosphere, about 2000 km deep. The temperature in the
layer makes an unexpected rise from 4800 K at the photosphere to 10,000 K at the outermost reaches of this
layer. Though the chromosphere lies above the photosphere it is obscured by the intense emission of light. The
red glow of the chromosphere can only be seen when this light is blocked during a total solar eclipse.
Corona: The final layer of the Sun extends millions of kilometers outward from the photosphere. The
temperature in this immense region averages at an amazing 2 million Kelvin. The exact mechanism for this
coronal heating is currently still a puzzle to astronomers and physicists. The coronal region is much less dense
than the other layers of the Sun and is therefore masked by the light emitted at the photosphere. As with the
chromosphere, the corona is only easily evident during times of solar eclipse.
The Sun's Energy
The basis for the activity of the Sun and the energy that it releases comes from the nuclear fusion reactions in
the core layer. In these reactions 700,000,000 tons of hydrogen are converted to 695,000,000 tons of helium and
5,000,000 tons of energy equivalent to 386 billion megawatts of energy. All of this energy does not reach the
earth - thankfully, as it travels it is constantly absorbed and re-emitted, losing a bit of energy each time. It
reaches the Earth's surface as visible light and ultraviolet light rays that can be harmful if too concentrated.
Sun Fact: Each square centimeter of the Sun emits as much light as a 6000 watt lamp
Another source of energy comes from the magnetic fields that are intimately related to the electronic fields
produced by the charged particles within the Sun's plasma. A magnetic field is produced by the flow of
electrically charged ions and electrons. On the Sun these currents are generated by the flow of the Sun's hot,
ionized gases. The magnetic field of the Sun is very strong and extends beyond the reaches of Pluto - so we feel
it too!
The Sun's Activity
The magnetic fields of the Sun are responsible for much of the solar activity that we see and experience. These
activities include sunspots, solar flares, and the solar wind.
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Sunspots occur in regions of cooler temperature on the Sun's surface and can be observed as darker regions on
the photosphere. It is thought that these regions are related to the variations of solar magnetic field. Sunspots
can be as large as 50,000 km in diameter.
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In the 1600s Galileo observed sunspots while viewing the Sun through his telescope. By the mid 1700's the
number of sunspots was being recorded daily from the observatory in Zurich. Since that time many new
advances in sunspot research have allowed us to keep an almost continual record of this solar phenomenon.
During the 17th century the sunspot activity was at a low - called the Maunder Minimum. This time coincides
with the Little Ice Age of northern Europe. Further correlations between solar activity, as measured through
sunspot number, and the climate on Earth are still being investigated.
Solar Flares appear as explosions from the Sun and are thought to be caused by magnetic field changes in areas
of high field strength. The occurrence of solar flares causes the release of gas, electrons, light, and x-rays. On
earth the effect of these flares is noted when the flare energy produces interference with power grids, causing
black outs of our home electricity. An excellent illustration of solar flares can be found at on the Virtual Tour of
the Sun
The solar wind is a low-density stream of charged particles (electrons and protons) that are emitted from the
Sun. The solar wind moves at a velocity of 450 km/sec. These highly energetic solar winds can affect Earth as
power surges in power lines, radio wave interference and the aurora borealis (the northern and southern lights).
How Long Will the Sun Last?
The Sun is the most important source of energy for life on Earth. Unfortunately it is not an infinite source. For
every time that helium is created there is a loss of the Sun's supply of hydrogen, its own fuel for nuclear fusion.
Scientists have estimated that over the past 4.5 billion years the Sun has used up half of its hydrogen source!
When it finally does run out of hydrogen fuel it will create radical changes leading to the destruction of planets
within the solar system, including Earth. But don't worry we have at least another 4.5 billion years of the Sun's
energy to bask in!
Learn More!
General Resources about The Sun
Online Resources
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Star Child
A learning center for young astronomers created by teachers for the NASA team.
Stanford Solar Center
This site presents a number of fun educational activities based on data from Stanford University and
NASA research projects.
Space Weather.com
Science news and information about the Sun-Earth environment
SOHO - Exploring the Sun
Web site that covers the progress of the SOHO (Solar & Heliospheric Observatory) project and provides
educational resources for many levels.
A Virtual Tour of the Sun
A multimedia presentation about the sun and solar activity.
Solar Physics
An educational Web site from the Marshall Space Flight Center Solar Group
The Nine Planets - The Sun
A clearly written summary of Sun facts followed by an excellent list of links for further research.
The Sun Has Spots
Background information for middle and high school teachers about sunspots.
How Stuff Works - How the Sun Works
Great in-depth explanations of solar activity and the structure of the Sun.
Zoom Astronomy's - The Sun
An easy to read and understand Web site about the Sun. Take the Sun quiz at the end!
Classification of solar spectra
A good site for advanced students to practice interpretation of graphs.
Astronomy in Motion Sun pages
Many great activities for exploring the sun!
Understanding Plasma and the Sun - the fourth state of matter.
A detailed definition of Plasma
Tour the Sun
The Yohkoh Movie Theater (YPOP) is designed to bring you images and movies depicting our nearest
star, the Sun, as seen by an X-ray telescope on board the Yohkoh satellite. Check out the Solar
Classroom!
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Books:
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Miller, Edward. The Sun: Our Nearest Star (Let's-Read-and-Find-Out Science). Harpercollins Juvenile
Books. ISBN 0060285346 (K-2)
Gallant, Roy. When the Sun Dies. Marshall Cavendish Corp/Ccb. ISBN 076145036X
Fowler, Allan. The Sun Is Always Shining Somewhere (Rookie Read-About Science Series). Children's
Press. ISBN 0516449060
Hillerman, Anne. Done in the Sun: Solar Projects for Children. Sunstone Press. ISBN 0865340188
LESSON 1:
Vocabulary of the Sun
Concepts:
You will learn about vocabulary of the sun.
Lesson:
Try the following activities with The Sun Vocabulary Worksheet. Depending on age and ability, you may be
able to complete assignments from multiple grade levels.
1-2: Copy the following words in alphabetical order, and describe each of the words verbally. Practice spelling.
3-5: Look up each word in a dictionary, and write down their definitions. Write each word in a sentence, or
write a paragraph using the words. Practice Spelling.*
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Views of the Sun
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Additional Resources:
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Find the definitions here
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6-8: Write a description or review about this topic using the vocabulary words. Above each vocabulary word,
write N if it is a noun, V if it is a verb; ADJ if it is an adjective; ADV if it is an adverb.
LESSON 2:
How do we Study the Sun
Concepts:
You will learn about technologies used to study the Sun.
Lesson:
You are a newspaper reporter writing an article for a science column. You have decided to interview one of the
leading solar scientists. To prepare, you start by learning about the techniques commonly used to study the Sun.
Here are some Web sites to get you started.
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Solar Physics
SOHO -Exploring the Sun
Yohkoh Public Outreach Project
Make a list of techniques used to study the Sun and write a short description of each technique including any
major discoveries. Here are some key instruments often used by scientist that you may want to look for.
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Telescope
Magnetoscope
Solar Spectrameter
Additional Resources:
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The Galileo Challenge
Research online information about Galileo and answer the online quiz questions.
LESSON 3:
Layers of the Sun
Concepts:
You will learn about the layers of the Sun and the characteristics of each layer.
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Lesson:
Make your own sun diagram using the information from the unit text and additional information from books
and Web sites. Make sure you include the following features in your diagram.
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Core, Radiative Zone, Convective Zone, Photosphere, Chromosphere, Corona, Sunspots, Solar Flares
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Add text to describe the features you have drawn.
Activity for younger students:
Print out one of the online diagrams of the Sun and color in the different parts of the Sun.
Additional Resources:
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From Core to Corona
Cut-away diagram of the Sun
LESSON 4:
History of the Sunspot
Concepts:
You will learn about the history and techniques of sunspot research.
Lesson:
Use the Web sites listed with this lesson to learn how Galileo first observed sunspots. Outline the history of
sunspot observation and recordings since that time. Discuss what use this information is to scientists today.