The document summarizes information about the Sun, including: 1) It is an average star that produces energy through nuclear fusion in its core and contains over 99% of the mass in the Solar System. 2) Significant variations in solar activity like sunspots and solar flares can be seen from day to day and year to year, arising from the bending and twisting of magnetic fields in the Sun's atmosphere. 3) Coronal mass ejections are powerful explosions that can eject large amounts of plasma from the Sun's corona and impact Earth's environment and technology.

1. PRIMARY SCHOOL "VASIL APRILOV" - RUSE
DIVE IN THE SKY

2. The Sun is average star, but without its energy we would not exist.
It contains more than 99.8% of the total mass in the Solar System, with the
planet Jupiter containing most of the rest. The atmosphere of the Sun is
primarily made up of hydrogen (75%) and helium (24%), together with small
amounts of oxygen, carbon, neon and iron.
All the energy from the Sun is produced in the core by nuclear fusion. It takes
about 50 million years for the Sun's energy to make its way out to the surface,
and then another 8 minutes for the light to reach us here on Earth.
Astronomers have calculated that the Sun is around 4,500 million years old
and should continue shining for another 5,000 million years or so before
running out of fuel and undergoing a relatively peaceful death.
The Sun is a very active place, as you can see from the SOHO animation
(see right), and significant variations can be seen from day to day, and year to
year. The activity arises from the bending and twisting of magnetic lines within
the atmosphere of the Sun, which can lead to sunspotsand solar flares.

3. Facts and Figures Sun
Radius 695,000 KM
Mass 1.99 x 1030 kg or 332,830 Earths
Rotation Period 25 Earth days at Equator
Surface gravity 27.9 g
Surface Temperature about 5,500 °C
Core Temperature about 15,000,000 °C
The Sun is a very active place, as you can see from the
SOHO animation (see right), and significant variations can be
seen from day to day, and year to year. The activity arises
from the bending and twisting of magnetic lines within the
atmosphere of the Sun, which can lead to sunspotsand solar
flares.

4. A CME, one type of "solar storm", erupts from
the Sun in January 2002. The actual disc of the
Sun, indicated by the white circle, is hidden in
this view through an instrument called a
coronagraph. The coronagraph creates an
artificial eclipse by blocking the too-bright light
from the Sun's surface, allowing us to view the
Sun's dimmer atmosphere.
Click on image for full size
Images courtesy SOHO (NASA & ESA).
Animation by Windows to the Universe staff .

5. A coronal mass ejection and prominence eruption
observed in white light from the SMM (Solar
Maximum Mission) spacecraft. The time of each
panel increases from left to right. The dashed
inner circle in each panel is the solar radius, the
occulting radius is at 1.6 solar radii.
Click on image for full size
Image courtesy of the High Altitude Observatory,
National Center for Atmospheric Research
(NCAR), Boulder, Colorado.

7. Astronomers use certain wavelengths of light and other electromagnetic
emissions as "windows" into different regions of the Sun. White light with
a wavelength between 400 and 700 nanometers (nm) shows the
photosphere, the visible "surface" of the Sun. Red light "magnetic maps"
reveal the magnetic field at the photosphere. Infrared "light" from helium,
red light from hydrogen, andultraviolet (UV) "light" from calcium all show
features in the chromosphere. The chromosphere is the Sun's lower
atmosphere, where temperatures rise to tens of thousands of kelvins.
Extreme ultraviolet (EUV) radiation from helium at 30.4 nm
wavelengthshow us the transition region between the lower atmosphere
and the much hotter corona (upper atmosphere). High energy EUV and
X-ray emissions come from the corona, where temperatures rise above a
million kelvins. Ultraviolet energy at wavelengths of 17.1, 19.5, and 28.4
nm from iron ions (Fe IX, Fe XII, etc.) show us hotter areas as our view
moves higher and higher above the photosphere. X-rays provide us with
a picture of the upper parts of the corona at temperatures of several
million kelvins.
Original artwork by Windows to the Universe staff (Randy Russell) using
images courtesy of SOHO (NASA and ESA), NCAR/HAO/MLSO, Big
Bear Solar Observatory, and SDO/AIA.

10. The sun is flinging 1 million tons of matter out into space every second! We call this
material solar wind. But just what makes up the solar wind, how dense is it, how fast
does it travel and just how hot is it?
The solar wind is made of Hydrogen (95%) and Helium (4%) and Carbon, Nitrogen,
Oxygen, Neon, Magnesium, Silicon and Iron (~1%). These atoms are all in the form
of positive ions which means they have lost electrons because the temperature is so
hot. So really, solar wind is positive ions and the electrons these ions have lost. We
call this plasma.
At the orbit of the Earth, the solar wind has an average density of about 6 ions/cm3.
This is not very dense at all! Take a look at this picture for comparison with Earth's
atmosphere. So even though the solar wind moves incredibly fast (normally in the
range from 300 to 600 km/s), it wouldn't even ruffle your hair if you were to stand
directly in the midst of a solar wind breeze! But, some explosive events like solar
flares or CME'son the Sun can produce speeds over 1000 km/s!
The solar wind goes all the way past Pluto. Scientists hope the Voyager spacecraft
will reach the end of the solar wind, the heliopause. Scientists sure are interested to
see what the solar wind is like that far out!
The temperature of solar wind plasma around the Earth is about 150,000°K.

12. "Without warning, the relatively calm solar atmosphere can be torn asunder by sudden
outbursts of a scale unknown on Earth. Catastrophic events of incredible
energy...stretch up to halfway across the visible solar surface, suddenly and
unpredictably open up and expel their contents, defying the Sun's enormous gravity."
(Sun, Earth, and Sky by Kenneth R. Lang)
These catastrophic events that the author is speaking about are coronal mass
ejections (CME's).
Coronal mass ejections are explosions in the Sun's corona that spew out solar
particles. A lot of material is thrown out into thesolar wind. Coronal mass ejections can
be dangerous when they hit the Earth.
CME's can seriously disrupt the Earth's environment. Intense radiation from the Sun,
which arrives only 8 minutes after being released, can alter the Earth's outer
atmosphere, disrupting long-distance radio communications. Very energetic particles
pushed along by the shock wave of the CME can endanger astronauts or fry satellite
electronics. These energetic particles arrive at the Earth (or Moon) about an hour later.
The actual coronal mass ejection arrives at the Earth one to four days after the initial
eruption, resulting in strong geomagnetic storms,aurorae and electrical power
blackouts.
Coronal mass ejections will become more and more frequent as we near solar
maximum. CME's, not discovered until the 1970's, are difficult to detect. That is why we
need satellites such as theACE satellite which acts as a spaceweather station while in
orbit. ACE can provide a one-hour advance warning of any geomagnetic storms that
would affect the Earth.

16. The gas in the solar coronais at very high temperatures (typically 1-2 million kelvins
in most regions) so it is almost completely in a plasma state (made up of charged
particles, mostly protons and electrons). Strong magnetic fields thread through the
corona. Where these magnetic lines of force are closed, the magnetic field is strong
enough to trap the solar plasma and keep it from escaping. Plasma accumulates in
these regions and forms the beautiful structures call helmet streamers seen during
solar eclipses. Prominences are often situated beneath helmet streamers, and
active regions occur beneath streamers near the equator (sometimes called active
regions streamers). In some regions, the coronal magnetic field cannot confine the
plasma, and the plasma expands outward, reaching supersonic velocities. Regions
on the Sun with these open magnetic field lines (which stretch far out into the solar
system) correspond to coronal holes and are the source of the solar wind, which
accelerates outward from the Sun and fills interplanetary space. The electrons in
the coronal hole plasma are typically cooler and less dense than streamers, and so
they show up as dark regions in both X-rays and white light.
Scientists try to understand the Sun (and other things as well) by developing
mathematical models. Frequently the equations that represent the solar plasma are
so complicated a computer must be used to solve them. The magnetic field lines on
the left image are from a computer simulation that solved the
magnetohydrodynamic (MHD) equations, which give a good representation of
many types of plasma behavior.

17. You may not know that humans have observed sunspots for a very long
time. These records have been around so long in fact, that we can link
sunspot number with solar activity. Large sunspotscan sometimes be seen
with just your eye, especially when the Sun is viewed through fog near the
horizon at sunrise or sunset. (WARNING: Never look directly at the Sun!
Even a brief glance can damage your eyes!)
The first written record of sunspots was made by Chinese astronomers
around 800 B.C. Court astrologers in ancient China and Korea, who
believed sunspots foretold important events, kept records off and on of
sunspots for hundred of years. An English monk named John of Worcester
made the first drawing of sunspots in December 1128.
Soon after the invention of the telescope, several astronomers used the
telescope to make observations of sunspots. This was around 1600.
Astronomers of that time weren't quite sure what to make of these spots on
the Sun. Some thought they were shadows of undiscovered planets
crossing the Sun, while others believed they were dark clouds in the Sun's
atmosphere. The movement of sunspots across the face of the Sun
allowed astronomers in the early 1600's to make the first estimates of the
Sun's rotation period (about 27 days).

18. In 1843 an amateur German astronomer named Samuel Schwabe discovered
the rise and fall of yearly sunspot counts we now call the sunspot cycle. He
first guessed the cycle's length at 10 years. Two French physicists, Louis
Fizeau and Léon Foucault, took the first photo of the Sun and sunspots in April
1845. Around 1852 four astronomers noted that the period of the sunspot
cycle was identical to the period of changes of geomagnetic activity at Earth,
giving birth to the study of Sun-Earth connections we now call "space
weather".
It would appear that sunspots not only have a connection to geomagnetic
activity at Earth, but they play a role in climate change as well. In the last
thousands of years, there have been many periods where there were not
many sunspots found on the Sun. The most famous is a period from about
1645 to 1715, called the Maunder Minimum. This period corresponds to the
middle of a series of exceptionally cold winters throughout Europe known as
theLittle Ice Age. Scientists still debate whether decreased solar activity
helped cause the Little Ice Age, or if the cold snap happen to occur around the
same time as the Maunder Minimum. In contrast, a period called the Medieval
Maximum, which lasted from 1100 to 1250, apparently had higher levels of
sunspots and associated solar activity. This time coincides (at least partially)
with a period of warmer climates on Earth called the Medieval Warm Period.
Sunspot counts have been higher than usual since around 1900, which has
led some scientists to call the time we are in now the Modern Maximum.

19. Magnetic field lines from a computer
simulation of the solar corona show
some of the complexity of the Sun's
magnetic field. Colors on the Sun's
surface show the strength of the
magnetic field (yellow is largest).

21. On average, people in the
U.S.A. are exposed to about
3.6 milliSieverts of radiation
each year. Windows to the
Universe original artwork using
data from the National Council
on Radiation Protection and
Measurements.

22. Radiation comes in two basic types: electromagnetic radiation
transmitted by photons, and particle radiationconsisting of
electrons, protons, alpha particles, and so forth.
Electromagnetic radiation, transmitted as photons, includes
everything from relatively benign radio waves to dangerous and
powerful X-rays and gamma rays. Energy levels across the
electromagnetic spectrum vary inversely with wavelength.
Particle radiation involves fast-moving sub-atomic particles, such
as electrons, protons, and nuclei (ions) of Helium and heavier
elements.
Exposure to too much radiation of various types can beharmful to
humans and other living things. Radiation can also fry the
electronics in spacecraft, disabling them.

24. The Sun is not a quiet place, but one that exhibits sudden releases of
energy. One of the most frequently observed events are solar flares:
sudden, localized, transient increases in brightness that occur in active
regions near sunspots. They are usually most easily seen in H-alpha and
X-rays, but may have effects in the entire elecromagnetic spectrum. The X-
ray brightness from a large flare often exceeds the X-ray output from the
rest of the Sun. Another type of event, the coronal mass ejection, typically
disrupt helmet streamers in the solar corona. As much as 1e13
(10,000,000,000,000) kilograms of material can be ejected into the solar
wind. Coronal mass ejections propagate out in the solar wind, where they
may encounter the Earth and influence geomagnetic activity. Coronal mass
ejections are often (but not always) accompanied by prominence
eruptions, where the cool, dense prominence material also erupts outward.
All of these forms of solar activity are believed to be driven by energy
release from the solar magnetic field. How this energy release occurs, and
the relationship between different types of solar activity, is one of the many
puzzles facing solar physicists today. The amount of solar activity on the
Sun is not constant, and is closely related to the typical number of
sunspots that are visible. The number of sunspots and the levels of solar
activity vary with an 11 year period known as the solar cycle.

25. An eclipse of the Sun occurs when the Earth passes through the
Moon's shadow. A total eclipse of the Sun takes place only during a
new moon, when the Moon is directly between the Sun and the
Earth.
When a total eclipse does occur, the Moon's shadow covers only a
small portion of the Earth, where the eclipse is visible. As the Moon
moves in its orbit, the position of the shadow changes, so total solar
eclipses usually only last a minute or two in a given location.
In ancient times, people were frightened by solar eclipses (even
back then people realized that the Sun was essential to life on
Earth). Now eclipses are of great interest to the public and to
astronomers. Eclipses provide an opportunity to view the Sun's
outer atmosphere, the solar corona.
If you ever get to view a solar eclipse, make sure to never look at
the Sun directly! Always use one of these safe techniques.

26. The Sun is flinging 1 million tons of matter out into space every
second! We call this material solar wind. Once the solar wind is blown
into space, the particles travel at supersonic speeds of 200-800
km/sec! These particles travel all the way past Pluto and do not slow
down until they reach the termination shock within the heliosphere.
The Heliosphere is the entire region of space influenced by the Sun.
The solar wind plasma is very thin. Near the Earth, the plasma is only
about 6 particles per cubic centimeter. So, even though the wind
travels SUPER fast, it wouldn't even ruffle your hair if you were to
stand in it because it's so thin! But, it is responsible for such unusual
things as:
• auroral lights
• fueling magnetospheric storms
• forming a planet's magnetosphere
The particles of the solar wind, and the Sun's magnetic field (IMF) are
stuck together, therefore the solar wind carries the IMF (interplanetary
magnetic field) with it into space.
Instruments like SWICS and SWOOPS onboard the Ulysses probe are
studying solar wind. They are hoping to make a 3-D map of solar wind
characteristics throughout the heliosphere.

30. Sunspots are dark, planet-sized regions that appear on the "surface" of the Sun.
Sunspots are "dark" because they are colder than the areas around them. A large
sunspot might have a temperature of about 4,000 K (about 3,700° C or 6,700° F).
This is much lower than the 5,800 K (about 5,500° C or 10,000° F) temperature of
the bright photosphere that surrounds the sunspots.
Sunspots are only dark in contrast to the bright face of the Sun. If you could cut an
average sunspot out of the Sunand place it in the night sky, it would be about as
bright as a full moon. Sunspots have a lighter outer section called the penumbra,
and a darker middle region named the umbra.
Sunspots are caused by the Sun's magnetic field welling up to the photosphere, the
Sun's visible "surface". The powerful magnetic fields around sunspots produce
active regions on the Sun, which often lead to solar flares andCoronal Mass
Ejections (CMEs). The solar activity of flares and CMEs are called "solar storms".
Sunspots form over periods lasting from days to weeks, and can last for weeks or
even months. The average number of spots that can be seen on the face of the
Sun is not always the same, but goes up and down in a cycle.Historical records of
sunspot counts show that this sunspot cycle has an average period of about eleven
years.
Our Sun isn't the only star with spots. Just recently, astronomers have been able to
detect "starspots" - "sunspots" on other stars.

32. Rising above the Sun's chromosphere , the temperature jumps
sharply from a few tens of thousands of kelvins to as much as a few
million kelvins in the Sun's outer atmosphere, the solar corona.
Understanding the reason the Sun's corona is so hot is one of the
many challenges facing solar physicists today. Because of the very
high temperatures, the corona emits high energy radiation and can
be observed in X-rays. The Earth's atmosphere absorbs X-rays, but
satellites above the atmosphere, such as the Yohkoh spacecraft, can
observe the Sun in these wavelengths. Shown on the left is a
blending of a Yohkoh X-ray image (reddish colors) with an eclipse
image taken by the High Altitude Observatory (gray-white colors) on
November 3, 1994. Near the poles of the Sun, the corona is dark for
both X-rays and white light. These regions are coronal holes and are
the source of the solar wind that extends out into interplanetary
space. The scattered white light shows the density of plasma in the
corona. The large white regions extending out far from the Sun are
helmet streamers, where the solar plasma has been trapped by the
Sun's magnetic field.

38. http://www.astro.bas.bg/sun/links_bg.html
http://portal.opendiscoveryspace.eu/search/site/sunspots
http://www.windows2universe.org/space_weather/sw_today/sun_tod
ay.html
http://www.crh.noaa.gov/fsd/?n=sunspots
http://www.windows2universe.org/sun/atmosphere/sunspots.html