DK Eyewitness - Astronomy
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The corona<br />
The outermost layer of the<br />
Sun’s atmosphere is called<br />
the corona. Even though it<br />
extends millions of miles<br />
into space, it cannot be<br />
seen during the day<br />
because of the brightness<br />
of the blue sky. During a<br />
total eclipse, the corona<br />
appears like a crown<br />
around the Moon. It is<br />
clearly seen in this picture<br />
of a total eclipse over<br />
Mexico in March 1970.<br />
Sun<br />
Moon<br />
Cast shadow<br />
Solar eclipse<br />
A solar eclipse happens when the Moon passes directly<br />
between Earth and the Sun, casting a shadow on the surface<br />
of Earth. From an earthly perspective, it looks as if the Moon<br />
has blocked out the light of the Sun. Total eclipses of the Sun<br />
are very rare in any given location, occurring roughly once<br />
every 360 years in the same place. However, several solar<br />
eclipses may occur each year.<br />
Prominence<br />
Earth<br />
Sunspots<br />
Sunspots are cooler areas on the Sun, where strong magnetic fields<br />
disturb the flow of heat from the core to the photosphere. Typical<br />
sunspots last about a week and are twice as big as Earth. They often<br />
form in pairs or groups. The number of sunspots appearing on the<br />
Sun rises and falls over an 11-year period. This is called the solar<br />
cycle. At sunspot maximum, the Sun also experiences large explosive<br />
eruptions called flares, which<br />
blast streams of particles<br />
into space.<br />
Corona Plotting the sunspots<br />
By observing the changing<br />
position of sunspots, we can<br />
see that the Sun is spinning.<br />
Unlike the planets, however, the whole<br />
mass of the Sun does not spin at the<br />
same rate because it is not solid. The<br />
Sun’s equator takes 25 Earth days to<br />
make one complete rotation. The Sun’s<br />
poles take nearly 30 days to accomplish<br />
the same task. These photographs are a<br />
record of the movements of a large spot<br />
group over 14 days in March/April 1947.<br />
coronal loops<br />
Huge loops of very hot gas surge through the<br />
Sun’s corona, guided by the magnetic field.<br />
These loops are about 30 times larger than<br />
Earth. This picture was taken from space<br />
in extreme ultraviolet light by NASA’s<br />
TRACE satellite, launched in 1998<br />
to study the Sun.<br />
Solar prominence<br />
Astronomers have learned much<br />
about the Sun from solar observatories<br />
operating in space, such as<br />
SOHO (the Solar and Heliospheric<br />
Observatory). This SOHO image<br />
of the Sun shows ultraviolet light<br />
from the chromosphere, a layer<br />
of hot gas above the yellow disk<br />
of the Sun we normally see. A<br />
huge prominence is erupting into<br />
the corona. Prominences like this<br />
usually last a few hours. They can<br />
fall back down or break off and<br />
cause gas to stream into space.<br />
Sometimes, the corona blasts<br />
huge clouds of gas into space.<br />
If one of these coronal mass<br />
ejections reaches Earth, it may<br />
cause a magnetic storm and<br />
trigger an aurora (northern<br />
or southern lights).<br />
facts about the sun<br />
Prominence<br />
Photosphere<br />
Core generating<br />
nuclear energy<br />
Radiative zone<br />
Convective zone<br />
• Equatorial diameter 0.86 million miles/1.4 million km<br />
• Distance from Earth 93 million miles/<br />
149 million km<br />
• Rotational period 25 Earth days<br />
• Volume (Earth = 1) 1,306,000<br />
• Mass (Earth = 1) 333,000<br />
• Density (water = 1) 1.41<br />
• Temperature at surface 9,900°F (5,500°C)<br />
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