The Sun
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The Sun is the star around which the Earth and the other planets revolve. It is 
the largest body in the solar system, some 750 times more massive than the 
combined bulk of all other objects. It is the Earth's source of light and heat. 
The Sun is an enormous ball of hydrogen and helium gas about 1.4 million km 
across, nearly 110 times the size of the Earth. Because the Sun is made of gas, 
different parts rotate at different speeds. At the equator, the Sun takes about 
24 days to make one complete spin. The Sun's visible surface, called the 
photosphere, has a temperature of 5,700 C. The gases heat up and become more 
compressed at deeper levels, until the temperature reaches 15 million C deep 
within the Sun's energy producing core. 

The planets are actually immersed in the outermost part of the sun, called the 
solar wind. This is a thin stream of electrically charged particles that flows 
out from the Sun and fills the space between the planets. The solar wind pushes 
back the glowing gases of a comet to create the comet's tail. The solar wind is 
really an extension of the Sun's hot outer atmosphere, called the corona. It 
stretches millions of miles above the solar surface. During a total solar 
eclipse, the corona appears as a strange milky glow around the Sun's darkened 
disk. 

The Sun's photosphere is only about 400 km deep, but it is a swirling, explosive 
mass of hot gases and powerful magnetic fields. Dark, mottled patches called 
sunspots are the photosphere's most familiar features. These are pockets of 
cooler gas sometimes half the temperature of surrounding regions. The number of 
sunspots increases and decreases in an orderly 11 year cycle, with the greater 
number of them meaning a greater potential for violent behavior from the sun. 
Sunspots contain intense magnetic fields that can trigger violent eruptions. 
These outbursts are called solar flares and are sudden energy releases that 
usually last no more than an hour. The energy released in this time by a large 
flare could power a major city for 200 million years! The blast from a large 
flare includes x-rays, ultraviolet and visible light, and waves of subatomic 
particles. These can damage satellites, interrupt radio and TV signals, and 
create beautiful sky glows called the aurora. The aurora occurs when fast-moving 
particles strike atoms in Earth's atmosphere, setting the maglow. 

Sometimes arcs of gas rise high above the Sun's photosphere and eject material 
into the corona. These are called prominences, when they are seen as bright 
arches at the Sun's edge, or filaments when seen as cool, dark threads on the 
bright solar disk. 

Each second, the Sun converts 584 million tons of hydrogen into 580 million tons 
of helium. The remaining four million is converted directly into high-energy 
radiation. Astronomers believe the Sun to be an average-size, middle-age star 
and at its current energy output, its hydrogen fuel will last another five 
billion years.
