|
|
The Sun
|
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 behaviour 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 skyglows called
the aurora. The aurora occurs when fast-moving particles strike atoms
in Earth's atmosphere, setting them aglow.
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.