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1994-02-18
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SUN
The sun (Sol) is the central body of the solar system and nearest star to
earth. The sun is a representative yellow dwarf. It is the only star whose
surface and outer layers can be examined in detail.
The sun is composed predominantly of hydrogen and helium (relative abundance by
mass approximately 3:1), with about 1% of heavier elements. It generates its
energy by nuclear fusion processes. From the center to the surface of the sun,
the temperature falls from around 15 000 00 K to 6 000 K.
The surface of the sun, i.e., the photosphere, represents the boundary between
the opaque convective zone and the transparent solar atmosphere. A permanent
feature of the photosphere is the granulation, which gives it a mottled
appearance. More striking are the sunspots and their associated faculae.
Immediately above the photosphere is the chromosphere. Between this and the
exceedingly rarified corona is the transition region. The corona itself
extends into the interplanetary medium, where the solar wind carries a stream
of atomic particles to the depths of the solar system.
The sun is thought to possess a w