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The Sun
© Copyright 2000, Jim Loy
The Sun (on the left) is a G2 star. This means that it emits most of
its light in the visible range of the spectrum, with greenish yellow being
slightly brighter than other colors. It is a main sequence star, meaning that
it is very normal for a star of its mass. The Sun was once called a dwarf star
(a yellow dwarf) (and it still is, on occasion) because it is much smaller than
most visible stars in the sky. But there are vastly more smaller stars out
there, stars which are too dim to see without a telescope (even the very
closest ones, see Known Space). So the Sun is larger
than the average star.
The Sun contains 99.85% of the mass of the Solar System (Sol=Sun, Solar
System=The Sun's System), and 0.5% of its angular momentum. The sun is large
enough to easily contain all of the rest of the objects of the Solar System. It
has about ten times the diameter of Jupiter, the largest planet, and about 100
times the diameter of the Earth (and 1,000,000 times the volume of Earth).
The Sun is composed of hydrogen (75%), helium (23%), and other elements
(2%). It produces energy at its core, by nuclear fusion of hydrogen into
helium. Its temperature is about 16 million Kelvins (about 30 million degrees
Fahrenheit) at the center, and about 6000 K (10,000 degrees Fahrenheit) at the
surface.
The most obvious visible feature on the
more or less uniform surface of the Sun is the sunspots (see the picture at the
top of this article). Galileo is often credited with their discovery, although
they were sometimes seen before the invention of the telescope. Sunspots are
like storms, caused by magnetic fields, which are often larger than the Earth.
They appear dark because they are slightly cooler than the rest of the surface.
On the left, we see a prominence, which erupts out of a sunspot (and loops back
into it). During a solar eclipse (when the moon just blocks our view of the
Sun, seen here on the right), we see the Sun's corona, the outer atmosphere of
the Sun. There is also a general outflow of gas from the sun, called the solar
wind. This increases during maximum sunspot activity.
The Sun rotates, in the same direction as the planets. It rotates faster
at the equator than it does at other latitudes. We can see this by observing
sunspots. Spots near the equator move east to west faster than spots to the
north or south. The poles rotate slowest of all. The equator rotates once in
about 25 days. Latitude 70 degrees (north or south) rotates once in about 35
days.
The number of sunspots follows a cycle (min to max and back to min) of
from 8 to 13 years. The average is 11 years. At a min, few spots are seen for
years. At such time, the Northern Lights (the Corona Borealis) and Southern
Lights (Corona Australis) become rare. These Northern and Southern Lights
result from the capture of gasses of the solar wind by the Earth's magnetic
field. The cycle is actually 22 years, as after 11 years the magnetic polarity
of the spots has reversed. It takes another 11 years for the sunspots to return
to their previous state, polarity and all.
The age of the Sun is estimated to be about 4.6 billion (thousand
million) years. It is about halfway (or less) through its life on the main
sequence. At the end of its main sequence life, it will expand into a red
giant, engulfing the Earth. Eventually, it will explode as a nova (it is not
massive enough to become a supernova), and the remnant (still with most of the
mass of the original Sun) will become a tiny white dwarf (about the size of the
Earth). Eventually, the white dwarf will die and become a black dwarf.
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