As the sun rushes through space at a speed of 150 miles per second, it takes
many smaller bodies along with it. The sun and its smaller companions together
are known as the solar system Together, these bodies are making a revolution
around the Milky Way that takes 225 million years. These other members of the
solar system range in size from the giant planet Jupiter to microscopic particles
called micrometeorites and even smaller particles -- atoms and molecules of
the interplanetary gar Earth is one of the largest bodies of the solar system,
although it is quite small when compared with the sun or Jupiter.
Astronomers do not know exactly how far out the solar system extends. When
it is at its farthest point from the sun (aphelion), some 4 and half billion
miles, Pluto is the most distant known planet. Many comets, however, have orbits
that take them even farther out, up to several hundred times the distance of
Flute Even at that distance the sun's gravitational force dominates and can
bring the comet back Some hundred billion comets form a tenuous halo in the
outer parts of the solar system. Each is like a giant snowball, 1,000 to 10,000
feet in diameter.
The Sun is the central member of the solar system. Its gravitational force
holds the other members in orbit and governs their motions. It far outweighs
all other components of the solar system combined. In fact the run contains
more than 99 percent of the mass of the entire solar system.
The sun is, however, only an average-sized star. If it were as far away from
Earth as most stars are, it would look no larger or brighter than its neighbors.
But since it is by far the nearest star and the only star whose surface details
may be observed, it is also one of the major sources of information that scientists
have about how stars behave.
The sun provides nearly all the heat and light and other forms of energy necessary
for life on our planet In fact, the sun provider virtually all the energy of
the solar system. Its gravitational attraction governs the motions (or kinetic
energy) of the planets and other bodies. Radiation from its surface bathes the
planets in all the electromagnetic radiation they receive, with some minor exceptions.
These exceptions include the faint light from stars, the disintegration of radioactive
materials on the planets, emissions of long-wave radiation by the planet Jupiter,
and the radio waves and X rays from remote space.
Although the sun is a rather ordinary star, it is very important to the inhabitants
of the Earth. The sun is the source of virtually all of the Earth's energy.
Yet the Earth receives only half a billionth of the energy that leaves the sun.
Because the sun's energy is so intense, there are some real dangers in studying
it. The intense heat of the sun's rays can destroy the retinal cells, causing
blindness. For this reason, the sun should never be viewed directly. Furthermore,
there is no safe way to view the sun through an ordinary telescope. Smoked glass
and dark glasses give no protection from the great concentration of heat and
light. The only safe way to study the sun is to protect its image through a
pinhole or a telescope onto a white screen.
The average distance of the sun from the Earth, arbitrarily called one astronomical
unit by astronomers, is 149,497,875 kilometers (92,958,350 miles). The sun's
radius is about 432,500 miles, or 109.3 times the volume of the earth. It has
been calculated the sun's mass, or quantity of matter, is some 333,400 times
as great as the Earth's mass.
Starts vary greatly in size and in color. They range from giant starts, which
are much larger than the sun, to dwarf stars, which can be much smaller. In
color they range from whitish-blue starts with very high surface temperatures
(over 30,000 degree Kelvin, or 53,500 degree F) to relatively cooler stars (about
2,500 degree K, or 4,000 degree F). The sun is a yellow dwarf star, a kind that
is common in Milky Way with a surface temperature of about 6,000 degree K (10,300
degree F). (The Kelvin temperature scale uses degrees of the same size as Celsius,
or centigrade, degrees, but it is numbered from absolute zero, which is -273.15
The sun looks like a burning sphere. In fact it is often pictured as a circle
with flames surrounding it. But the sun is actually too hot for an Earth-type
chemical reaction like burning to occur on its surface. Besides, if burning
produced its energy, it would have run out of fuel many millions of years ago.
Various theories have been advanced to explain the sun's tremendous energy
output. One said that all the bits of matter in the sun were exerting gravitational
attraction on each other and causing the sun to shrink and become more tightly
packed. This process, called gravitational contraction, does occur in some stars
and can release a great deal of energy. However, gravitational contraction could
produce energy for only 50 million years at most, while the sun's age must be
at least as great as the Earth's age of 4 and half billion years.
Atomic theory finally provided an explanation. Scientists now agree that thermonuclear
reactions are the source of solar energy. Albert Einstein's theoretical calculations
showed that a small amount of mass could be converted to a great amount of energy.
The vast amount of matter in the sun could provide fuel for billions of years
of atomic reactions.
The sun's core is believed to be a superhot, extremely dense mass
of atomic nuclei and electrons. Its temperature is calculated to
be about 15,000,000 K (27,000,000¢XF). Under these conditions,
nuclei can collide and fuse into new and heavier nuclei. This is
a type of theomonuclear reaction called a fusion reaction. During
such a reaction some of the mass of the nuclei changes to energy.
Two specific processes, the carbon cycle and the proton-proton-
reaction, occur most often.
| 1: the core 2: where the energy
maintains 3: the convection zone 4: photosphere 5: solar surface
The Photosphere and Sunspots.
The sun's surface, called the photosphere (sphere of light) is the lowest
visible layer of the sun. The temperature of the photosphere ranges from 7,500
K (13,000¢XF) at the bast to 4,700 K (8,000¢XF) at the top. Its average
temperature is 5,800 K (10,000¢XF). The photosphere has a definite texture.
Many small, luminous grains are separated by dark areas that look like nets
or canals. High altitude photographs of the photosphere's granulation show that
the grains are hundreds of miles in diameter. They are continually forming and
disappearing. According one hypothesis, the grains are the tops of gas columns
that ascend and descend through the photosphere.
The uniformity of the granulation indicates that a relatively calm condition
exists at the solar surface. This is periodically subject to violent disturbances.
Generally, these disturbances appear as darker points, called pores, on the
more luminous background of the photosphere. The pores usually grow rapidly
in number and in size to form a large single sunspot or a group of sunspots.
Typical sunspots have a dark, circular center, called the umbra, surrounded
by a lighter area, the penumbra. Rays issuing from the center of the umbra form
the penumbra. Sunspots vary greatly in size but are always small compared to
the size of the sun. When they appear in groups, they may extend over thousands
of miles. The darkness of the umbra is a sign that the sunspots are cooler than
the photosphere. The umbras appear to be some 2,000 K (3,100¢XF) cooler
than the photosphere. Furthermore, when they approach the sun's edge, the umbras
appear to be lower than the photosphere as well.
The regular observations from 1750 to the present reveal that the spots appear
and disappear in a definite cycle, and that they are limited to the two zone
of the sun contained between latitudes 40¢X and 5¢X of its northern
and southern hemispheres. Their cycle lasts an average of 11 years. At the beginning
of a cycle a few spots appear at around 35¢X latitudes, then they rapidly
increase in number, reaching a maximum in the course of around five years. At
the same time they move slowly toward the equator. During the next six years
their number decreases while they continue to approach the equator. There the
cycle ends and at the same time another cycle starts immediately.
The layer above the photosphere is called the chromosphere (sphere of color)
because of its reddish color, visible during total eclipses of the sun. The
lower chromosphere absorbs some of the light that is emitted from the photosphere,
causing the dark absorption lines of the sun's spectrum. This absorption occurs
because the lower chromosphere is cooler than the photosphere. However, its
temperature rises with height until at the upper boundary it has reached almost
1,000,000 K (1,800,000¢XF).
Much of the sun's "weather" takes place in the chromosphere. When the chromosphere
is viewed under hydrogen light or under the violet light of calcium, bright
areas called plages appear. The plages are usually located above or near sunspots
and may be extensions of bright patches - faculae - that occur on the photosphere
A far more violent phenomenon is the solar flare, a sudden chromospheric eruption
from a plage area. The solar flare may emit high energy radiation and highly
energetic charged particles. Solar flares usually form very rapidly, reaching
their maximum brilliance within minutes, after which they slowly die out. Very
strong solar flares may emit X rays, radio waves, and swarms of charged particles.
These enormous spurts of energy could be very dangerous for space travelers
above the protection of the Earth's atmosphere because such fast-moving radiation
can penetrate spaceship walls and damage body cells.
The solar corona
The chromosphere is surrounded by the solar corona, a faintly luminous outer
atmosphere. As this atmosphere is thousands of times dimmer than the sun's disk,
it is usually invisible. Before the invention of the coronagraph, it could be
seen only when the sun was totally eclipsed by the moon. When the photosphere
is blocked out, the corona appears as a silvery halo with long arcs and streamers.
The arcs are usually visible above disturbed regions, especially where prominence
are present. When sunspots are at a minimum, the corona has long streamers along
the equator with shorter rays at the poles. Its shape then resembles that of
force lines around a magnetic sphere. This shape changes when sunspots are at
a maximum. The corona then appears almost circular, with streamers distributed
uniformly around the disk.
The solar wind is a continuous outward flow of ionized gas (plasma) from the
corona of the sun, which extends beyond the earth's orbit and into interstellar
space. It consists chiefly of a mixture of protons and electrons, plus the nuclei
of some heavier elements in smaller numbers. The particles that make up the
solar wind are formed when the coronal gases expand and evaporate. About a million
tons of gas per second flow away from the sun by this process. The high temperatures
of the corona accelerate the particles to speeds great enough to allow them
to escape from the sun's gravitational field. As they leave, the particles carry
part of the sun's magnetic field along with them. Because of the sun's rotation
and the steady outflow of particles, the lines of the magnetic field carried
by the solar wind trace curves in space. The solar wind is responsible for deflecting
the tails of passing comets away from the sun. when the solar wind encounters
the earth's magnetic field, a shock wave results. In the vicinity of the Earth,
the solar wind produces magnetic storms, fading of radio transmissions, and
the polar auroras.
The Properties of Sun.
| 8.64¡Ñ105 miles
|109.3 times Earth's diameter
|333,400 times Earth's mass
|1.4¡Ñ1033 cubic centimeters
|50 octillion cubic feet
|1,3000,000 times Earth's diameter
|1.41 grams/cubic centimeter
|88 pounds/cubic foot
|one fourth Earth's density
|26.9 Earth Days
|31.1 Earth Days
Chemical Composition of the Sun
Logarithm of relative number of atoms
article: Pot of Gold?