|
CHAPTER THREE
FORMATION OF THE
EARTH
Stars are believed to form by "gravitational collapse." This is the process whereby a
large cloud, or nebula, of hydrogen and helium (and to a lesser extent the heavier
elements) in space, collapses inward due to the pull of its own gravity. As the gasses
occupy less space, the density of the cloud begins to increase. Since pressure also
increases, the temperature rises.
The temperature of this collapsing gas continues to increase to millions of degrees
until the hydrogen atoms reach a critical point whereby they fuse together and form
helium. Tremendous energy and light are emitted through this process, which is known
as nuclear fusion. This is same energy that is unleashed by the hydrogen bomb. The
exploding outward forces which are produced by the nuclear fusion reactions are countered
by the immense gravitational pull inward, which is due to the tremendous mass of the star.
These two opposing forces are in a state of general equilibrium during most of the life
of the star. That is why the size of a star such as our sun remains relatively constant.
When a star converts most of its hydrogen into helium, it begins to run out of
its primary fuel. It is believed that the star continues to burn hotter until the higher
temperatures are reached whereby the heavier elements can be formed by the fusion process.
At some point, a large star (one that is much larger than our sun) will collapse upon
itself and explode - spewing its raw materials into space. Heavier elements, such as the
metals, are among the debris of the explosion. This exploding star is called a "super nova."
Our solar system began to form about five billion years ago. It was created from a
nebular cloud that was the remains of a super nova. Our sun is thus considered a "second
generation" star because it formed from the dust and gas of an earlier star.
While our sun was formed by the same gravitational collapse described above, the
planets that orbit the sun were formed much differently, by a process called "accretion."
As the nebular cloud that formed our solar system began to collapse, it also
flattened into a narrow elliptical disk. Most of the matter of the disk collapsed inward
and formed the sun. But some of the dust and gas remained in a flattened plane that rotated
around the sun. Throughout the universe, it is common to find flattened planes of matter
orbiting larger objects. Many galaxies possess this flattened property, and in our own solar
system we can observe this feature in the rings of Saturn.
The plane of dust and gas soon began to coalesce into large particles, which in turn
collided and formed even larger particles. It is believed that only a few million years
were required before the foundations of planets, called "planetesimals," were formed. The
planetesimals continued to draw in much of the remaining nebular debris that rotated around
the sun.
The young Earth was bombarded in its early life by dust, meteors, comets, and
even smaller planetesimals. These raw materials continuously pummeled the Earth and
increased the mass of our planet. The Earth's surface was kept in a near molten state
from the energy released by the continuous barrage of nebular debris, and the release
of radiation from the radioactive decay of elements in the Earth's interior. But despite
its hostile, violent beginnings, and its barren, inhospitable nature, a new planet had taken
form.
Four and half billion years ago, the Earth was born.
(These are selected paragraphs within this chapter. This is not the entire chapter.)
RETURN
|
|