Lecture 9: Threshold 4—The Earth and the Solar System
What followed should be familiar by now. Gravity drew the cloud together.
As it became denser, it heated up—particularly in the center. When the center
reached 10 million degrees Celsius, hydrogen fusion began and our Sun lit
up. It acquired the typical structure of stars, with fusion reactions in the
center, a middle layer containing reserves of hydrogen, and a surface from
which energy radiates into space. We can see areas of star formation in our
galaxy even today. Similar processes
occur even at smaller scales. For
example, Jupiter is mainly gaseous
and is large enough for its center to
be extremely hot, but not quite hot
enough for fusion. It is almost a star,
but not quite.Now we shift away from the Sun to
the debris surrounding it. Through
a process known as accretion, the
planets and other bodies of our solar system were created from the 0.1% of
the solar nebula that was not incorporated in the Sun. As it contracted, the
solar nebula spun faster, like an ice skater doing a pirouette. Centrifugal force
À attened the spinning nebula into a disk, so that the material not gobbled
up by the Sun orbited the Sun in a single plane. Saturn’s rings consist of
orbiting debris, so they may give us a good idea of the shape and structure of
the solar nebula.In each orbit, particles of matter were drawn together by electrostatic forces
or crushed together in the course of violent collisions. Gradually, larger
objects appeared and began colliding with each other and sometimes merging
with each other. They grew like large snowballs. Stray bodies in the solar
system, such as comets, are thought to be remnants of these early stages of
accretion. Within a few million years, the large bodies in each orbit formed
“planetesimals.” The largest planetesimals drew in most of the remaining
material through their gravitational pull until a single large body appeared in
each orbit.We cannot determine the
age of the solar system by
dating Earth rocks because
the Earth is so geologically
active that its original surface
is now unrecognizable.