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The Nebula Dust Cloud Hypothesis
( Modern theory's best guess )
The Solar System's Dust Cloud
Interstellar space is known to be inhabited by many
large dust clouds (Oort clouds )
Dust particles form when random collisions between
free atoms result in the formation of chemical compounds.
Particles form clouds as a result of light pressure
from stars. The shadow cast by one dust particle falling on another nearby
dust particle will result in the lowest light pressure being along the line
between the two particles.
As the cloud gets bigger, the gravitational force
within the cloud begins to dominate over the differential light pressure. Most
of the observed interstellar clouds are about the size at which gravity begins
The total mass of these clouds is comparable to the
total mass of all stars and planets.
Clouds are truly nebulous, spread out more thinly
than the highest vacuum attainable on Earth.
Chemical composition is relatively familiar: Hydrogen, Helium, Nitrogen,
Oxygen, Carbon etc.
Roughly 4 billion years ago, one such dust cloud
started a process of collapse that was to eventually lead to the formation of
the Solar System.
This dust cloud was roughly spherical in shape, about
10 to 20 percent more massive than the present mass of the Sun, and revolving
about the center of the Milky Way near the end of one of the spiral arms.
The cloud begins to collapse under its own gravity,
this collapse could be initiated by a nearby supernova or by random
fluctuations of density within the cloud.
Because it is revolving around the centre of the
galaxy, the outer edge of the cloud (i.e., the one farthest from the galactic
centre) moves more slowly than the inner part. Hence the collapse of the cloud
would cause it to rotate, and in order to conserve angular momentum the speed
of rotation would increase as the cloud contracts.
As the cloud collapses, gravitational energy is
converted into heat. The cloud heats up and compresses in the center. It heats
enough for the dust to vaporize. The initial collapse is supposed to take less
than 100,000 years.
As the contraction continues, the cloud flattens, as
it is easier for matter to follow the attraction of gravity along the axis of
rotation than perpendicular to it, where the opposing centrifugal force is
The center compresses enough to become a protostar
and the rest of the gas orbits/flows around it. Most of that gas flows inward
and adds to the mass of the forming star.
As gas and dust are pulled in toward the central condensation, their gravitational
energy is converted to heat and the temperature of the material rises.
Ultimately the temperature becomes great enough in the interior of the
condensation for nuclear reactions to begin, thereby giving birth to a
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