Download How the Solar System Formed: The Nebular

How the Solar System Formed: The Nebular Hypothesis
Collapsing Clouds of Gas and Dust
A great cloud of gas and dust (called a nebula) begins to collapse. For billions of
years it had be stable and drifted in space but something disturbed its equilibrium
(perhaps a passing star or a blast wave of energy from a supernova explosion). The
equilibrium came from the force of gravity trying to collapse the nebular being
equal to the gas pressure pushing back to expand the nebular. Now, gravity takes
over and the nebular begins to collapse and all the atoms and mass that had been
spread out through out the nebular fall (collapse) to the center. As it collapses
inward, it begins to spin faster (think of a spinning ice skater pulling in her arms,
concentrating her mass in the center, to make herself spin faster). This is called
conservation of angular momentum.
In the Nebular Hypothesis, a cloud of gas and dust collapsed by gravity
begins to spin faster because of angular momentum conservation
It is unlikely that such a nebula would be created with no angular momentum, so it
is probably initially spinning slowly. Because of conservation of angular momentum,
the cloud spins faster as it contracts.
The Spinning Nebula Flattens
Because of the competing forces associated with gravity, gas pressure, and rotation,
the contracting nebula begins to flatten into a spinning pancake shape with a bulge
at the center, as illustrated in the following figure. The bulge will become the infant
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sun, or proto-sun, where 99.8% of all the mass of the collapsing nebular is
concentrated, and the flattening disk, or pancake, will be where .15% of the left over
atoms (matter) will be distributed into orbiting rings. The orbiting rings will be
cooler, the further away from the center, and as these areas cool, solids will
condense. These rings will sweep up all the solids into larger and larger bodies that
will make up the core of planets.
The collapsing, spinning nebula begins to flatten into a rotating pancake
Condensation of Protosun and Protoplanets
As the nebula collapses further, instabilities in the collapsing, rotating cloud cause
local regions to begin to contract gravitationally. These local regions of condensation
will become the Sun and eventually be swept into infant planets (proto-planets or
planetesimals) as well as their moons and other debris in the Solar System.
While they are still condensing, the incipient Sun and planets are called the protosun
and protoplanets, respectively.
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As the nebula collapses further, local regions begin to contract
gravitationally on their own because of instabilities in the collapsing,
rotating cloud
Evidence for the Nebular Hypothesis
The orbits of the planets lie nearly in a plane with the sun at the center. The planets
all revolve in the same direction. All debris (matter) orbits the sun (except moons).
The most dense materials are closest to the Sun. All the planets and most of the
moons and asteroids are about the same age. The same materials (but in different
proportions) make up all the objects in the solar system. We can observe with
telescopes the formation of other stars in nebulae, and we have detected other
planets orbiting stars.
The nebular hypothesis explains many of the basic features of the Solar System, but
we still do not understand fully how all the details are accounted for by this
hypothesis.
Resources on the Nebular Hypothesis
Animations of Formation of Solar System and Origin of the Moon (this one)
http://csep10.phys.utk.edu/astr161/lect/solarsys/nebular.html
The Nebular Hypothesis: Origin of the Solar System (helpful)
http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html
Nebular Hypothesis: Origins of the Solar System (more advanced)
http://student.ccbcmd.edu/courses/astm101c/unit2/solneb.html
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