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Chapter 8

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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Chapter 8: Formation of the Solar System
8.1: The Search for Origins
Four Criteria For Success of Solar System Formation Theory: (1) Must explain
patterns of motions discussed in chap 7. (2) Must explain why planets fall into
terrestrial(small, rocky, near Sun) and jovian(large, hydrogen-rich) categories. (3) Must
explain existence of huge numbers of asteroids + comets + why they reside in asteroid
belt, Kuiper Belt, and Oort Cloud. (4) Must explain gen. pats while allowing exceptions
AKA: Uranus Tilt + Earth’s large moon.
Nebular Hypothesis: Prop’d by Kant + Laplace SS formed from grav collapse of
interstellar cloud of gas.
Other prop was close encounter hypo Sun + other star collided Failed b/c (1)
Couldn’t account 4 orbtial motions of planets or division of planets into two major
categories (2) Collision btw. Sun and star significantly improbable – Stars separated too
far. Chance of encounter so small that would be difficult to imagine happening once to
form SS and doesn’t account for other planetary systems observed.
Nebular hypothesis became Nebular Theory Successfully predicts success of other
planetary objects.
8.2: The Birth of the Solar System
Idea of Solar Nebula: Cloud of gas that collapsed under its own gravity to form our SS.
Gas that made of SN contained apprx 98% H and He + 2% of all other elements.
Three Processes of SN Alteration: (1) Heating: Temp of SN increased as collapsed 
Grav pot energy conv 2 kin energy of indiv gas particles falling inward These partcls
crashed into each other, convs kin energy of inw fall 2 random motions of term energy.
Sun formed in centre where temps + densities highest. (2) Spinning: SN rotated faster
and faster till shrank in radius Rapid rot helped ensure not all materials in SN
collapsed into centre: Greater ang momentum of rot cloud, more spread out will be. (3)
Flattening: SN flattened into disk. Cons o’ collisions btw. particles in spinning cloud.
Diff clumps o’ fas within cloud move in random dircts at random speeds. – clumps
colliede + marge as cloud collapses, + each new clump has avg vel o’ clumps that formed
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