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AST101 - L20 [Nov 25]

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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AST101 – L20 – [Nov 25]
8.1 The Search for Origins
What properties of our solar system must a formation theory explain?
oPatterns of motion of the large bodies
Orbit in same direction and plane
Generally spin in same direction (some exceptions)
oPlanets fall into two major categories:
Small, rocky terrestrial planets
Large, hydrogen-rich jovian planets
oSwarms of asteroids and comets populate the solar system
Asteroid belt
Kuiper Belt
Oort Cloud
oSeveral notable exceptions to these trends stand out:
Uranus’s odd tilt
Earth’s relatively large moon
What theory best explains the features of our solar system?
oThe nebular theory states that our solar system formed from the
gravitational collapse of a giant interstellar gas cloud: the solar nebula
A large amount of evidence supports this hypothesis
oClose Encounter Hypothesis states that the planets formed from debris
torn off the Sun by a close encounter with another star
Cannot explain observed motions and types of planets
Very unlikely except, maybe, at the earliest stages of formation in
a cluster
8.2 The Birth of the Solar System
Galactic Recycling
oElements that formed planets were made in stars and then recycled
through interstellar space
Stars are born in clouds of gas and dust
Stars produce heavier elements from lighter ones
Stars return material to space when they die
oWe can see stars forming in other interstellar gas clouds
Supports nebular theory
What caused the orderly motions of the planets?
oConservation of Angular Momentum
Rotation speed of the cloud from which our solar system formed
must have increased as the cloud contracted
Rotation of a contracting cloud speeds up for the same reason a
figure skater’s rotation speeds up as he/she pulls in his/her arms
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