Post on 01-Oct-2020
transcript
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Lec 13: 10 October 2011 Chapter 8: Formation of the Solar System Last Time – Finish 7, Start 8
• Terrestrial v. Jovian Planets • The Interstellar Medium; Nebulae • Chemical Composition of Pre-Solar Nebula
Today – Formation of Sun and Solar System • “Trigger” for Collapse • Formation of Disk • Ignition and Clearing Phase • Accretion v. Fragmentation
Wednesday: Chapter 16 (The Sun)
The Pre-Solar Nebula • Relative abundance of the
elements was pre-determined • This occurred 4.56 billion
years ago (as determined by radioactive age-dating)
• Cool, dark, tenuous. • Dust + Gas
• initially stable: pressure balances gravity; slowly rotating
• might be within larger region of star formation; maybe isolated
• total mass of cloud at least 1.1 solar masses
• Trigger? supernova, stellar winds, motion through galaxy, spiral arm
• once collapse begins, gravity “wins” (but only if it radiates enough to stay cool --> dust is critical)
• once collapse begins, most mass falls to center (protosun); dense and hot in center, cooler as you get farther from center
• cloud rotates faster to conserve angular momentum
• material falls into equatorial plane from above and below
• flattens into a disk in which all the material orbited the center in the same direction
• Sun “turns on” when center of it is hot and dense enough
• extra pressure from sunlight halts the collapse
Slowly-Rotating Cloud Collapses Into Rapidly-Rotating Disk
• After about 108 years, temperatures at the protosun’s center became high enough to ignite nuclear reactions that convert hydrogen into helium
• rest of solar system forms out of rotating disk and leftover debris • everything in disk is already in roughly circular orbits in same direction!
We See This Happening Around Other Stars
• Inner disk too hot for “volatiles” to freeze solid. Contains only heavy elements (metals) and lots of gas
• Outer disk contains solid metals and solid ice as well as gas
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Accretion of Planetesimals into Protoplanets • fragmentation v. accretion (depends on relative velocity)
• outer planets start first and have metallic and icy solid cores, so they become massive enough to capture Hydrogen and Helium gas
• too hot in inner solar system for ices; only metals form solids
Outer planets also form mini “solar-systems” of moons orbiting in their equatorial plane
Inner planets don’t have them (their moons are “captured” later)
“Clearing Phase”
• strong solar “wind” blows remaining gas away
• we see Jets and cavities blown by young stars
Clearing Phase: “Era of Heavy Bombardment” • outer planets scatter debris into Kuiper belt
• protoplanets collide, melt, differentiate, reduce in number
• surfaces get heavily cratered; formation of our Moon; re-melting
• impacts destroy and create (e.g. water to Earth from comets?)
Evolution of the Solar System:���How Did We Get Here From There?
• Some aspects are pre-determined by origin – planets orbiting in ecliptic plane in same direction – large, gas giants in outer solar system – small, rocky planets in inner solar system – 3 main “reservoirs” of debris
• But many things change or “evolve” – gradual changes, building on previous changes – cyclic changes – catastrophic changes (often from impacts)