The Pre-Solar Nebula - College of Charlestonneffj.people.cofc.edu/ASTR129/Notes/lec13.pdf · – large, gas giants in outer solar system! – small, rocky planets in inner solar system!

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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)