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Chapter 18: The Formation of Stars and Planets Basic Observations Star Formation favors smaller stars (more smaller stars observed than larger stars) favors binary or multiple star systems young stars tend to be found in clusters (=> formation occurs in localized regions bound together by gravity) associations (formed near each other, but too far apart to be bound by mutual gravitation) a1c18:1 Basic Observations (cont’d) Solar system clues all orbital planes of planets and equatorial plane of sun (approximately) the same planets orbit in the same direction, most rotate same direction planetary orbits are nearly circular regular satellites of outer planets display same patterns as planets Sun has 99% of solar system mass, but less than 1% of the angular momentum (“Angular Momentum Problem”) a1c18:2 Star Birth Molecular clouds: concentrations of gas and dust [Fig18.1,2-4(orion nebula),5 (IRAS view of molecular cloud), eagle_nebula_GPN-2000-000987.jpg] relatively cool => contain molecules (vs. atoms and ions) => radio telescopes used to measure spectrum Giant Molecular Clouds ~10 pc across ~ 1 million M in gas and dust Clumps ~1000 -10,000 M with cloud cores Cloud Cores (formation of cores still a puzzle) cores collapse gravitationally, slowed by magnetic fields [fig 18.6] cores contain intense infrared sources => protostars warmer cores => larger stars, multiple stars, a1c18:3 cooler cores =>smaller stars Evolution of a protostar central region: gravity strongest, collapse fastest collapse generates heat (and infrared radiation) density increases until material becomes opaque gas temperature increases, pressure balances gravity continued accumulation of material by protostar rotation of core causes material to accumulate in disk around protostar (angular momentum) [Figure 18.7-9,13, 12(disk edge on vs flat) pancake.avi, 0708.mov] friction transfers angular momentum to exterior regions eventually dispersed by winds, or forms planetary system/binary companion a1c18:4 Young Stars H-R diagram [Figure 18-10,11] T-Tauri stars: light (less than 3M ) pre main sequence Ae, Be stars: more massive pre-main sequence stars gravitational contraction → PE converted to KE (heat) → density/pressure in center of protostar start hydrogen fusion more massive stars collapse, start fusion more quickly reach main sequence before dust and gas dispersed a1c18:5 Young Stars (cont’d) strong stellar winds possibly caused by strong magnetic activity caused by initiation of fusion (large energy release in core → convection) strong wind halts infalling material, eventually blowing away surrounding cloud [Figure 18-15] bipolar outflows [ Fig 18.15, 16,beta_pictoris_01l.jpg] strong wind blows away surrounding gas/dust → stars become visible most massive stars reach main sequence before becoming visible a1c18:6 Planetary Systems [2507.mov] planets formed from disk of material: the solar nebula abundance of radioactive isotopes provide timeline most meteorites formed within 20 million years of each other earth formed within the next 100 million years => solar system formed ~ 4.6 billion years ago, took ~10 to 100 million years to form solar nebula has same chemical composition as outer region of the sun. planets contain more heavier elements (small percentage of solar mix) => solar nebula was many times the mass of the planets ~ 1/10 to 1/100 x M a1c18:7 Cooling and Condensation nebula began to cool as accumulation of infalling material by sun ceased inner regions hottest as nebula cools, materials condense (at condensation temperatures) the condensation sequence [Figure 18.17,18] determines what materials condense => chemical makeup of the planet a1c18:8 Planet formation [ejs_gravitation2.jar] microscopic grains of condensed materials accumulated into larger bodies: planetesimals (up to 1000’s kms in size) largest bodies continue to accumulate material (accrete) bombardment era two models for giant planets [Figure 18.19] protoplanets accumulate gas from nebula gas forms unstable rings which form protoplanets possible final collisions (formation of earth’s moon, stripping of mercury’s silicate exterior, etc.) satellite systems generally miniature versions of solar system formation some satellites are captured planetesimals a1c18:9 evidence for Extrasolar planetary systems pulsars (timing of pulses affected by star/large planet orbiting each other, similar to doppler shift) periodically doppler shifted stellar spectra see, http://exoplanets.org/ a1c18:10