1Announcements1. HW 6 is due on December 1 beforemidnight2. Final exam: 12/15, 10:30 AM,Hasbrouck 20Astro 101: 11/25/09Today’s topic:Formation of the Solar SystemPlanetary System Formation:The Nebular HypothesisThe physics that occurs when a gascloud collapses governs theproperties of the star and planets thatform at the end of the collapse.Question: how abundant are theseinterstellar clouds from which thestars are hypothesized to form?Answer: there are large numbers ofthese clouds, and some are large andmassive, as shown on the followingslides.The Orion Nebula - the nearestregion of vigorous starformationThe Orion Nebula - locationof huge clouds of moleculesVisible light Radio light(overlaid on visible light)2The Orion Nebula - locationof huge clouds of moleculesVisible light Radio light(overlaid on visible light)Colors show regions where the spectral fingerprint of carbon monoxide emission isdetected. Carbon monoxide is the secondmost common moleculein the universe (aftermolecular hydrogen,H2)Notice howlarge theseclouds ofmoleculesare…Orion Nebula: visible vs. infrared light• The visible light is fromthermal radiation from stars,which have surfacetemperatures of a fewthousand degrees Kelvin andcore temperatures of millionsof degrees!• The infrared light frommolecular clouds aremuch colder, at about 10degrees Kelvin!! (Notfar above absolute zero)We must be careful when we investigate theseclouds, however. This one is an example ofmatter ejected by a star. This is not a cloudof molecules that will turn into a star. Bystudying the characteristics of this object,we see that it is an entirely different thing.3Distant galaxyNearby star (like our Sun, butolder) that it is ejecting itsouter shells as it approachesthe end of its life.Comparing this nearby object with the very distant galaxy, wecan see how there was a debate during Hubble’s time aboutwhether the “spiral nebulae” were nearby objects or distantgalaxies. Using techniques such as the Cepheid star analysis thatwe discussed previously, Hubble showed convincingly that thespirals are distant galaxies.PRS. Which of the followingprinciples of physics is NOTimportant for understanding theformation of the Solar System?1. Conservation of angular momentum2. Conservation of linear momentum3. Conservation of energy4. Newton’s law of gravity5. Actually, all of these laws of physics areimportant factorsThis is an extra-credit prs, as explained in lecture.Collapse ofan interstellarcloudViewed from above, the cloud is rotating counter-clockwise41. Collapse under its own gravity• The mass inside the cloudexcerts a gravitational forcethat pulls the cloud toward itscenter.• If the cloud is somehowcompressed a bit, the force ofgravity becomes even strongerbecause gravity is inverselyproportional to distance• As the cloud collapses, itsdensity increases.• But what triggers the initialcompression of the cloud?The blast wave sent out when a nearby star explodes could do the trick!What will happen to thetemperature of the cloud asit collapses?1. The cloud will cool down.2. The cloud will warm up.3. The cloud temperature will remain thesame.4. This cannot be answered withoutadditional information.Consider a gas cloud in space, suchas the one shown at right. Supposethat it starts to collapse suddenly.PRSPRSPRSIMPORTANT!!!!!!!!one form of energyAnother form …Energy is conserved!2. The cloud heats up as itcollapses• Conservation of energy!• As it collapses, gravitional potential energy kinetic energy• The particles within the cloud collide with each otherand the motions are randomized, so kinetic energy thermal energy• The cloud temperature increases as itcollapses53. As the cloud collapses,it spins more rapidly• angularmomentum =m x v x r• As r decreases, thevelocity mustincrease in order toconserve angularmomentum4. As the cloud collapses,it flattens into a disk• Partly due to conservation of angularmomentum ( = m x v x r)Consider a spinning sphericalobject, e.g., a rotating gas cloud.(each trace shows the path of a clump of gas)If we move the blue clumpdown but keep the radiusthe same, angular momentumdoes not change.However, if wemove the blueclump left/right,then somethingmust change toconserve angularmomentum!4. As the cloud collapses,it flattens into a disk• Partly due to conservation of angularmomentum ( = m x v x r)• Also, as clumps of gas collide and merge, themotions become more orderly and circularLinear Momentum is ConservedP = m·vP1 = m1·v1 P2 = m2·v2If two objects collide and stick together, thefinal momentum must be the same as theinitial momentum.Pfinal = P1 + P2We can describe a momentum as a sum of two components:Total momentum:Component 1Component 26Momenta cancel outperpendicular to diskMomentum 1Momentum 2Net momentum is zero, so ifthese clumps collide and sticktogether, after the collision thevelocity of the final clump iszero too.Momentum 1Momentum 2In this lower example, afterthe clumps collide and stick,they still have a net velocity to the right.The same effect changeshighly elliptical orbitsinto circular orbitsWhen two gas clumps collide, these components oftheir momentum cancel out.On the other hand, these momentumcomponents add togetherThe components of motion thatmake an orbit elliptical tend tocancel outOrbit of gas clump 1Orbit of gas clump 1Momentum componentsin opposite directionscancel outMomentum componentsin the same direction addtogether and increase thenet momentumMOMENTUMTRANSFERCHANGES ANELLIPTICAL ORBITINTO A CIRCULARORBITWhy does the cloud stop collapsing?If the cannonball is fired with sufficiently high velocity, then themotion parallel to the ground compensates for the pull of gravity;the ball enters a state of “constantly falling” toward the groundwithout ever getting closer to the ground.• Recall our discussion of objects orbitingaround a planet:7The collapsing disk is the oppositesituation• As matter in the disk movescloser to the center, the diskmust spin more rapidly toconserve angular momentum.• Eventually, the velocitybecomes so great that theclump settles into “orbit”, and the collapse stops. That is, the velocity eventually becomes largeenough so that it enters that“constantly falling” situationMatter falling infrom a larger distanceAs the nebula collapses, it heatsup, spins faster, and flattens.• Patterns of motion.Orbits are1) mostly circular2) in the same direction(counterclockwise)3) in (mostly) the