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TAMU ASTR 101 - Lecture15

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Copyright © 2009 Pearson Education, Inc.Sun-grazing comets from SOHOCopyright © 2009 Pearson Education, Inc.Shadows of Saturn’s MoonsPanEpimetheusCopyright © 2009 Pearson Education, Inc.Chapter 12Star StuffCopyright © 2009 Pearson Education, Inc.12.1 Star BirthOur goals for learning:• How do stars form?• How massive are newborn stars?Copyright © 2009 Pearson Education, Inc.How do stars form?Copyright © 2009 Pearson Education, Inc.Star-Forming Clouds• Stars form in darkclouds of dusty gasin interstellar space.• The gas between thestars is called theinterstellarmedium.Copyright © 2009 Pearson Education, Inc.Gravity Versus Pressure• Gravity can create stars only if it can overcomethe force of thermal pressure in a cloud.• Gravity within a contracting gas cloud becomesstronger as the gas becomes denser.Copyright © 2009 Pearson Education, Inc.Mass of a Star-Forming Cloud• A typical molecular cloud (T~ 30 K, n ~ 300particles/cm3) must contain at least a few hundredsolar masses for gravity to overcome pressure.• The cloud can prevent a pressure buildup byconverting thermal energy into infrared and radiophotons that escape the cloud.Copyright © 2009 Pearson Education, Inc.Fragmentation of a CloudThis simulation beginswith a turbulent cloudcontaining 50 solarmasses of gas.Copyright © 2009 Pearson Education, Inc.Fragmentation of a CloudThe random motionsof different sections ofthe cloud cause it tobecome lumpy.Copyright © 2009 Pearson Education, Inc.Fragmentation of a CloudEach lump of the cloudin which gravity canovercome pressure cango on to become a star.A large cloud canmake a whole clusterof stars.Copyright © 2009 Pearson Education, Inc.Glowing Dust GrainsAs stars begin to form,dust grains that absorbvisible light heat upand emit infrared light.Copyright © 2009 Pearson Education, Inc.Glowing Dust GrainsLong-wavelengthinfrared light isbrightest fromregions where manystars are currentlyforming.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhat would happen to a contracting cloud fragmentif it were not able to radiate away its thermal energy?A. It would continue contracting, but its temperature would not change.B. Its mass would increase.C. Its internal pressure would increase.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhat would happen to a contracting cloud fragmentif it were not able to radiate away its thermal energy?A. It would continue contracting, but itstemperature would not change.B. Its mass would increase.C. Its internal pressure would increase.Copyright © 2009 Pearson Education, Inc.Solar system formation is a good example of star birth.Copyright © 2009 Pearson Education, Inc.Cloud heats up as gravity causes it to contract due toconservation of energy. Contraction can continue ifthermal energy is radiated away.Copyright © 2009 Pearson Education, Inc.As gravity forces a cloud to become smaller, it begins tospin faster and faster, due to conservation of angularmomentum.Copyright © 2009 Pearson Education, Inc.As gravity forces a cloud to become smaller, it begins tospin faster and faster, due to conservation of angularmomentum. Gas settles into a spinning disk because spinhampers collapse perpendicular to the spin axis.Copyright © 2009 Pearson Education, Inc.Rotation of acontractingcloud speedsup for thesame reason askater speedsup as she pullsin her arms.Collapse of the Solar NebulaCopyright © 2009 Pearson Education, Inc.Collisions between particles in the cloud causeit to flatten into a disk.FlatteningCopyright © 2009 Pearson Education, Inc.Collisionsbetween gasparticles in acloudgraduallyreduce randommotions.Formation of Circular OrbitsCopyright © 2009 Pearson Education, Inc.Collisionsbetween gasparticles alsoreduce upand downmotions.Why Does the Disk Flatten?Copyright © 2009 Pearson Education, Inc.The spinningcloud flattensas it shrinks.Formation of the Protoplanetary DiskCopyright © 2009 Pearson Education, Inc.Formation of JetsRotation also causes jetsof matter to shoot outalong the rotation axis.Copyright © 2009 Pearson Education, Inc.Jets areobservedcoming fromthe centers ofdisks aroundprotostars.Copyright © 2009 Pearson Education, Inc.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhat would happen to a protostar that formedwithout any rotation at all?A. Its jets would go in multiple directions.B. It would not have planets.C. It would be very bright in infrared light.D. It would not be round.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhat would happen to a protostar that formedwithout any rotation at all?A. Its jets would go in multiple directions.B. It would not have planets.C. It would be very bright in infrared light.D. It would not be round.Copyright © 2009 Pearson Education, Inc.Protostar to Main Sequence• A protostar contracts and heats until the coretemperature is sufficient for hydrogen fusion.• Contraction ends when energy released byhydrogen fusion balances energy radiated from thesurface.• It takes 30 million years for a star like the Sun(less time for more massive stars).Copyright © 2009 Pearson Education, Inc.Summary of Star Birth1. Gravity causes gas cloud to shrink and fragment.2. Core of shrinking cloud heats up.3. When core gets hot enough, fusion begins and stopsthe shrinking.4. New star achieves long-lasting state of balance.Copyright © 2009 Pearson Education, Inc.How massive are newborn stars?Copyright © 2009 Pearson Education, Inc.A cluster of many stars can form out of a single cloud.Copyright © 2009 Pearson Education, Inc.Verymassivestars arerare.Low-massstars arecommon.Copyright © 2009 Pearson Education, Inc.Upper Limit on a Star’s Mass• Photons exert aslight amount ofpressure when theystrike matter.• Very massive starsare so luminous thatthe collectivepressure of photonsdrives their matterinto space.Copyright © 2009 Pearson Education, Inc.Upper Limit on a Star’s Mass• Models of starssuggest thatradiation pressurelimits how massivea star can be withoutblowing itself apart.• Observations havenot found stars moremassive than about150MSun.Copyright © 2009 Pearson Education, Inc.Lower Limit on a Star’s Mass• Fusion will not begin in a contracting cloud if somesort of force stops contraction before the coretemperature rises above 107 K.• Thermal pressure cannot stop contraction because thestar is constantly losing thermal energy from itssurface through radiation.• Is there another form of pressure that can


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TAMU ASTR 101 - Lecture15

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