Sun grazing comets from SOHO Copyright 2009 Pearson Education Inc Shadows of Saturn s Moons Pan Epimetheus Copyright 2009 Pearson Education Inc Chapter 12 Star Stuff Copyright 2009 Pearson Education Inc 12 1 Star Birth Our 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 dark clouds of dusty gas in interstellar space The gas between the stars is called the interstellar medium Copyright 2009 Pearson Education Inc Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud Gravity within a contracting gas cloud becomes stronger as the gas becomes denser Copyright 2009 Pearson Education Inc Mass of a Star Forming Cloud A typical molecular cloud T 30 K n 300 particles cm3 must contain at least a few hundred solar masses for gravity to overcome pressure The cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud Copyright 2009 Pearson Education Inc Fragmentation of a Cloud This simulation begins with a turbulent cloud containing 50 solar masses of gas Copyright 2009 Pearson Education Inc Fragmentation of a Cloud The random motions of different sections of the cloud cause it to become lumpy Copyright 2009 Pearson Education Inc Fragmentation of a Cloud Each lump of the cloud in which gravity can overcome pressure can go on to become a star A large cloud can make a whole cluster of stars Copyright 2009 Pearson Education Inc Glowing Dust Grains As stars begin to form dust grains that absorb visible light heat up and emit infrared light Copyright 2009 Pearson Education Inc Glowing Dust Grains Long wavelength infrared light is brightest from regions where many stars are currently forming Copyright 2009 Pearson Education Inc Thought Question What would happen to a contracting cloud fragment if 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 Question What would happen to a contracting cloud fragment if 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 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 to conservation of energy Contraction can continue if thermal energy is radiated away Copyright 2009 Pearson Education Inc As gravity forces a cloud to become smaller it begins to spin faster and faster due to conservation of angular momentum Copyright 2009 Pearson Education Inc As gravity forces a cloud to become smaller it begins to spin faster and faster due to conservation of angular momentum Gas settles into a spinning disk because spin hampers collapse perpendicular to the spin axis Copyright 2009 Pearson Education Inc Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms Collapse of the Solar Nebula Copyright 2009 Pearson Education Inc Flattening Collisions between particles in the cloud cause it to flatten into a disk Copyright 2009 Pearson Education Inc Collisions between gas particles in a cloud gradually reduce random motions Formation of Circular Orbits Copyright 2009 Pearson Education Inc Collisions between gas particles also reduce up and down motions Why Does the Disk Flatten Copyright 2009 Pearson Education Inc The spinning cloud flattens as it shrinks Formation of the Protoplanetary Disk Copyright 2009 Pearson Education Inc Formation of Jets Rotation also causes jets of matter to shoot out along the rotation axis Copyright 2009 Pearson Education Inc Jets are observed coming from the centers of disks around protostars Copyright 2009 Pearson Education Inc Copyright 2009 Pearson Education Inc Thought Question What would happen to a protostar that formed without any rotation at all A B C D Copyright 2009 Pearson Education Inc Its jets would go in multiple directions It would not have planets It would be very bright in infrared light It would not be round Thought Question What would happen to a protostar that formed without any rotation at all A B C D Copyright 2009 Pearson Education Inc Its jets would go in multiple directions It would not have planets It would be very bright in infrared light It would not be round Protostar to Main Sequence A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface It takes 30 million years for a star like the Sun less time for more massive stars Copyright 2009 Pearson Education Inc 1 2 3 4 Summary of Star Birth Gravity causes gas cloud to shrink and fragment Core of shrinking cloud heats up When core gets hot enough fusion begins and stops the shrinking 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 Very massive stars are rare Low mass stars are common Copyright 2009 Pearson Education Inc Upper Limit on a Star s Mass Photons exert a slight amount of pressure when they strike matter Very massive stars are so luminous that the collective pressure of photons drives their matter into space Copyright 2009 Pearson Education Inc Upper Limit on a Star s Mass Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart Observations have not found stars more massive than about 150MSun Copyright 2009 Pearson Education Inc Lower Limit on a Star s Mass Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation Is there another form of pressure that can stop contraction Copyright 2009 Pearson Education Inc Degeneracy Pressure Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place Copyright 2009
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