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Lecture 22

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 Lecture 22: March 1 , 2013st  halting collapse o as contraction packs molecules & dust in a cloud fragment closer together it becomes more difficult for infrared & radio radiation to escape o pressure begins to build up in interior, balancing gravity o contraction slows down & we call the object a protostar o it continues to grow by accreting new material  a contracting cloud fragment always has some small, overall rotation  conservation of angular momentum ensures that the rotation speeds up as the cloud shrinks & flattens  think of the figure skater bringing their arms in to make themselves smaller to spin faster  in late stages of collapse, the central protostar may fire jets of high speed gas along its rotation axis  we think that the Milky Way collapsed into a disk; our solar system is a disk  when yo  when you have a disk you have solar jets that come out of it in opposite directions  we don’t know how these things form, but many think it may be due to magnetic fields that shoot materials out  jets are tight beams of material like electrons that shoot at the speed of light  you can’t see through protostellar disks  they are very dark since made of dust  there is now a beginning of nuclear fusion which moves a protostar to a main sequence star  these steps don’t really matter for this class!* o 1. a protostar assembles from a collapsing cloud fragment; it’s concealed beneath a shroud of dusty gas o 2. a protostar shrinks & heats as gravitational potential energy is converted to thermal energy o 3. surface temperature rises when radiation becomes dominant mode of energy flow within protostar o 4. fusion rate increases until it balances the energy radiated from the star’s surface  *point
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