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Chapter 16

Chapter 16

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PHYS 183
Tracy Webb

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PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 16: Star Birth 16.1 Stellar Nurseries  youngest star clusters always associated with dark clouds of gas & dust  birthplace of stars  dark patches in the Milky Way are interstellar gas clouds  interstellar medium: the gas & dust that fill the spaces between stars within a galaxy; consists of 70% H, 25% He & 2% heavier elements o virtually all has the same chemical composition but can look very different place to place based on differences in temperature & density  molecular clouds: interstellar clouds that are particularly cold & dense that stars are born in o H 2s the most abundant molecule but hard to detect since usually too cold for2H to produce emission lines, so usually use CO which produces radio emission lines  interstellar dust: half of the atoms of elements heavier than He in a molecular cloud are tiny, solid grains; constitute ~1% of molecular cloud’s mass  detect reddening from Doppler shift since it doesn’t change wavelength of stars spectral lines so determine amount of reddening by comparing observed colour to colour expected of that spectral type  amount of reddening tells how much dusty gas lies between earth & star  infrared allows us to see directly through molecular clouds & also show new stars in clouds  most radiation produced by young stars in molecular cloud can’t escape cloud since dust grains absorb it & heat up themselves, emitting thermal radiation in infrared & microwaves  this is why clouds that appear dark in visible light glow in infrared light  stars form when gravity causes molecular cloud to contract until central object become hot enough to sustain nuclear fusion in core  molecular clouds are the only places in space that the force of gravity overcomes the form of pressure  pressure depends on density & temperature  thermal pressure: temperature dependant pressure o can resist gravity in most interstellar gas clouds due to low gas densities keeping gravity weak  degeneracy pressure: pressure that is not temperature dependant  gravity is stronger in molecular clouds since more mass in each cubic cm of volume  gravitational contraction: regions of molecular cloud in which gravitational attraction is stronger than thermal pressure so forced to contract o this converts some of the gravitational potential energy into thermal energy o if can’t get rid of thermal energy quickly, it builds up inside of cloud which raises temperature & pressure halting formation of star  doesn’t usually happen  stars form in clusters since gravity stronger in high mass gas clouds  individual gas clumps within molecular clouds move at substantially different speeds indicating overall gas motion is turbulent  magnetic fields can help the cloud resist gravity  large clouds of gas form many individual stars instead of one extremely massive star due to battle between gravity and pressure o gravity follows inverse square law  strength of gravity increases as cloud shrinks in size PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013  molecular cloud doesn’t have to be very massive to form a star as long as it is cold & dense  composition of gas clouds in galaxy haven’t changed much during last 5 billion years  must have changed somewhat since all elements heavier than He produced through life & death of stars  first generation stars can’t have been born in the same way they are today since heavy elements didn’t exist then & CO didn’t exist to radiate away thermal energy o theory: emission of photons from H2could have helped them cool o determined clouds back then not as cool as they are today since 2 needs temperatures at least 100 K to excite 16.2 Stages of Star Birth  cloud fragments continue to collapse on itself as long as it remains cold & it will remain cold as long as photons emitted by molecules within cloud can carry away energy released by gravitational contraction  contraction makes it hard for photons to escape since density increases  photon absorbed which increases thermal energy and therefore temperature & pressure  central region of cloud fragment grows dense to trap radiation  temperature & pressure rise which slows contraction  birth of a protostar  protostar: the dense centre of the cloud fragment made of gas that will become a new star; not yet a true star since core isn’t hot enough for nuclear fusion  mass of protostar grows with time until gas surrounding protostar gone (either ran out or something blows remaining
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