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PHYS 183 (51)
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Chapter 18

Chapter 18

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PHYS 183
Tracy Webb

18: The Bizarre Stellar Graveyard 18.1 White Dwarfs  white dwarf: exposed core of a star that has died and shed its outer layers in a planetary nebula o stellar in mass, small in size o gravity is very strong near surface  if it was unopposed, would crush to even smaller but pressure is pushing back o no fusion to maintain heat & pressure, so pressure comes from electron degeneracy pressure o composition reflects products of star’s final nuclear-burning stage o usually has mass of sun compressed into volume size of earth  more mass = smaller size (greater gravity)  there is a fundamental limit on the maximum mass of a white dwarf  electron speeds can’t be faster than the speed of light (they reach this speed with a mass of 1.4 M sun  this is known as the white dwarf limit/Chandrasekhar limit  many known white dwarfs in binary systems, so we can measure their masses  white dwarf not in a binary system will continue to cool  white dwarf in a binary system can gain mass of companion if it is a main sequence or giant star o when mass first spills over to white dwarf, has some orbital velocity  must orbit faster & faster as it falls towards white dwarfs surface (conservation of angular momentum) o this is called accretion; the mass rotating is called an accretion disk  differences between protostellar disks & accretion disks = size, orbital speed, temperature  accretion can provide a “dead” white dwarf with a new energy source as long as its companion keeps feeding matter into accretion desk  white dwarf has strong gravity which compresses H gas from companion into thin surface layer; pressure & temperature rise as layers build up; once temperature of bottom layer reaches 10 million K H fusion ignites  white dwarf is now a nova for a few weeks  far less luminous than a supernova but still bright; generates heat & pressure  accretion resumes after nova explosion subsides so entire process can repeat itself  nova: a relatively minor detonation of H fusion on the surface of a white dwarf in a close binary system  supernova: total explosion of a star  white dwarf supernova: as white dwarf nova contains to become more massive approaching the white dwarf limit (1.4 Msuntemperatures rise enough for C fusion; ignites almost instantly creating “C bomb” detonation, exploding the white dwarf completely  distinguish between white dwarf supernova & massive star supernovas through light  white dwarfs fade faster; white dwarf supernova lack H lines  use white dwarf supernovas to measure large distances  explosions occur at the same mass, so light curves look very similar & maximum luminosities too; can determine the galaxy’s distance by using the inverse square law for light 18.2 Neutron Stars  neutron star: ball of neutrons created by collapse of iron core in massive star supernova; essentially giant atomic nuclei made almost entirely of neutrons & held together by gravity; resists the crush of gravity with neutron degeneracy pressure  escape velocity is half the speed of light  paper clip with the density of a neutron star would outweigh Mount Everest  state of matter predicted to vary with depth - + o surface  similar to white dwarf; has crust composed of e & p o inside  pressure of overlying layers ensures individual atomic nuclei disintegrated & almost all e have combined with p+
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