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10/29 Lecture Notes (Exam 2) NOT FINISHED

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Alexandra Pope

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White Dwarfs ● White dwarfs are the remaining cores of low-mass stars. ● Electron degeneracy pressure supports then against the crush of gravity. ● White dwarfs cool off and grow dimmer with time. Size of a White Dwarf ● White dwarfs with the same mass as Sun are about same size as Earth. ● Higher-mass white dwarfs are smaller. The White Dwarf Limit ● Quantum mechanics says that electrons must move faster as they are squeezed into a very small space. ● As a white dwarf’s mass approaches 1.4 Msun, its electrons must move at nearly the speed of light. ● Because nothing can move faster than the speed of light, a white dwarf cannot be more massive than 1.4 the white dwarf limit. (....) ………... ● Astar that started with less mass gains mass from its companion. ● Eventually the mass-losing star will become a white dwarf… ● ….. Accretion Disks ● Mass falling toward a white dwarf from its close binary companion has some angular momentum. ● The matter therefore obits the white dwarf….. ● Kepler’s law: matter in the inner part of the disk moves faster than that in the outer disk. ● Friction between orbiting rings of matter in the disk transfers angular……. Nova ● The temperature of accreted matter eventually becomes hot enough for hydrogen fusion. ● Fusion begins suddenly and explosively, causing a nova. ● The nova star system temporarily appears much brighter. ● The explosion drives accreted matter out into space. Two Types of Supernova ● Massive Star Supernova: ○ Iron core of a massive star reaches white dwarf limit and collapses into a neutron star, causing a total explosion. ● White Dwarf Supernova: ○ Carbon fusion suddenly begins as a white dwarf in a close binary system reaches the white dwarf limit, causing total explosion. ● One way to tell supernova types apart is with a light curve showing how luminosity changes with time. Supernova Type: Massive Star or White Dwarf? ● Light curves differ ● Spectra differ (exploding white dwarfs don’t have hydrogen absorption lines) Nova or Supernova? ● Supernovae are much more luminous (about 100 thousand times) ● Nova: H to He fusion of a layer of accreted matter white dwarf left intact. ● ….. REVIEW Neutron Star ● Aneutron star is the ball of neutrons left behind by a massive star supernova. ● Degeneracy pressure of neutrons supports a neutron star against gravity. ● Electron degeneracy pressure goes away because electrons combine with proteins, making neutrons and neutrinos. ● Neutrons collapse…… ● Aneutron star is about the same size as the distance fromAmherst to Northampton, but is more massive than the sun. Discovery of Neutron Stars ● Using radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission…. ● …… ● ● Pulsar at the center of the Crab Nebula pulses 30 times per second. Pulsars ● Apulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation disk ● Pulsars spin fast because a stellar core’s spin speeds up as it collapses into a neutron star due to Conservation ofAngular Momentum. ● The radiation beams sweep through space like lighthouse beams as the netron st
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