17. Neutron Stars and Black Holes

17. Neutron Stars and Black Holes - Neutron Stars and Black...

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Neutron Stars and Black Holes Outline Our goals for this section: To explain what neutron stars and pulsars are and identify some of their key properties To discuss how a black hole forms, and what the basic properties of a black hole are To introduce some concepts from special and general relativity to help explain black holes
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After a supernova occurs, what is left of the original star? In the case of a carbon-detonation (type I) supernova, theoretical calculations indicate that the whole star ought to be destroyed, leaving no dense remnant In the case of a core-collapse (type II) supernova, recall that the explosion occurs when neutronization converts protons and electrons to neutrons and neutrinos Rebound from the neutron core blows away the outer layers What is left is a “star” composed almost entirely of neutrons, called a neutron star A neutron star is supported by neutron degeneracy Neutron Stars
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Like white dwarfs, neutron stars are “dead” in the sense that they no longer generate much energy by nuclear fusion Neutron stars are extremely small and dense – their typical radii are ~ 20 km (about the size of Manhattan) – the mean density of a typical neutron star is ~10 18 kg/m 3 , so one teaspoonful would be about the same mass as a mountain!
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Beyond their extremely high densities and small radii, neutron stars have two other important properties: extremely rapid rotation (roughly between 3 and 30 times per second !) strong magnetic fields Why do neutron stars spin so quickly? conservation of angular momentum, mvr neutron stars have to spin much faster than the stars they formed from as r shrinks, v goes up
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The combined effect of fast rotation and a strong magnetic field gives rise to what are known as pulsars The Discovery of Pulsars In 1967, then graduate student Jocelyn Bell observed an astronomical object emitting radio radiation in the form of short, rapid pulses. The cycle was 0.01s pulse, 1.34s of nothing, another 0.01s pulse, and so on. The timing was exceedingly consistent. In 1974, Bell’s thesis supervisor , Anthony Hewish, shared in the Nobel Prize in physics for explaining this strange phenomenon in terms of spinning neutron stars.
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Hewish’s explanation for pulsars is sometimes called the lighthouse model – pulsars have two “hot spots” on their surface, located near their magnetic poles – the spots are heated by charged particles which are accelerated to very high energies by the star’s rotating magnetic field – the hot spots emit intense radio radiation essentially constantly – if the neutron star happens to be oriented such that one or both of the hot spots point toward Earth during its rotation, we see a series of pulses as the radio emission sweeps over us Pattern of radio emission from a pulsar.
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blue), infrared (red), and X-ray (light blue in the centre). This one exploded in 1054 A.D. and was
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17. Neutron Stars and Black Holes - Neutron Stars and Black...

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