lecture22_08 - Lecture 22 Neutron stars Black Holes...

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Lecture 22 Neutron stars Black Holes Galaxies
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TA office hours after class today: Q. 1-2 : AB221 Q. 3-4 : MP1203A (and pickup) Q. 5-6 : MP 1416 Tutorial/Help sessions: Thurs. 6 th , Thurs. 11 th , Mon.15 th all in Koffler KP108, 4-5pm.
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Neutron Stars Theory: Collapse of a Massive Star 1. A hypothesis worked out in the 1930s predicted that after the mass of a star’s core increased beyond the Chandrasekhar limit, the star will collapse further and its electrons and protons will combine to form neutrons. 2. A neutron star is a star that has collapsed to the point at which it is supported by neutron degeneracy.
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Theory: Collapse of a Massive Star (cont.) 3. The diameter of a typical neutron star is only 0.2% of the diameter of a white dwarf and the neutron star is a billion times denser than a white dwarf. 4. Neutron stars have masses between 1.4 and about 3.2 solar masses. The mass of a typical neutron star is 1.5 solar masses, its diameter is 20 km (width of small city), its density is 10 15 g/cm 3 , a nd its temperature is 10,000,000 K.
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Observation: The Discovery of Pulsars 1. In 1967 Jocelyn Bell discovered an unknown source of rapidly pulsating radio waves. Subsequent discoveries of similar sources gave rise to the name pulsar. 2. A pulsar is a pulsating radio source with a regular period (between a millisecond and a few seconds) believed to be associated with a rapidly rotating neutron star.
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Discovery of pulsars: radio signal intensity
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Observation: The Discovery of Pulsars (cont.) 3. The first few pulsars discovered had pulse duration of about 0.001 second. Objects that emit pulsing signals with such duration cannot have a diameter any greater than 0.001 light-seconds, which is 300 kilometers. 4. Such a small size ruled out white dwarfs (Earth-sized objects), leaving the hypothesized neutron star as the explanation for pulsars.
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Illustration of light-time effect due to size of object
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1. A few months after the discovery of the first pulsar, a pulsar was discovered in the Crab Nebula with a period of 0.033 second (i.e., it blinks 30 times a second). 2. The Crab pulsar emits radiation in all parts of the spectrum, from radio waves to X-rays. Its total energy output is more than 100,000 times that of the Sun. Fig. 15-15a Observation: The Crab Pulsar and Others
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Moderately Massive Stars— Conclusion 1. We know that neutron stars have masses more than 1.4 solar masses but we can only estimate that the upper limit is between 2 and 4 solar masses. 2. The evolution of a moderately massive star takes it through the steps shown in Fig. 15-18 on the right. Fig. 15-18 +
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1. Very massive stars differ from moderately massive stars primarily in what happens to them when their core is compressed to a density greater than electron degeneracy can support. In a moderately massive star, the resulting
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lecture22_08 - Lecture 22 Neutron stars Black Holes...

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