f10_nov16 - Ast 307 - Nov. 16 , 2010 Neutron Stars &...

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Ast 307 - Nov. 16 , 2010 ch. 21 - 22 A neutron star is about the same size as a small city Structure of Neutron Stars Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission from one place in the sky When several similar sources were discovered in known supernova remnants (the Crab Nebula), it was realized that they were coming from a spinning neutron star—a pulsar. The Discovery of Neutron Stars The pulsar at center of the Crab Nebula “flashes” 30 times per second
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Nature of Pulsars A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with its rotation axis The radiation beams sweep around like lighthouse beams, as the neutron star rotates The reason the star rotates so fast is because of conservation of angular momentum. How the Lighthouse Model Works When the beam is briefly pointed at us, the observers, we see a brighter star image than when it has swept past and points in a different direction. A time sequence of pictures looks like this: http://relativity. livingreviews .org/open? pubNo=lrr-1998-10 For an animation, see the link below. Synchrotron Emission Electrons spiral around the magnetic field lines; this “twisting” motion is a form of acceleration, so the electrons radiate photons (strongly at radio wavelengths). Pulsar Slow Downs
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! Evolution of Binary Stars When two stars are in a close binary system, they interfere with each other’s evolution. Any matter in the vicinity feels forces from both stars, in different directions. One can draw a sequence of surfaces with different values of total gravitational energy; the surface where the two stars have equal effects is called the Roche lobe. ! “Roche Lobes” in a Binary The “Roche lobes” can be thought of as the gravity domains of the two stars. They represent positions where the gravity effects of the two stars balance. “A” is the separation of the stars (semi-major axis of the orbit). The star on the left is more massive. Fig. 8 of Iben, Astrophysical Journal Supplements, Vol. 76, page 64. ! Mass Transfer in a Binary
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f10_nov16 - Ast 307 - Nov. 16 , 2010 Neutron Stars &...

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