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Lecture 27

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McGill University
PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 Lecture 27: March 20 , 2013  second midterm will be Monday, 6 pm in leacock 132  same length  pulsars: cosmic lighthouses o highly magnetized spinning neutron star o visual result of neutron stars o all neutron stars spin, including pulsars o it sends radiation (charged particles) out of the north and south pole of a neutron star o axis of rotation is misaligned with beams (axis of magnetic fields) o all neutron stars are pulsars to some people, but not to us  must be aligned correctly as they are visual  when you look in the centre of the Crab Nebula you see a pulsar  why are pulsars neutron stars? o circumference is 60 km  size of a city o pulsars rotate at ~1000 times per second o surface rotation velocity ~ 60 000 km/s (or 0.2 speed of light) o anything but a neutron star would be torn apart o why do they spin so fast? conservation of angular momentum  this will continue until the angular momentum is taken away but we don’t have any way to do that  neutron stars exist in binary systems  there are xray bursts from He fusion on the surface o like with white dwarfs, there is an upper limit on the total mass neutron degeneracy pressure can support it  3 solar masses  this means it will collapse again o mass accretion from a companion star forms an accretion disk (hotter & brighter than a white dwarf system) o can estimate the masses by observing it rotating around other masses  black holes o for really massive stars (> 12 solar masses) nothing can stop the gravitational collapse of the supernova o collapses to an object with zero radius & infinite density o black holes are
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