lecture18_ch13a - White Dwarfs & Neutron Stars Life...

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Unformatted text preview: White Dwarfs & Neutron Stars Life of a 20 M Sun star Life of a 1 MSun star What is a white dwarf? White dwarfs are the cores of dead stars Electron degeneracy pressure supports them against gravity They become dimmer and redder as they cool off • Small: faint, get fainter • Hot: but cools steadily with time. A white dwarf is about the same size as Earth 1 Origin of Degeneracy Pressure Origin of Degeneracy Pressure Exclusion Principle: Two electrons in the same position cannot have the same momentum Exclusion Principle: Electrons (or protons or neutrons) canʼt be pushed together past a certain limit. Uncertainty Principle: An electronʼs momentum and position cannot both be specified with infinite precision Uncertainty Principle: If the space an electron can be is very small, its momentum can be very LARGE. Also true for protons and neutrons The White Dwarf Limit Einsteinʼ s theory of relativity says that nothing can move faster than light. When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can no longer support it. Chandrasekhar found (at age 20!) that this happens when a white dwarfʼs mass reaches 1.4 MSun White dwarfs shrink when you add mass to them S. Chandrasekhar (1910-1995) Stars in Algol are close enough that matter flows from subgiant onto main-sequence star What can happen to a white dwarf in a close binary system? 2 Star that started with less mass gains mass from its companion Eventually the masslosing star will become a white dwarf What happens next? White dwarfʼs gravity pulls matter off of giant companion, but angular momentum prevents matter from falling in Friction in disk makes it hot, causing it to glow Infalling matter forms an accretion disk around white dwarf Friction also removes angular momentum from inner regions of disk, allowing them to sink onto white dwarf Hydrogen that accretes onto a white dwarf builds up in a shell on the surface When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space 3 Two Types of Supernova Supernova that Chinese observed in 1006 A.D. was probably an exploding white dwarf Massive star supernova: Iron core of massive star reaches the white dwarf limit and collapses into a neutron star or BH, causing explosion White dwarf supernova: Carbon fusion suddenly begins as white dwarf in close binary system reaches the white dwarf limit, causing total explosion Which is it: a supernova or a nova? • Supernova are MUCH MUCH more luminous! (about 10 million times) • Nova: H to He fusion of a layer, white dwarf left intact • Supernova: Complete detonation of the entire white dwarf core, nothing left. One way to tell supernova types apart is checking the light curves, showing how luminosity changes with time A neutron star is the ball of neutrons left behind by a massive-star supernova What is a neutron star? Degeneracy pressure of neutrons supports a neutron star against gravity 4 Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos Neutron Star Neutrons collapse to the center, forming a neutron star A neutron star is about the same size as Lansing The first neutron star was discovered by Bell Burnell in 1967 Using a radio telescope she noticed very regular pulses of radio emission coming from a single part of the sky How were neutron stars discovered? Her advisor got the Nobel Prize. J. Bell Burnell Pulsar at center of Crab Nebula pulses 30 times per second Pulsars are neutron stars that give off very regular pulses of radiation 5 Pulsars are rotating neutron stars that act like lighthouses Beams of radiation coming from poles look like pulses as they sweep by Earth A pulsarʼs rotation axis is not aligned with its magnetic poles Pulsar “Sounds” Why Pulsars must be Neutron Stars Pulsar 0329+54 (1.4 pulses/sec): Circumference of NS = 2π (radius) ~ 60 km Pulsar 0833-45 (11.2 pulses/sec): Spin Rate of Fast Pulsars ~ 1000 cycles per second Crab Pulsar (30 pulses/sec): Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS Pulsar 1937+21 (640 pulses/sec): Anything else would be torn to pieces! Pulsars spin fast because coreʼs spin speeds up as it collapses into neutron star Conservation of angular momentum What happens to a neutron star in a close binary system? 6 Accretion disks around neutron stars get hot enough to emit X-rays Sudden fusion events on neutron starʼs surface lead to X-ray bursts (like nova, except on a neutron star) Fastest pulsars get their angular momentum from accreting matter Matter falling onto surface is orbiting at close to speed of light 7 ...
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