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Death of Stars III Physics 113 Goderya Chapter(s): 14 Learning Outcomes:
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Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point – a singularity: => A Black Hole!
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Escape Velocity Velocity needed to escape Earth’s gravity from the surface: v esc ≈ 11.6 km/s. v esc Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. v esc If you could compress Earth to a smaller radius => higher escape velocity from the surface.
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The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: V esc = c R s = 2GM____ c2c2 R s is called the Schwarzschild Radius. G = Universal const. of gravity M = Mass
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Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light We have no way of finding out what’s happening inside the Schwarzschild radius. => nothing (not even light) can escape from inside the Schwarzschild radius “Event horizon”
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Schwarzschild Radius of Black Hole (SLIDESHOW MODE ONLY)
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Black Holes in Supernova Remnants Some supernova remnants with no pulsar / neutron star in the center may contain black holes.
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Schwarzschild Radii
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“Black Holes Have No Hair” Matter forming a black hole is losing almost all of its properties. Black Holes are completely determined by 3 quantities: Mass Angular Momentum (Electric Charge)
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General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the BH will be stretched vertically (tidal effects) and squeezed laterally.
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General Relativity Effects Near Black Holes (2) Time dilation Event Horizon Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the BH): Clocks closer to the BH run more slowly. Time dilation becomes infinite at the event horizon.
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General Relativity Effects Near Black Holes (3) Gravitational Red Shift Event Horizon All wavelengths of emissions from near the event horizon are stretched (red shifted). Frequencies are lowered.
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Observing Black Holes No light can escape a black hole => Black holes can not be observed directly. If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity. Mass > 3 M sun => Black hole!
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End States of Stars (SLIDESHOW MODE ONLY)
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Candidates for Black Hole Compact object with > 3 M sun must be a black hole!
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Compact Objects with Disks and Jets Black holes and neutron stars can be part of a binary system. => Strong X-ray source! Matter gets pulled off from the companion star, forming an accretion disk. Heats up to a few million K.
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X-Ray Bursters Several bursting X-ray sources have been observed: Rapid outburst followed by gradual decay Repeated outbursts: The longer the interval, the stronger the burst
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The X-Ray Burster 4U 1820-30 In the cluster NGC 6624 Optical Ultraviolet
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Black-Hole vs. Neutron-Star Binaries Black Holes: Accreted matter disappears beyond the event horizon without a trace. Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the neutron star surface.
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Black Hole X-Ray Binaries Strong X-ray sources Rapidly, erratically variable (with flickering on time scales of less than a second) Sometimes: Quasi-periodic oscillations (QPOs) Sometimes: Radio-emitting jets Accretion disks around black holes
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Radio Jet Signatures The radio jets of the Galactic black- hole candidate GRS 1915+105
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Model of the X-Ray Binary SS 433 Optical spectrum shows spectral lines from material in the jet. Two sets of lines: one blue-shifted, one red-shifted Line systems shift back and forth across each other due to jet precession
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Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-rays Later discovered with X-ray and optical afterglows lasting several hours – a few days GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars.
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