lecture19_ch13b - Black Holes “Supermassive” Black...

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Unformatted text preview: Black Holes “Supermassive” Black Holes Gamma Ray Bursts What is a black hole? Escape Velocity A black hole is any object whose gravity is so powerful that not even light can escape it. Light would not be able to escape Earthʼ s surface if you could shrink it to < 1 cm Initial Kinetic Energy = (escape velocity)2 = 2 Final Gravitational Potential Energy G (mass) (radius) The “surface” of a black hole is the radius at which the escape velocity equals the speed of light. This spherical surface is known as the event horizon. Escape velocity and radius The radius of the event horizon is known as the Schwarzschild radius. = 3 km (Mass/Msun) 1 No Escape Neutron Star Limit Nothing can escape from within the event horizon because nothing can move faster than light. • Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 Msun • A massive-star supernova can make a black hole if enough mass falls back onto the core after the explosion 3 M Sun Black Hole Neutron Star Beyond the neutron star limit, no known force can resist the crush of gravity. As far as we know, gravity crushes all the matter into a single point known as a singularity. The event horizon of a 3 MSun black hole is also about as big as Lansing Black Holes bend space and time The immense gravity of a black hole strongly warps space and alters time in its vicinity. Einsteinʼ s General Theory of Relativity explains gravity through the warping of space and time. Einsteinʼ s theory explains how gravity works: matter curves space. 2 If the Sun shrank into a black hole, its gravity would be different only near the event horizon Thought Question What would you see if you could move as fast as a beam of light? Black holes donʼt suck! General Relativity: Light signals from person in rear of an accelerating rocket take extra time to catch up with person in front Light pulses take extra time to climb out of a deep hole in spacetime Tidal forces near the event horizon of a 3 M Sun black hole would be lethal to humans Time passes more slowly near the event horizon Tidal forces would be gentler near a supermassive black hole because its radius is much bigger 3 Would falling in to a black hole kill you? Do black holes really exist? Some X-ray binaries contain compact objects of mass exceeding 3 M Sun which are likely to be black holes One famous X-ray binary with a likely black hole is in the constellation Cygnus Is the companion a black hole? No star(s) there Mass > 3 solar masses Too compact to be a star or a cluster of stars. Beware: Analogies are not physics • “left the universe” means no longer in contact with the outside universe, but the mass is still there (it doesn’t go somewhere else). • “bottomless pit” is the shape of spacetime around a black hole, but the curvature of spacetime is caused by the presence of mass, not a “pit”. 4 Galactic center in infrared light Galactic center in radio What lies in the center of our galaxy? (Ch 14.4) Galactic center in radio Strange radio sources in galactic center Strange radio sources in galactic center Stars appear to be orbiting something massive but invisible … a black hole? Stars at galactic center Black hole having a meal? Orbits of stars indicate a mass of about 4 million MSun 5 GRBs are detected from every direction The birth of black holes: gamma-ray bursts? For latest SWIFT map; http://grb.sonoma.edu/ Some gamma ray bursts come from supernovae in very distant galaxies Gamma ray bursts may signal the births of new black holes What would it be like to visit a black hole? Every time gamma ray burst occurs, a black hole is born… 6 Approaching the black hole: light bends around it Moving around the black hole: the images passing closest to the event horizon are distorted the most http://www.astro.psu.edu/users/nnp/grb-amnh07-med.mov SWIFT and Gamma Ray Burst movie from Penn State 7 ...
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This note was uploaded on 11/13/2011 for the course ISP 205 taught by Professor Donahue during the Fall '08 term at Michigan State University.

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