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Ch 13--Black Holes 16 Nov 2000 ASTR103, GMU, Dr. Correll 1 What do you think? • Are black holes just holes in space? • What exists at the surface of a black hole? • What power or force enables black holes to draw things in? • Do black holes last forever? 16 Nov 2000 ASTR103, GMU, Dr. Correll 2 Beyond Neutron Stars • Neutron degeneracy (a quantum mechanical “gas”) pressure can support neutron stars of mass up to 3 Msun – about 8 km in diameter • For Neutron stars with mass > 3 Msun, neutron degeneracy is overwhelmed and the star collapses • No other forces known which can impede this collapse--apparently, collapse continues to the point of nearly infinite density--a singularity! 16 Nov 2000 ASTR103, GMU, Dr. Correll 3 Beyond Neutron Stars • Regardless of the final endpoint of collapse, a region around the star is created with a gravitational field so strong that nothing, not even electromagnetic radiation (light, radio waves, x-rays, etc) can escape – spacetime curvature is so strong that all paths lead toward the singularity--a black hole has formed! 16 Nov 2000 ASTR103, GMU, Dr. Correll 4 Einstein’s Special Relativity • In 1905 Einstein proposed his new theory of relative motion-special relativity – Your description of reality (physics) is the same regardless of the constant velocity at which you move (an important but common notion) – Regardless of your speed and direction, you always measure the speed of light to be the same (an important but paradigm breaking assumption) 16 Nov 2000 ASTR103, GMU, Dr. Correll 5 Einstein’s Special Relativity • Leads to profound conclusions! – length contraction of moving objects – time dilation of moving clocks – mass of an object increases with speed, requiring infinite energy to reach the speed of light – equivalence of mass and energy: E=mc2 – led to the concept of an integrated “spacetime” which is more fundamental than “space and time” 16 Nov 2000 ASTR103, GMU, Dr. Correll 6 Einstein’s General Relativity • In 1915 Einstein proposed his theory of General Relativity which combined the concepts of special relativity with his new idea of the force of gravity being described as the curvature of spacetime – massive bodies curve the fabric of spacetime – curved spacetime determines how the bodies will move 16 Nov 2000 ASTR103, GMU, Dr. Correll 7 Mass Curving Spacetime • Small mass star produces mild depression or curvature of spacetime • Massive star produces large depression or curvature of spacetime 16 Nov 2000 Curvature of 2-dim space by small mass star Curvature of 2-dim space by large mass star 8 ASTR103, GMU, Dr. Correll Einstein’s General Relativity 16 Nov 2000 ASTR103, GMU, Dr. Correll 9 Einstein’s General Relativity • Confirmation of GR – starlight deflected by the Sun’s gravity – perihelion shift of mercury agrees with GR predictions (Newtonian theory was inaccurate!) – high altitude clocks run slower than clocks on the ground – spectra of some starlight observed to be gravitationally redshifted 16 Nov 2000 ASTR103, GMU, Dr. Correll 10 Einstein’s General Relativity (figure exaggerated) 16 Nov 2000 ASTR103, GMU, Dr. Correll 11 Einstein’s General Relativity 16 Nov 2000 ASTR103, GMU, Dr. Correll 12 Black Holes in Binary Systems • By studying the Dopplershifted spectra of binary stars we can measure the mass • Black hole companions would exhibit strong x-ray emissions as the infalling gas is compressed and heated • Rapid fluctuations in x-ray signal indicate small diameter of source – light travel time 16 Nov 2000 ASTR103, GMU, Dr. Correll 13 Binary Black Holes • Coalescence and Merger of binary black holes would emit enormous amount of gravitational radiation • Laser Interferometry Gravitational Observatory (LIGO) under construction in Washington and Louisiana 16 Nov 2000 ASTR103, GMU, Dr. Correll 14 Black Holes in Binary Systems • Cygnus X-1 discovered by Uhuru xray satellite – subsequent optical observations revealed blue supergiant (with no x-ray emission) – mass of blue supergiant about 30Msun – mass of x-ray source about 7Msun – x-ray fluctuations in hundredths of a second indicate source smaller than earth 16 Nov 2000 ASTR103, GMU, Dr. Correll 15 Supermassive Black Holes • The centers of galaxies appear to contain supermassive black holes--millions of solar masses in size! 16 Nov 2000 ASTR103, GMU, Dr. Correll 16 Minuscule Black Holes? • During the earliest moments of the Big Bang, tiny pockets of matter-energy may have been so dense that they formed minuscule black holes – Theoretically possible, but not yet observed, these small primordial black holes are an open question 16 Nov 2000 ASTR103, GMU, Dr. Correll 17 Inside a Black Hole • The mass of a black hole is concentrated at the singularity • The size of the black hole is defined by the surface defining the trapped interior region form which nothing can escape--the event horizon – The Schwarzschild radius 16 Nov 2000 ASTR103, GMU, Dr. Correll 18 Inside a Black Hole 16 Nov 2000 ASTR103, GMU, Dr. Correll 19 Inside a Black Hole • Spinning black holes, those with angular momentum have additional structure – The singularity is an annular ring – The event horizon is spherical – An larger, oblate spheroid defines the ergosphere where space is dragged around with the spinning black hole 16 Nov 2000 ASTR103, GMU, Dr. Correll 20 Falling into a Black Hole • The tidal forces of gravity near a black hole distort any matter that falls into it 16 Nov 2000 ASTR103, GMU, Dr. Correll 21 Black Holes Evaporate • According to quantum mechanics, the vacuum is actually a sea of particles and antiparticles spontaneously appearing and then annihilating each other • Near the event horizon of a black hole, some virtual particles are trapped while their counterparts escape. Via this Hawking process the black hole loses mass 16 Nov 2000 ASTR103, GMU, Dr. Correll 22 What do you think? • Are black holes just holes in space? – Possible, in theory, but most often they contain highly compressed matter at their centers with additional matter continually being accreted • What exists at the surface of a black hole? – Empty space, no stationary matter exists their. It’s the unmarked boundary between the trapped region and the outside • What power or force enables black holes to draw things in? – Gravity! • Do black holes last forever? – No, they evaporate (but it takes a tens/hundreds of billions of years) 16 Nov 2000 ASTR103, GMU, Dr. Correll 23