Black Holes Over the top? What if the remnant core is very massive? M core > 2-3 M sun (original star had M > 18 M sun ) Neutron degeneracy pressure fails. Nothing can stop gravitational collapse. Collapses to zero radius and infinite density. Becomes a Black Hole. Gravity s Final Victory Core would collapse into a singularity, an object with zero radius infinite density Black Holes Close to a singularity: Gravity is so strong that nothing, not even light, can escape. Infalling matter is shredded by powerful tides and crushed to infinite density. Becomes a Black Hole Black : neither emits nor reflects light Hole : nothing entering can ever escape Near Black holes escape speed is greater than the speed of light Einstein s Gravity Space tells matter how to move. Matter tells space how to curve
Einstein s General Theory of Relativity Gravity is a result of curvature in the fabric of space-time. Mass tells the space-time how to curve. Larger the mass, larger the curvature. Light also needs to follow the curved path near a massive object light bending Newton vs. Einstein Newton s laws of gravity work in everyday experiences. Einstein s General Relativity needs to be taken into account only in the case of -- extremely strong gravity -- speeds close to c. -- extremely high accuracy Schwarzschild Radius Light cannot escape from a Black Hole if it comes from a radius less than the Schwarzschild Radius: R S = 2GM 2 c M = Mass of the Black Hole G = Gravitational constant For M = 1 M sun, R S 3 km
Neutron Star vs. Black Hole Event Horizon Manhattan Neutron Star M=1.5 M sun R=10 km Black Hole M=1.5 M sun R S =4.5 km R S defines the Event Horizon surrounding the singularity: Events occurring inside R S are invisible to the outside universe. Anything closer than R S can never leave. Hides the singularity from the outside universe. The Point of No Return for a Black Hole Journey to a Black Hole: A Thought Experiment Two observers: Jack & Jill Jack, in a spacesuit, is falling into a black hole. He carries a laser beacon that flashes a beam of blue light once a second Jill is orbiting the black hole in a starship at a safe distance in a stable circular orbit. She watches Jack fall in by monitoring the flashes from his laser beacon. Jack Laser Flash Jill He Said, She Said From Jack s point of view: Sees the ship getting further away. Flashes his blue laser once a second by his watch. From Jill s point of view: Each flash takes longer to arrive, and is redder and fainter than the one before it. Near the Event Horizon... Jack Sees: His blue laser flash every second by his watch The outside world looks distorted Jill Sees: Flashes are redshifted to radio wavelengths Flashes are coming with longer intervals
Jill s View from far away Jack s view from 10R S Down the hole... Jill Sees: Sees one last laser flash after a long delay Flash is faint and at long radio wavelengths She never sees another flash from Jack Jack Sees: Gets shredded by strong tides near the singularity and crushed to infinite density. Jill s Conclusions: The powerful gravity of a black hole warps space and time around it: Time appears to stand still at the event horizon as seen by a distant observer. [Time dilation] Time flows as it always does as seen by an infalling astronaut. Light emerging from near the black hole is Gravitationally Redshifted to longer (red) wavelengths Jack s Conclusions: I wish I stayed back home! Gravitational redshift Photons loose energy climbing out of the gravitational well
Gravity around Black Holes Far away from a black hole: Gravity is the same as for a star of the same mass. If the Sun became a black hole, the planets would all orbit the same as before. Close to a black hole: R < 3 R S, no stable orbits - all matter gets sucked in. At R = 1.5 R S, photons orbit in a circle! Seeing what can t be seen Artist s Conception of an X-Ray Binary Q: If no light gets out of a black hole, how can we ever hope to find one? A: Look for the effects of their gravity on their surroundings: A star orbiting around an unseen massive companion. X-rays emitted by gas heated to extreme temperatures as it falls into the hole. X-Ray Binaries Bright, variable X-ray sources identified by X-ray observatory satellites: Spectroscopic binary with only one set of spectral lines - the companion is invisible. Gas from the visible star is dumped on the companion, heats up, and emits X-rays. Estimate the mass of the unseen companion from the orbit. Black Hole Candidates X-ray binaries with unseen companions of mass > 3 M sun, too big for a Neutron Star. Candidates: Cygnus X-1: M = 6-10 M sun V404 Cygni: M > 6 M sun LMC X-3: M = 7-10 M sun None are as yet iron-clad cases.
Black hole at the Galactic center Black Holes are not (totally) Black Classical General Relativity says: Black Holes are totally black Can only grow in mass and size Last forever (nothing gets out once inside) But, General Relativity does not include the effects of Quantum Mechanics. Hawking Radiation Stephen Hawking asked what quantum effects would occur near the event horizon of a Black Hole. Showed that: Black holes slowly leak particles Each particle carries off a little of the black hole s mass The smaller the mass of the black hole, the faster it leaks. Evaporating Black Holes Black Holes evaporate slowly by emitting Hawking Radiation. Rate is VERY slow: A 3 M sun black hole would require about 10 63 years to completely evaporate. ~10 53 times the present age of the Universe. Mini black holes (mountain sized) would just now be evaporating in a burst of gamma rays