Copyright © 2009 Pearson Education, Inc.Chapter 13The Bizarre Stellar GraveyardCopyright © 2009 Pearson Education, Inc.Cosmic Race• Graininess of the Unvierse is at 2x 10-33 cm• If string theory is true, thereshould be a difference in arrivaltime of photons• δt ~ 0.8s• Not enough lagCopyright © 2009 Pearson Education, Inc.White Dwarf• 106 gm cm-3, or 1 ton per cm-3• Chandrasehkar mass 1.4M• As M increases, R decreases• At 1.4M, the star’s C&Osuddenly explode in athermonuclear runawayCopyright © 2009 Pearson Education, Inc.13.2 Neutron StarsOur goals for learning:• What is a neutron star?• How were neutron stars discovered?• What can happen to a neutron star in a closebinary system?Copyright © 2009 Pearson Education, Inc.What is a neutron star?Copyright © 2009 Pearson Education, Inc.A neutron staris the ball ofneutrons leftbehind by amassive-starsupernova.The degeneracypressure ofneutronssupports aneutron staragainst gravity.Copyright © 2009 Pearson Education, Inc.Electron degeneracypressure goes awaybecause electronscombine with protons,making neutrons andneutrinos.Neutrons collapse to thecenter, forming aneutron star.Copyright © 2009 Pearson Education, Inc.A neutron star is about the same size as a small city.Copyright © 2009 Pearson Education, Inc.Neutron star• Density is around 4 x 1014 gm cm-3• Density is the same as atomic nucleus• Or take every human and pack them up into 1 cm-3• Because of GR, the weight of a ns is greater thanits mass, and you can see more than 1/2 of it.Copyright © 2009 Pearson Education, Inc.Discovery of Neutron Stars• Using a radio telescope in 1967, JocelynBell noticed very regular pulses of radioemission coming from a single part ofthe sky.• The pulses were coming from a spinningneutron star—a pulsar.– And the Nobel prize went to Hewishand Ryle…Copyright © 2009 Pearson Education, Inc.Pulsar at centerof Crab Nebulapulses 30 timesper secondCopyright © 2009 Pearson Education, Inc.X rays Visible lightCrab Nebula Movie-CHANDRACopyright © 2009 Pearson Education, Inc.PulsarsA pulsar is aneutron star thatbeams radiationalong a magneticaxis that is notaligned with therotation axis.Copyright © 2009 Pearson Education, Inc.PulsarsThe radiationbeams sweepthrough space likelighthouse beams asthe neutron starrotates.Copyright © 2009 Pearson Education, Inc.Neutron Star Limit• Quantum mechanics says that neutrons in thesame place cannot be in the same state.• Neutron degeneracy pressure can no longersupport a neutron star against gravity if its massexceeds about 3MSun.Copyright © 2009 Pearson Education, Inc.What have we learned?• What is a neutron star?— A ball of neutrons left over from a massivestar supernova and supported by neutrondegeneracy pressure• How were neutron stars discovered?— Beams of radiation from a rotating neutronstar sweep through space like lighthousebeams, making them appear to pulse.— Observations of these pulses were the firstevidence for neutron stars.Copyright © 2009 Pearson Education, Inc.13.3 Black Holes: Gravity’s UltimateVictoryOur goals for learning:• What is a black hole?• What would it be like to visit a black hole?• Do black holes really exist?Copyright © 2009 Pearson Education, Inc.What is a black hole?Copyright © 2009 Pearson Education, Inc.A black hole is an object whose gravity is sopowerful that not even light can escape it.Some massive star supernovae can make a blackhole if enough mass falls onto the core.What is a black hole?Copyright © 2009 Pearson Education, Inc.Escape Velocityinitial kineticenergyfinal gravitationalpotential energy==(escape velocity)2 G × (mass)2 (radius)Copyright © 2009 Pearson Education, Inc.Light wouldnot be able toescape Earth’ssurface if youcould shrink itto <1 cm.Relationship Between Escape Velocity and Planetary RadiusCopyright © 2009 Pearson Education, Inc.Surface of a Black Hole• The “surface” of a black hole is the radius at whichthe escape velocity equals the speed of light.• This spherical surface is known as the event horizon.• The radius of the event horizon is known as theSchwarzschild radius.Copyright © 2009 Pearson Education, Inc.The event horizon of a 3MSun black hole is also aboutas big as a small city.3 MSun black holeNeutron starCopyright © 2009 Pearson Education, Inc.A black hole’s massstrongly warpsspace and time inthe vicinity of theevent horizon.Spacetime, Mass,Radius and OrbitsCopyright © 2009 Pearson Education, Inc.No Escape• Nothing can escape from within the eventhorizon because nothing can go faster than light.• No escape means there is no more contact withsomething that falls in. It increases the hole’smass, changes its spin or charge, but otherwiseloses its identity.Copyright © 2009 Pearson Education, Inc.Singularity• Beyond the neutron star limit, no known force canresist the crush of gravity.• As far as we know, gravity crushes all the matter intoa single point known as a singularity.Copyright © 2009 Pearson Education, Inc.What would it be like to visit ablack hole?Copyright © 2009 Pearson Education, Inc.If the Sun shrankinto a black hole, itsgravity would bedifferent only nearthe event horizon.Black holes don’t suck!Copyright © 2009 Pearson Education, Inc.Light waves take extra time to climb out of a deep hole inspacetime, leading to a gravitational redshift.Copyright © 2009 Pearson Education, Inc.Time passes more slowly near the event horizon.Copyright © 2009 Pearson Education, Inc.Thought QuestionIs it easy or hard to fall into a black hole?A. EasyB. HardCopyright © 2009 Pearson Education, Inc.Thought QuestionIs it easy or hard to fall into a black hole?A. EasyB. Hard(Hint: A black hole with the same mass as the Sunwouldn’t be much bigger than a college campus.)Copyright © 2009 Pearson Education, Inc.Tidal forces near theevent horizon of a3MSun black holewould be lethal tohumans.Tidal forces would begentler near asupermassive blackhole because its radiusis much bigger.Copyright © 2009 Pearson Education, Inc.Do black holes really exist?Copyright © 2009 Pearson Education, Inc.Black Hole Verification• Need to measure mass— Use orbital properties of companion— Measure velocity and distance of orbiting gas• It’s a black hole if it’s not a star and its massexceeds the neutron star limit (~3MSun).Copyright © 2009 Pearson Education, Inc.Some X-ray binaries contain compact objects of massexceeding 3MSun that are
View Full Document
Unlocking...