Chapter 13 The Bizarre Stellar Graveyard Copyright 2009 Pearson Education Inc Cosmic Race Copyright 2009 Pearson Education Inc Graininess of the Unvierse is at 2 x 10 33 cm If string theory is true there should be a difference in arrival time of photons t 0 8s Not enough lag 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 O suddenly explode in a thermonuclear runaway Copyright 2009 Pearson Education Inc 13 2 Neutron Stars Our goals for learning What is a neutron star How were neutron stars discovered What can happen to a neutron star in a close binary system Copyright 2009 Pearson Education Inc What is a neutron star Copyright 2009 Pearson Education Inc A neutron star is the ball of neutrons left behind by a massive star supernova The degeneracy pressure of neutrons supports a neutron star against gravity Copyright 2009 Pearson Education Inc Electron degeneracy pressure goes away because electrons combine with protons making neutrons and neutrinos Neutrons collapse to the center forming a neutron 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 than its 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 Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky The pulses were coming from a spinning neutron star a pulsar And the Nobel prize went to Hewish and Ryle Copyright 2009 Pearson Education Inc Pulsar at center of Crab Nebula pulses 30 times per second Copyright 2009 Pearson Education Inc X rays Crab Nebula Movie CHANDRA Copyright 2009 Pearson Education Inc Visible light Pulsars A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis Copyright 2009 Pearson Education Inc Pulsars The radiation beams sweep through space like lighthouse beams as the neutron star rotates Copyright 2009 Pearson Education Inc Neutron Star Limit Quantum mechanics says that neutrons in the same place cannot be in the same state Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3MSun Copyright 2009 Pearson Education Inc What have we learned What is a neutron star A ball of neutrons left over from a massive star supernova and supported by neutron degeneracy pressure How were neutron stars discovered Beams of radiation from a rotating neutron star sweep through space like lighthouse beams making them appear to pulse Observations of these pulses were the first evidence for neutron stars Copyright 2009 Pearson Education Inc 13 3 Black Holes Gravity s Ultimate Victory Our 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 What is a black hole A black hole is an object whose gravity is so powerful that not even light can escape it Some massive star supernovae can make a black hole if enough mass falls onto the core Copyright 2009 Pearson Education Inc Escape Velocity initial kinetic energy final gravitational potential energy escape velocity 2 G mass 2 radius Copyright 2009 Pearson Education Inc Light would not be able to escape Earth s surface if you could shrink it to 1 cm Relationship Between Escape Velocity and Planetary Radius Copyright 2009 Pearson Education Inc Surface of a Black Hole 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 The radius of the event horizon is known as the Schwarzschild radius Copyright 2009 Pearson Education Inc Neutron star 3 MSun black hole The event horizon of a 3MSun black hole is also about as big as a small city Copyright 2009 Pearson Education Inc A black hole s mass strongly warps space and time in the vicinity of the event horizon Spacetime Mass Radius and Orbits Copyright 2009 Pearson Education Inc No Escape Nothing can escape from within the event horizon because nothing can go faster than light No escape means there is no more contact with something that falls in It increases the hole s mass changes its spin or charge but otherwise loses its identity Copyright 2009 Pearson Education Inc Singularity 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 Copyright 2009 Pearson Education Inc What would it be like to visit a black hole Copyright 2009 Pearson Education Inc If the Sun shrank into a black hole its gravity would be different only near the event horizon Black holes don t suck Copyright 2009 Pearson Education Inc Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift Copyright 2009 Pearson Education Inc Time passes more slowly near the event horizon Copyright 2009 Pearson Education Inc Thought Question Is it easy or hard to fall into a black hole A Easy B Hard Copyright 2009 Pearson Education Inc Thought Question Is it easy or hard to fall into a black hole A Easy B Hard Hint A black hole with the same mass as the Sun wouldn t be much bigger than a college campus Copyright 2009 Pearson Education Inc Tidal forces near the event horizon of a 3MSun black hole would be lethal to humans Tidal forces would be gentler near a supermassive black hole because its radius is 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 mass exceeds the neutron star limit 3MSun Copyright 2009 Pearson Education Inc Some X ray binaries contain compact objects of mass exceeding 3MSun that are likely to be black holes Copyright 2009 Pearson Education Inc One famous X ray binary with a likely black hole is in the constellation Cygnus Copyright 2009 Pearson Education Inc What have we learned What is a black hole A black hole is a massive object whose radius is so small that the escape
View Full Document
Unlocking...