6White dwarfs shrink when you add mass to them because their gravity gets stronger. The larger the star the smaller the dwarf
7Shrinkage of White Dwarfs Quantum mechanics says that electrons in the same place cannot be in the same stateAdding mass to a white dwarf increases its gravity, forcing electrons into a smaller spaceIn order to avoid being in the same state some of the electrons need to move fasterIs there a limit to how much you can shrink a white dwarf?
8The White Dwarf LimitEinstein’s theory of relativity says that nothing can move faster than lightWhen electron speeds in white dwarf approach speed of light, electron degeneracy pressure can no longer support itChandrasekhar found (at age 20!) that this happens when a white dwarf’s mass reaches1.4 MSunS. Chandrasekhar
9Hydrogen that accretes onto a neutron star builds up in a shell on the surface When base of shell gets hot enough, hydrogen fusion suddenly begins leading to a nova
10Nova explosion generates a burst of light lasting a few weeks and expels much of the accreted gas into space
11Two Types of Supernova Massive star supernova: Iron core of massive star reacheswhite dwarf limit and collapses into aneutron star, causing explosionWhite dwarf supernova:Carbon fusion suddenly begins as whitedwarf in close binary system reaches white dwarf limit, causing total explosion
12One way to tell supernova types apart is with a light curve showing how luminosity changes
13Nova or Supernova?Supernovae are MUCH MUCH more luminous!!! (about 10 million times)Nova: H to He fusion of a layer, white dwarf left intactSupernova: complete explosion of white dwarf, nothing left behind
14Supernova Type: Massive Star or White Dwarf? Light curves differSpectra differ (exploding white dwarfs don’t have hydrogen absorption lines)
15What have we learned? What is a white dwarf? A white dwarf is the core left over from a low-mass star, supported against the crush of gravity by electron degeneracy pressure.• What can happen to a white dwarf in a close binary system?A white dwarf in a close binary system can acquire hydrogen from its companion through an accretion disk. As hydrogen builds up on the white dwarf’s surface, it may ignite with nuclear fusion to make a nova.
1613.2 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?
28Why Pulsars must be Neutron Stars Circumference of NS = 2π (radius) ~ 60 kmSpin Rate of Fast Pulsars ~ 1000 cycles per secondSurface Rotation Velocity ~ 60,000 km/s~ 20% speed of light~ escape velocity from NSAnything else would be torn to pieces!
29Conservation of angular momentum Pulsars spin fast because core’s spin speeds up as it collapses into neutron starConservation of angular momentumCollapse_solar_nebula.swf
30Thought QuestionCould there be neutron stars that appear as pulsars to other civilizations but not to us?A. YesB. No
31What have we learned? What is a neutron star? A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova.• How were neutron stars discovered?Neutron stars spin rapidly when they are born, and their strong magnetic fields can direct beams of radiation that sweep through space as the neutron star spins. We see such neutron stars as pulsars, and these pulsars provided the first direct evidence for the existence of neutron stars.
3213.3 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?
34A black hole is an object whose gravity is so powerful that not even light can escape it.
35Thought QuestionWhat happens to the escape velocity from an object if you shrink it?A. It increasesB. It decreasesC. It stays the same
36Thought QuestionWhat happens to the escape velocity from an object if you shrink it?A. It increasesB. It decreasesC. It stays the sameHint:
37Thought QuestionWhat happens to the escape velocity from an object if you shrink it?A. It increasesB. It decreasesC. It stays the sameHint:
38Light would not be able to escape Earth’s surface if you could shrink it to < 1 cm Escape_velocity_and_r.swf
39The “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.
40A black hole’s mass strongly warps space and time in vicinity of event horizon
41No EscapeNothing 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 mass, changes the spin or charge, but otherwise loses its identity.
42Neutron Star LimitQuantum mechanics says that neutrons in the same place cannot be in the same stateNeutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3 MsunSome massive star supernovae can make black hole if enough mass falls onto core
43Beyond 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.
44Neutron star 3 MSun Black Hole Fill in your favorite city map here.The event horizon of a 3 MSun black hole is also about as big as a small city
45Thought QuestionHow does the radius of the event horizon change when you add mass to a black hole?A. IncreasesB. DecreasesC. Stays the same
46Thought QuestionHow does the radius of the event horizon change when you add mass to a black hole?A. IncreasesB. DecreasesC. Stays the same
56Black Hole Verification Need to measure massUse orbital properties of companionMeasure velocity and distance of orbiting gasIt’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)
57Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes
58One famous X-ray binary with a likely black hole is in the constellation Cygnus
59What have we learned? What is a black hole? A black hole is a place where gravity has crushed matter into oblivion, creating a true hole in the universe from which nothing can ever escape, not even light.
60What have we learned? What would it be like to visit a black hole? You could orbit a black hole just like any other object of the same mass. However, you’d see strange effects for an object falling toward the black hole:Time would seem to run slowly for the objectIts light would be increasingly redshifted as it approached the black hole.The object would never quite reach the event horizon, but it would soon disappear from view as its light became so redshifted that no instrument could detect it.
61What have we learned? • Do black holes really exist? No known force can stop the collapse of a stellar corpse with a mass above the neutron star limit of 2 to 3 solar masses, and theoretical studies of supernovae suggest that such objects should sometimes form. Observational evidence supports this idea.
6213.4 The Mystery of Gamma-Ray Bursts Our Goals for Learning• What causes gamma ray bursts?
64Gamma ray bursts may signal the births of new black holes
65At least some gamma ray bursts come from supernovae in very distant galaxies
66What have we learned? • What causes gamma ray bursts? Gamma-ray bursts occur in distant galaxies and are the most powerful bursts of energy we observe anywhere in the universe. No one knows their precise cause, although at least some appear to come from unusually powerful supernovae.