white dwarfs cool off and grow dimmer with time
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White dwarfs cool off and grow dimmer with time. The White Dwarf Limit. A white dwarf cannot be more massive than 1.4 M Sun , the white dwarf limit (or Chandrasekhar limit ). Star that started with less mass gains mass from its companion - PowerPoint PPT PresentationTRANSCRIPT
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White dwarfs cool off and grow dimmer with time
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The White Dwarf Limit
• A white dwarf cannot be more massive than 1.4MSun, the white dwarf limit (or Chandrasekhar limit)
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Star that started with less mass gains mass from its companion
Eventually the mass-losing star will become a white dwarf
What happens next?
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Nova• The temperature of
accreted matter eventually becomes hot enough for hydrogen fusion
• Fusion begins suddenly and explosively, causing a nova
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Two Types of Supernova
Massive star supernova:
Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing explosion
White dwarf supernova:
Carbon fusion suddenly begins as whitedwarf in close binary system reaches white dwarf limit, causing total explosion
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One way to tell supernova types apart is with a light curve showing how luminosity changes with time
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A neutron star is the ball of neutrons left behind by a massive-star supernova
Degeneracy pressure of neutrons supports a neutron star against gravity
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Electrons combine with protons, making neutrons and neutrinos
Neutrons collapse to the center, forming a neutron star
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A neutron star is about the same size as a small city
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How were neutron stars discovered?
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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
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Pulsar at center of Crab Nebula pulses 30 times per second
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Pulsars• A pulsar is a
neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis
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Pulsars• The radiation beams
sweep through space like lighthouse beams as the neutron star rotates
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Why Pulsars must be Neutron Stars
Circumference of NS = 2π (radius) ~ 60 km
Spin Rate of Fast Pulsars ~ 1000 cycles per second
Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS
Anything else would be torn to pieces!
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What can happen to a neutron star in a close binary system?
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Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary
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Accreting matter adds angular momentum to a neutron star, increasing its spin
Episodes of fusion on the surface lead to X-ray bursts
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A black hole is an object whose gravity is so powerful that not even light can escape it.
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Supernovae/Supernova Remnants
• Massive stars fuse heavier elements, up to Iron (Fe)
• Core is billions of Kelvin and greater than Chandrasekhar Limit (1.4 Msun)
• Rapid collapse to neutron star
• Rebound of core results in expulsion of outer layers Supernova Remnant
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Before/After!
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Tycho SNR (1572)
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Supernova 1987A (light took 170,000 years to get here!)