Neutron Stars and Black Holes

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Neutron Stars and Black Holes

• Physical Astronomy
• Professor Lee Carkner
• Lecture 19

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Question

• Should there be a lower limit for the mass of
  observed white dwarfs? Why or why not?

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Neutron Stars

• Proposed in 1934 (after the discovery of the
  neutron)
• Very small (10 km), and thus very low luminosity
• rapidly rotating neutron star produces beamed
  radio emission

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Neutron Degeneracy

• Neutron degeneracy pressure only kicks in at
  nuclear densities
• Star is like a big ball of 1057 nucleons
• Also follows a mass X volume constant
  relationship
• Mass limit of 3 Msun

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Neutron Star Formation

• Process hard to model
• At high densities electrons become relativistic
  and combine with protons to produce neutrons (and
  neutrinos)
• As density increases neutrons drip outside of
  nuclei

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Neutron Star Structure

• inner crust of heavy nuclei and free neutrons
• interior mostly neutrons
• maybe a core of sub-nuclear particles?

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Rotation

• Ratio of initial and final periods
• Pf/Pi (Rf/Ri)2
• Periods milliseconds

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Flux Freezing

• Magnetic fields get frozen into core material
  and concentrated as core shrinks
• Bf/Bi (Ri/Rf)2
• Again, hard to know initial core magnetic field
• B 108 T

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Pulsars

• P 1 sec
• Only something very small and compact could
  change that fast
• from asymmetrical supernova
• Can be found in the center of SNR

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Pulsar Model

• Changing magnetic field produces magnetic dipole
  radiation
• If the cone intersects the Earth, we see the
  radio pulse
• Energy is drawn from rotation and the pulsar
  slows down over time

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Black Hole

• Above 3 Msun, core must form a black hole
• Point occurs at the Schwarzschild radius
• RS 2GM/c2
• Marks the event horizon
• At the center is the singularity
• Even outside of the event horizon, tidal forces
  are very strong
• Material nearing a black hole is violently ripped
  apart
• Can heat up material causing it to emit

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Rotation

• Maximum angular momentum is
• Lmax GM2/c
• May cause frame dragging of local spacetime

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Types of Black Holes

• 3-15 Msun, stellar remnant black holes
• 100-1000 Msun intermediate mass black holes
• possible explanation for superbright X-ray
  sources
• 105-109 Msun, supermassive black holes
• Create AGN when active, hard to find if not active

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NS and BH Binaries

• Called an X-ray binary
• If the mass of the compact object is greater than
  3 Msun, it is a black hole
• More than anything else but annihilation

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Types of Binaries

• X-ray pulsar
• Matter falls onto pulsar, heating it up to X-ray
  temperatures (107 K)
• X-ray hot spot may be eclipsed
• Mass transfer may spin-up the pulsar, decreasing
  the period
• X-ray burster
• If the magnetic field is too week the material
  will build up in a layer on the surface

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Next Time

• Read 24.2-24.4
• Homework 17.11, 17.17, 17.18, 24.15, 24.32