Lecture 31 - Neutron Stars and Pulsars (4/3/96)

Seeds: Chapter 11

1. Supernova Recap
   • The end result of a star's life depends upon the mass of the star:
     • M < 0.01 M_sun: Planet (big Jupiter)
     • 0.01 < M < 0.08 M_sun: Brown Dwarf (failed star)
     • 0.08 < M < 0.25 M_sun: Helium White Dwarf
     • 0.25 < M < 8 M_sun: C-N-O White Dwarf -> planetary nebula
     • 8 < M < 10 M_sun: O-Mg-Ne White Dwarf -> planetary nebula
     • 10 < M < 40 M_sun: Neutron Star -> supernova
     • M > 40 M_sun: Black Hole -> supernova
   • SN1987A began as a 20 M_sun star, evolved to a red giant of luminosity 60000 L_sun, burned increasingly heavy elements in its core, exploded in a supernova, and was observed here on Earth on February 24, 1987.
2. Neutron Stars
   • Neutron stars have typical masses of 1.5 Msun within a radius of 10-15 km.
   • Neutron stars are very hot soon after their creation, but have a tiny surface area and so are very faint.
   • During collapse, conservation of angular momentum causes neutron star to spin up in rotation frequency by the inverse ratio of the radii squared.
   • Also during collapse, the magnetic field is amplified by the ratio of the densities (inverse radius cubed).
   • There is tremendous spin and magnetic energy in the neutron star, not to mention the gravitational energy in the immense surface gravity.
   • Thus, we might hope to detect emission that taps one of these energy sources.
3. Pulsars
   • In 1967, Hewish and Bell discover "pulsed" radio emission from unknown astronomical objects. These are later identified as from rapidly rotating neutron stars. Hewish later received the Nobel Prize for this discovery.
   • These objects are called pulsars after the pulses received once every rotation.
   • The pulses are due to a beam of radiation emitted from the magnetic poles of the pulsar (which like the Earth's are not aligned with the rotational axis) which sweep by the Earth like a searchlight.
   • This radio emission comes from synchrotron radiation from high energy particles caught in the strong magnetic fields coming out of the pole of the pulsar.
   • There is a pulsar in the center of the Crab Nebula. This is what provides the power for the nebula emission, through its magnetic and rotational energy.
   • The Crab pulsar is rotating with a period of 0.033 seconds (!), but is slowing down due to the loss of energy to the nebula. Eventually it will be spinning too slowly to emit much radiation and will disappear from the skies.
   • Currently over 500 pulsars are known, with periods ranging from just over 0.001 seconds to tens of seconds. Pulsars have been seen pulsing at optical, X-ray, and Gamma ray wavelengths. Some are even in binary systems (how did they survive the supernova?).
4. The Structure of Neutron Stars
   • A neutron star is made, basically, of neutrons. It is like a giant solar-mass atomic nucleus!
   • The conditions inside a neutron star are far removed from what we can study in our laboratories here on Earth. There is much interest, theorising, and speculation concerning the internal state of neutron star matter.
   • However, there is some internal structure to the neutron star.
   • The outer kilometer or so is a crust made of heavy nuclei (like iron) and electrons.
   • Below this, there is a zone 3-5 kilometers thick of superfluid neutrons. Superfluidity is a strange state of matter where quantum effects allow bizarre behavior to occur.
   • In most of the central zone of the neutron star, superfluid neutrons are joined by a small fraction of superconducting protons and electrons.
   • Some astrophysicists speculate that at the heart of a neutron star is a core made of an even stranger state of matter.
5. Accretion Disk Radiation
   • The pulsar emission taps into the magnetic and rotational energy of the neutron star. There is also a great deal of energy in the gravitational field also.
   • The surface gravity of the neutron star is so great that a few grams of matter released from 1 AU away will impact the surface with the energy of a several megaton nuclear explosion!
   • Gaseous material gravitationally swept up near a neutron star will be accelerated to high velocities.
   • In addition tidal forces near the neutron star will be strong enough to rip apart solid or gaseous bodies, and disperse the matter into a rotating disk surrounding it.
   • This accretion disk, which is similar to the protostellar disks we studied earlier, will be heated by the gravitational acceleration and tidal forces to high temperatures (millions of degrees K).
   • This high temperature gas will emit X-rays, which can be seen with X-ray telescopes on Earth-orbiting satellites. We have located many neutron star systems this way.
   • We have found a number of neutron-star / normal star binary systems where gas from the normal star has made its way to an accretion disk.
   • Neutron stars are pretty extreme objects, but we believe there are even more extreme objects out there!

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