Chapter 23 Neutron Stars

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Chapter 23Neutron Stars

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Neutron stars

• Inspired by the discovery of the Neutron in 1932,
  two Astronomers Fritz Zwicky (Clatech) and Walter
  Baade (Mount Wilson Observatory) proposed the
  following -
• When a massive star reaches the end of its life,
  the gravitational contraction can overcome the
  degenerate electron pressure that held the core
  during the last stages of the star.
• Then the electrons and protons can combine under
  this pressure and transform into neutrons.

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Neutron stars

• What would be left is a core that consists of
  closely packed neutrons - A Neutron Star
• Further compression will be resisted by the
  degenerate neutron pressure (due to Pauli
  exclusion principle which prohibits two neutrons
  from occupying the same energy state).
• Since this pressure is much stronger than the
  degenerate electron pressure (that keeps white
  dwarfs intact) it could support more massive
  stellar corpses than white dwarfs.

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Discovery of Neutron Stars(1960s)

• No one took much notice of Zwicky Baades
  postulate of 1930s
• Until a grad. Student at Cambridge university in
  U.K. working on an array of radio antennas
  discovered in 1967 a very regular pulsating radio
  source (period 1.3373011 sec) coming from one
  location in the sky. The came to be known as
  Pulsars.
• Initially, it was thought to be E.T.

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Pulsars are rotating neutron stars

• Astronomers tried to find what pulsars were but
  eventually had to rule out all known candidates
  including white dwarfs.
• Finally astronomers had accepted the more exotic
  idea that pulsars are actually neutron stars.
  
• The discovery of the Crab pulsar in the center of
  the Crab nebula (a supernova remnant) convinced
  astronomers that stellar corpses can take two
  forms -White dwarfs and Neutron stars
• Crab Pulsar Period 0.33 seconds

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Rapidly Rotating Neutron stars

• Why do neutron stars emit radiation at all? Why
  is it pulsed? How can it pulse so rapidly (30
  times per second in the case of Crab Pulsar)
• Neutron star spins rapidly due to conservation of
  angular momentum during shrinking
• When a star many times bigger than our Sun
  collapses to a Neutron star ( 30 km diameter) it
  will rotate extremely rapidly (due to
  conservation of angular momentum).

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Rapidly Rotating Neutron stars

• All stars possesses magnetic fields.
• When a star collapse and form a neutron star, the
  magnetic field surrounding the neutron star
  becomes extremely intense
• One trillion times(1012) that of the Sun.
• So, a neutron star is rapidly spinning dense
  object with an intense magnetic field.
• But, why does it radiate and why does it pulsate?

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Pulsars Rapidly rotating Neutron Stars
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Pulsars Rapidly rotating Neutron Stars

• Consider a neutron star whose magnetic axis is
  inclined to the rotation axis (true for all
  planets in the solar system)
• The magnetic field is so strong that it
  spontaneously creates pairs of electrons and
  positrons(anti-electrons) and these particles are
  pushed into the magnetic fields.
• As these particles spiral along the magnetic
  field lines they emit EM radiation.
• This produces two narrow beams of radiation
  streaming out of the magnetic poles.

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Pulsars Rapidly rotating Neutron Stars

• If the neutron star is oriented in such away that
  one of the beams sweep across Earth, then we will
  detect a brief flash (pulse) as it sweeps across
  Earth
• A radio telescope therefore, will detect regular
  pulses of radiation and the period of the pulses
  equal to the period of rotation.
• This is very similar to how a ship at sea will
  see the beam of light from a lighthouse.

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Pulsars
Crab pulsar
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Pulsars
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Neutron Stars

• Why is the Crab nebula so luminous?
• Usually emission nebulae(or H II regions) shine
  due to the presence of an O or B type star close
  by.
• The Crab pulsar(the neutron star at the center)
  is the energy source that make the Crab nebula
  shine.
• Some of the rotational energy of the neutron
  star must get transferred to the surrounding gas.
• As a result the rate of rotation of the neutron
  star is decreasing - it is slowing down!

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Neutron Stars

• This slowing down is observed in the Crab nebula
• How does this energy transfer take place?
• The answer is Synchrotron Radiation
• Radiation produced when relativistic (high speed)
  electrons ejected by the neutron star (forming
  immense jets at the poles) spiral their way
  through the strong magnetic field.
• Spinning neutron stars slow down as it radiated
  its rotational energy.
• Old neutron stars spin more slowly

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Neutron Stars

• X-Ray image shows where the relativistic
  electrons are ejected from the neutron star.

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Neutron Star Interior

• Although the light coming from pulsars provide
  information as to the surrounding, scientist
  construct theoretical models to study the
  internal structure.

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Neutron Star Interior

• The interior of a neutron star is a sea of
  densely packed degenerate neutrons.
• These neutrons exhibit exotic physical
  properties
• Superfluidity degenerate neutrons can flow
  without any friction.
• Superconductivity electrical currents inside the
  neutron star can travel around without
  experiencing any electrical resistance.
  Therefore, once you start an electrical current
  it keeps going forever. This drives the large
  magnetic field.

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Millisecond Pulsars

• Pulsars with rotation periods in the millisecond
  region (i.e. they spin several hundred times a
  second) are called millisecond pulsars.
• The extremely rapid rotation and the very gradual
  slowing down observed in these pulsars lead to
  one conclusion
• Mass transfers from a close by binary companion
  star(probably a redgiant) feeds matter into the
  neutron star making it spin faster.
• Therefore, a slow aging pulsar can be spun-up
  by mass transfer from its bloated companion.

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Neutron Star Interior

• The interior of a neutron star is a sea of
  densely packed degenerate neutrons.
• These neutrons exhibit exotic physical
  properties
• Superfluidity degenerate neutrons can flow
  without any friction.
• Superconductivity electrical currents inside the
  neutron star can travel around without
  experiencing any electrical resistance.
  Therefore, once you start an electrical current
  it keeps going forever. This drives the large
  magnetic field.

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Pulsars
The Black Widow Pulsar the black widow pulsar is
slowly eating away its companion star by blasting
it with energetic particles.
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Pulsating X-ray sources
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Pulsating X-ray sources

• If material from the companion star is drawn into
  the magnetic poles, then there will be large
  quantities of charged particles spiraling in the
  intense magnetic field
• This will produce intense hot spots that will
  emit copious amounts of X-ray radiation.
• X-ray luminosity of such a Pulsating X-ray source
  is 100,000 time the luminosity of the Sun (over
  all wavelengths)

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Upper mass limit of Neutron Stars

• In Neutron stars the gravity is balanced by two
  forces.
• Degenerate neutron pressure
• Strong nuclear force.
• If the mass of a neutron star has a mass larger
  than the maximum limit of 2 to 3 M? then the
  gravitational collapse will be too strong to be
  resisted by these two forces.
• There is no force in nature known to physicists
  that could stop a massive star from
  collapsing.into a Black Hole !!!