Chapter23 Neutron Stars

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Neutron Star and Pulsars Chapter 18.2
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Chapter 18.2 Neutron Stars (NS)Road Map

• History
• Neutron stars were predicted in 1933 (Zwicky and
  Baade)
• Discovered accidentally in 1967 (Bell and Hewish)
  as radio pulsars
• Origin
• Neutron stars are the remnants of massive stars
  after they explode as supernovae (pre-SN mass M
  gt3 Msun)
• Final NS remnant has a mass between 1.4 Msun and
  3Msun.
• Composition
• NS extremely small (30km diameter) and dense
  (100 million tons per cm3 !)
• Interior is mostly degenerate neutrons in a
  superconducting superfluid
• Intense gravitational force balanced by
  degenerate neutron gas pressure
• There is a thin (500 m) solid crust on surface
• Intense magnetic field implies currents, so must
  have protons and electrons
• All NS are rapid rotators (Plt10sec) due to
  angular momentum conservaton
• Some NS are in binary systems
• Mass transfer can produce an accretion disk,
  observed in x-rays
• When helium on surface reaches 3107 K, fusion
  produces x-ray burst

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Road Map for Pulsars

• All pulsars are neutron stars, but are all NS
  pulsars?
• At least 1,000 known (but only 3 in SN remnants)
• Lighthouse (oblique rotator) model for emission
• Spin axis, magnetic field axes are misaligned
• Particles (produced by pair production) radiate
  in narrow cone along magnetic field axis (beamed
  radiation)
• Radiation is most commonly detected as radio
  emission , but sometimes optical, x-ray youngest
  pulsars
• Pulsar rotation periods
• Range from 0.0015 sec to 3 sec (most gt 0.1 sec)
• Most are slowing down (rotational energy -gt
  radiation)
• Hence spin period gt age (slower older)
• Occasionally observer timing glitch results
  from starquake
• Millisecond pulsars
• Found mostly in binary systems
• Old pulsars but fast rotation paradox?
• Solution spin up angular momentum transfer
  from companion

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HST image of an isolated neutron star
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Structure of a Neutron star
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For a neutron star to have a magnetic field, it
must also have a few protons scattered throughout
to create the magnetic field.
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History Fritz Zwicky (and W. Baade) propose the
existence of neutron stars in 1933.

• The atoms in normal, every day objects are
  separated by electrons in adjacent atoms.
• Called degenerate electron pressure.
• Astronomers Zwicky and Baade proposed that a tiny
  stellar core crushed by a supernova could be
• supported by degenerate neutron pressure.
• Such tiny stars are called neutron stars.
• A star compacted to neutron density would be very
  small.
• A 1 solar mass would be only 30 km across.

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SidebarFritz Zwicky A brilliant, but peculiar
astrophysicist and mountain climber
Zwicky (L), Reichstein (R)
"Non-alpinists again and again ask, why we run up
the mountains like mad. Many answers have been
given to this question the greatness of nature,
to use the forces of the body, to run away from
the daily life or the joy of the adventure
(Schiller "If you don't risk your life, you will
never win the life") and so on. But never, I
heard the answer which applied to me and my
mountain friend Reichstein from the university of
Basel, which is "In daily life, as well as in
science, one finds almost never problems, which
one can solve alone, complete and in a short
time. Also if one tackles them successfully,
there are always new aspects popping up, which
occupy us for a long time, sometimes for our
whole life. We are eager therefore to pursue
achievements, which can be closed as a
masterpiece, which can be done alone and which
nobody will question. The climb of a new mountain
or a new difficult path in the mountains is such
an achievement".
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The discovery of pulsars in the 1960s stimulated
interest in neutron stars.

• First detected in 1967 by Cambridge University
  graduate student Jocelyn Bell.
• Radio source with an regular on-off-on cycle of
  exactly 1.3373011 seconds.

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Film Excerpt Discovery of Pulsars by Bell
Hewish
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The discovery of pulsars in the 1960s stimulated
interest in neutron stars.

• First detected in 1967 by Cambridge University
  graduate student Jocelyn Bell.
• Radio source with an regular on-off-on cycle of
  exactly 1.3373011 seconds.
• Some scientists speculated that this was evidence
  of an alien civilizations communication system
  and dubbed the source LGM
• Little Green Men
• Today, we know pulsars are rapidly spinning
  neutron stars.

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Pulsars are rapidly rotating neutron stars with
intense magnetic fields.

• Early ideas about pulsars were that there were
  pulsating white dwarfs however, even a white
  dwarf (Earth-sized) is too big to oscillate in
  less than one second.
• When the Crab Pulsar was detected at the center
  of the Crab Nebula supernova remnant, astronomers
  knew pulsars had to be related to supernovae and
  the stellar core crushed to neutron degeneracy.

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Pulsar model
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Radio images showing a stellar core (pulsar)
ejected during the supernova.
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PRS EC The physical size of a neutron star is
closest to

• 1 km
• 10 km
• 1,000 km
• Size of Earth
• Size of Sun

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PRS EC Which statement about pulsars is
incorrect?

• All pulsars are neutron stars
• Pulsars are remnants of supernova
• Pulsar occasionally have starquakes
• All neutron stars become pulsars
• Some pulsars spin many time per second

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Pulsars gradually slow down as they radiate
energy into space.

• The Crab Pulsar is slowing 3 x 10-8 seconds per
  day.
• Electrons moving in a circular path at enormously
  high speed release energy in the form of
  synchrotron radiation.
• The age of a pulsar can be measured by how fast
  it is currently spinning.

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Superfluidity and superconductivity are among the
strange properties of neutron stars.

• Current models of neutron stars suggest that
  neutrons stars have a solid crust overlying a sea
  of neutrons that can flow without any friction
  whatsoever, called superfluidity.

• In addition to a general slowing of pulsars over
  time, they sometimes exhibit a sudden speed-up
  called a glitch caused by an instability in a
  slowing crust but a still rapidly rotating
  interior.

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The fastest pulsars were probably created by mass
transfer in close binary systems.
Astronomers have cataloged at least 50 super fast
pulsars, called millisecond pulsars, that have
been sped up by mass from a companion star that
hits the neutron star and speeds it up.
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Pulsating X-ray sources are also neutron stars in
close binary systems.
Far more luminous than typical pulsars, pulsating
X-ray sources are close binary systems where the
neutron star in the pair steals matter from its
companion and quickly heats it at its magnetic
pole.
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PRS EC As a spinning pulsar ages, it

• Slows down
• Speeds up
• Remains the same
• Varies periodically
• Depends on size larger pulsars slow down,
  smaller pulsar speed up

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Explosive thermonuclear processes on white dwarfs
and neutron stars produce novae and bursters.

• A NOVA is a sudden brightening and slow dimming
  of a binary star that has rapidly burned off
  excess mass obtained from its companion.

• This can occur repeatedly as the more mature star
  alternately burns off stolen matter and then
  collects more material.

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PRS EC The person who discovered pulsars was

• A graduate student in England
• A professor at Cambridge University
• An Astronomer at Caltech
• An amateur astronomer in New Zealand
• An engineer at NASA

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Like a white dwarf, a neutron star has an upper
limit on its mass.

• White dwarfs will collapse if they exceed the
  Chandrasekhar limit of 1.4 M?
• Neutron star upper mass limits are due to
• Degenerate nature of neutrons.
• Strong nuclear force holding neutrons together.
• If a neutron star exceeds 3 M? then even photons
  cannot escape the stars gravity and the object
  is a bizarre object called a black hole (subject
  of next lecture!)