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21. Neutron Stars
• Neutron stars were proposed
• Pulsars
in the 1930’s
were discovered in the 1960’s
• Pulsars are rapidly rotating neutron stars
• Pulsars slow down as they age
• Neutron stars are superfluid & superconductive
• The fastest pulsars are in close binary systems
• Pulsating X-ray sources are also neutron stars
• White dwarfs, neutron stars ⇒ Novae, bursters
• Neutron stars have upper mass limits
Neutron Stars Proposed in the 1930s
• The neutron
is discovered
1932
– Discovered by James Chadwick
– Basic properties of the neutron
•
•
•
•
No electrical charge
~ 1,837 times the mass of an electron
Proton mass + Electron mass = Neutron mass
Proton charge + Electron charge = Neutron charge
• The neutron star is proposed
– Hypothesized by Fritz Zwicky & Walter Baade
• Proposed the neutron star as supernovae leftovers
• Two possible stellar corpses
– White dwarfs & neutron stars
• Supported by degenerate neutron pressure
– Basic properties at 1.0 M☉
• Diameter of
~ 30 km
• Escape velocity of ~ 0.5 . c
1934
Pulsars Discovered in the 1960s
• Radio telescope array constructed
1967
– Jocelyn Bell et al. search for random radio twinkling
– Jocelyn Bell et al. discover regular radio pulsing
• Pulses 1.3373011 seconds apart
• Others have periods ranging from ~ 0.25 to ~ 1.5 seconds
• Three possible explanations rejected
– Eclipsing binary stars
• Star edges would have to overlap to orbit fast enough
– Variable stars
• Rapid diameter changes would tear apart any
star
– Rotating white dwarfs with hot spots
• Rapid
rotation rate
would tear apart any white dwarf
• One conclusion accepted
– Radio source must be far smaller than a white dwarf
The Crab Pulsar: “On” & “Off”
Intensity Variations of a Pulsar
Pulsars: Rapidly Rotating Neutron Stars
• Four important questions about neutron stars
– Why do neutron stars emit
any radiation ?
– Why do neutron stars emit radio wavelengths ?
– Why do neutron stars emit pulses of radiation ?
– Why do neutron stars emit
pulses so fast ?
• Important physical aspects of neutron stars
– They are very small
• Their mass exceeds the Chandrasekhar limit
• Their surface area is 10–10 times their ZAMS surface area
– They rotate very fast
• Conservation of angular momentum insures fast rotation
– They have intense magnetic fields
• Magnetic field is 10+10 times their ZAMS magnetic field
• Approximately 1012 Gauss
Different Rotational & Magnetic Axes
• Prior observations
– Sun’s rotational & magnetic axes are not aligned
– No planet’s rotational & magnetic axes are aligned
• Basic physical processes
– No fundamental reason why they should be aligned
– Electric generators
• Rotation in a strong magnetic field
– Neutron stars should be huge electric generators
• Spontaneous production of e– & e+ pairs
• One example of the conversion of energy into mass
– Magnetic field lines accelerate the e– & e+ pairs
• These behave just like a radio antenna
– Narrow radio energy beams leave magnetic poles
• Typically ~ 2° wide
A Pulsar’s Rotational & Magnetic Axes
Pulsar Model
http://en.wikipedia.org/wiki/File:Pulsar_schematic.svg
Pulsars Do Not Pulse; They Rotate!
• Observations
– These objects are observed to blink on & off
– These objects were assumed to turn on & off
• Underlying processes
– These objects actually emit radiation constantly
– These objects behave like a lighthouse beam
• The light is on constantly
• The light is focused into a narrow beam
• The beam of light rotates & is seen only intermittently
Radio-Wavelength View: Crab Nebula
Optical/X-Ray View: The Crab Nebula
© NASA (2002)
Pulsars Emit at Multiple Wavelengths
• Basic physical process
– No fundamental reason to emit only radio l’s
• Wide variety of energy levels exist
• Wide range of l’s should be emitted
• Additional observations
– The Crab pulsar
• X-ray l’s
• Visible l’s
Recorded by orbiting Einstein Observatory
Recorded by Earth-bound optical telescopes
– Other pulsars
• > 1,000 identified since 1968
• > 100,000 probably exist in the Milky Way galaxy
• Probable source
– Type II supernovae
• Core collapse of massive stars
A Pulsar Seen at Three Wavelengths
Ejected Pulsars
• Basic observations
– Many fast-moving pulsars have been identified
• Basic conclusions
– Many Type II supernovae must be asymmetric
• Resultant neutron star cannot remain within the remnant
• Resultant neutron star
penetrates
supernova remnant
Neutron Star Ejected by a Supernova
Pulsars Slow Down As They Age
• Basic observations
– Supernova remnants emit huge amounts of energy
– Rotation rate of neutron stars gradually decreases
• Energy expended is same as energy emitted by remnant
– Many supernova remnants emit unusual blue light
• Synchrotron radiation similar to particle accelerators
• Relativistic electrons in a powerful magnetic field
• Basic physical process
– Energy transferred to electrons from magnetic field
• Similar to the Sun’s coronal heating
Superfluidity & Superconductivity
• Some basic properties of neutron stars
– Thought to have solid crust of ordinary neutrons
– Thought to have fluid interior of degenerate neutrons
• Two special properties of neutron stars
– Superfluidity
• Matter can flow without friction under some conditions
• Convection in a neutron star can continue indefinitely
– Superconductivity
• Currentcan flow without friction under some conditions
• Neutron star’s p+ & e– produce electric currents
• Interactions in a neutron star
– Glitches occur
Sudden rotational speed increase
• Faster spinning fluid core “grabs onto” slowing solid crust
Internal Structure of a Neutron Star
Glitches in Neutron Stars
Protons & Electrons In Neutron Stars
• Basic physical processes
– Pressure & temperature are
extremely high
• Great majority of p+ & e– are forced to join as neutrons
– Pressure & temperature are average properties
• A few particles will have quite low actual values
• These particles can remain separated as free particles
• Consequences
– Some p+ & e– are free to generate a magnetic field
Close Binary Systems: Fastest Pulsars
• Discovery of the fastest pulsar
1982
– Period of 1.558 milliseconds
• Rotates ~ 642 times per second
– One implication
• Rapid rotation ⇒ Rapid energy loss ⇒ Rapid slow-down
– The reality
• The slow-down rate is far less than expected
– The cause
• This pulsar is part of a very close binary system
–
–
–
–
–
The two stars were of substantially different mass
The high-mass star evolved quickly & died in a supernova
The low -mass star survived to the red giant phase
The low -mass star over-fills its Roche lobe
Mass transfer “spins up” the companion neutron star
• Other millisecond pulsars
– Some are not part of binary systems: Still a mystery
The Black Widow Eclipsing Pulsar
X-Ray Binary Pulsars
• Discovered in 1971 by the Uhuru spacecraft
– High-energy pulsars are in close binary systems
• Deduced from cyclical Doppler shift every 1.7 days
• Pulsing period of ~ 1.24 seconds
– More than 20 have been discovered
• Basic physical processes
– Mass transfer from ordinary star to neutron star
•
•
•
•
Channeled by magnetic field to the magnetic poles
Accelerated by gravity to ~ 0.5 . c
Hot spots form at ~ 108 K
Intense X-ray emission ~ 105 . L☉
– Pulsar beam sweeps past the Earth
X-Ray Pulses From Centaurus X-3
Height variations are an artifact of sensor orientation.
Model of an X-Ray Binary Pulsar
Novae & Bursters
• Novae
– Brighten by a factor of 104 to 108 in hours to days
– Reach a peak luminosity of ~ 109 . L☉
– White dwarfs in close binary systems
• Gradual mass transfer of H onto the white dwarf’s surface
• Highly compressed & heated to ~ 107 K by strong gravity
• Runaway surface H fusion is the nova
• X-Ray Bursters
– Brighten by a factor of 101 for ~ 20 seconds
– Neutron stars in close binary systems
• Relatively weak magnetic field allows surface H
accumulation
• Fusion converts some H into He
• Runaway surface He fusion is the burster
Novae & Type Ia Supernovae
• Similarities
– Both occur in close binary systems
– One star is always a white dwarf
• Differences
– Novae are much less energetic than supernovae
• Novae ~ 1037 joules
– Novae
Supernovae ~ 1044 joules
accrete relatively small amounts of gas
• Runaway fusion occurs on the surface
• The white dwarf is not destroyed
• This event can happen repeatedly
– Supernovae accrete relatively large amounts of gas
• Runaway fusion occurs in the interior
• The white dwarf is destroyed
• This event can happen only once
Light Curve of Nova Cygni 1975
Light Curve of an X-Ray Burster
Neutron Stars Have Upper Mass Limits
• Degenerate electron pressure
– Capable of supporting < ~1.4 . M☉
• Chandrasekhar limit
– End result is a white dwarf
• Escape velocity < c
• Degenerate neutron pressure
– Capable of supporting <~ 3.0 . M☉
– End result is a neutron star
• Escape velocity ~ c
Important Concepts
•
Pulsars are discovered
1967
•
– Strongly emits at radio l’s
– Periods from ~ 0.25 to ~ 1.5 seconds
– Object much smaller than white dwarf
•
•
– Superfluidity & superconductivity
• Interact to produce glitches
– P+ & n– can exist in neutron stars
• Generate the magnetic field
Pulsars must be neutron stars
–
–
–
–
Supported by degenerate n pressure
Very
small diameter
Very
rapid rotation
Very strong magnetic field
Basic physical processes
– Offset rotational & magnetic axes
• Behave just like a lighthouse beam
– Magnetic field channels
e–
&
e+
pairs
• Produces ~ 2° wide radio beam
– Pulsars rotate, not turn “on” & “off”
– Pulsars emit at multiple l’s
• X-Ray & visible l’s
– Pulsars gradually slow down
Special properties of neutron stars
– Millisecond pulsars
• Accelerated by accreting gas
•
Other unusual phenomena
– X-Ray binary pulsars
• Neutron stars in close binary systems
• Accretion causes radiating hot spots
– Novae
• White dwarfs in close binary systems
• Runaway surface H fusion
– Bursters
• Neutron stars in close binary systems
• Runaway surface He fusion
– Novae & Type IA supernovae
• Repeatable vs. non-repeatable events