1. 21. Neutron Stars Neutron stars were proposed in the 1930s
Pulsars were discovered in the 1960s
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 make novae & bursters
Neutron stars have upper mass limits

2. 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
Mass of 1 proton + Mass of 1 electron = Mass of 1 neutron
Charge of proton + Charge of electron = Charge of neutron
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 MSun
Diameter of ~ 30 km
Escape velocity of ~ c

3. Pulsars Discovered in the 1960s
Radio telescope array constructed
Jocelyn Bell et al. search for random radio twinkling
Jocelyn Bell et al. discover regular radio pulsing
Pulses 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 much smaller than a white dwarf

4. The Crab Pulsar: “On” & “Off”

5. Intensity Variations of a Pulsar

6. Pulsars: Rapidly Rotating Neutron Stars
Important questions about neutron stars
Why should neutron stars emit any radiation?
Why should neutron stars emit radio wavelengths?
Why should neutron stars emit pulses of radiation?
Why should 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 they rotate fast
They have intense magnetic fields
Their magnetic field is times their ZAMS magnetic field
Approximately 1012 Gauss

7. Different Rotational & Magnetic Axes
Prior observations
The 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 beams of radio energy leave both magnetic poles
Typically ~ 2° wide

8. A Pulsar’s Rotational & Magnetic Axes

9. 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

10. Radio-Wavelength View: Crab Nebula

11. Optical/X-Ray View: The Crab Nebula
© NASA (2002)

12. 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 Recorded by orbiting Einstein Observatory
Visible l’s Recorded by 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

13. A Pulsar Seen at Three Wavelengths

14. Ejected Pulsars Basic observations Basic conclusions
Many fast-moving pulsars have been identified
Basic conclusions
Many Type II supernovae must be asymmetric
The resultant neutron star cannot remain within the remnant
The resultant neutron star penetrates the supernova remnant

15. Neutron Star Ejected by a Supernova

16. 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 the 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 the magnetic field
Similar to the Sun’s coronal heating

17. Superfluidity & Superconductivity
Some basic properties of neutron stars
Thought to have a solid crust
Thought to have a fluid interior of degenerate neutrons
Two special properties of neutron stars
Superfluidity
Matter can flow without friction under some conditions
Superfluids can flow up the side of their container
Convection in a neutron star can continue indefinitely
Superconductivity
Current can flow without friction under some conditions
This assumes there is no “load” on the current
Protons & electrons in a neutron star produce electric currents
Interactions in a neutron star
Glitches occur Sudden increase in rotational speed
Faster spinning fluid core “grabs onto” slowing solid crust

18. The Internal Structure of a Neutron Star

19. Glitches in Neutron Stars

20. 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

21. Fastest Pulsars in Close Binary Systems
Discovery of the fastest pulsar 1982
Period of milliseconds
Rotates ~ 642 times per second
One implication
Fast rotation should lead to fast 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 A mystery

22. The Black Widow Eclipsing Pulsar

23. X-Ray Binary Pulsars Discovered in 1971 by the Uhuru spacecraft
These 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 ~ c
Hot spots form at ~ 108 K
Intense X-ray emission ~ LSun
Pulsar beam sweeps past the Earth

24. X-Ray Pulses From Centaurus X-3
Height variations are an artifact of sensor orientation.

25. Model of an X-Ray Binary Pulsar

26. Novae & Bursters Novae X-Ray Bursters
Brighten by a factor of 104 to 108 in hours to days
Reach a peak luminosity of ~ LSun
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

27. 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 Supernovae ~ 1044 joules
Novae 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

28. The Light Curve of Nova Cygni 1975

29. The Light Curve of an X-Ray Burster

30. Neutron Stars Have Upper Mass Limits
Degenerate electron pressure
Capable of supporting < ~1.4 . MSun
Chandrasekhar limit
End result is a white dwarf
Escape velocity < c
Degenerate neutron pressure
Capable of supporting <~ MSun
End result is a neutron star
Escape velocity ~ c

31. 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
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
Superfluidity & superconductivity
Interact to produce glitches
P+ & n– can exist in neutron stars
Generate the magnetic field
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
Runaway surface He fusion
Novae & Type IA supernovae
Repeatable vs. non-repeatable events