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 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 1934– 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

Pulsars Discovered in the 1960s• Radio telescope array constructed 1967

– Jocelyn Bell et al. search forrandomradiotwinkling– Jocelyn Bell et al. discover regularradio 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• 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

© N

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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

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

• Current can 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 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

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

• Pulsars are discovered1967

– 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• Neutron stars in close binary systems• Runaway surface He fusion

– Novae & Type IA supernovae• Repeatable vs. non-repeatable events

Important Concepts