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21. Neutron Stars • Neutron stars were proposed • Pulsars in the 1930s 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 – 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 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 © 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 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