 When found in stars between 8 and 25x M sun, what happens to the core?  Cores are stopped by the outer layers of the star  Cores recontract and remain intact  Cores form highly compressed clumps of neutrons = neutron stars  Isolated neutron stars are stable

 Have an incredible density, equal to that of an atom’s nucleus  200 million times more dense than a white dwarf  A neutron star with twice the mass of our Sun would have a diameter of 20 km on Earth  Surface gravity is exceptionally strong

 Neutron stars were first predicted in 1932  Conditions seemed outrageous so predictions were widely ignored  1968: Jocelyn Bell – noticed that a particular region in space was emitted radio pulses at a regular frequency (once every sec)  First thought was that aliens were responsible so source was named LGM1 (Little Green Men 1)  Once further sources were detected, they were renamed pulsars

 Crab Pulsar – first pulsar discovered and studied in detail (1968)  1054: Native Americans and Chinese saw and recorded observations of a “guest star” in Taurus  At this location now is the remnants of a supernova - the Crab Nebula  At the nebula’s center is the Crab pulsar  Pulsar is spinning 30 times per second  In order to stay intact at this rotational rate, it must be small (much smaller than a white dwarf)

 Why do they rotate so fast?  Conservation of Angular Momentum – as the size decreases, the rotational rate must increase to conserve angular momentum (skater)  Most stars also have a magnetic field  Sun: the magnetic field is spread out over millions of square km  When a star condenses, its magnetic field becomes very concentrated  Magnetic field is so strong that a similar magnet on the moon would lift iron rods off Earth

 How do they emit pulses of EM radiation?  Axis of rotation is not the same as the line between the north and south poles  As it rotates, its magnetic field changes direction which generates electric fields  The electric fields act on charged particles near the surface, sending them outward  Explanation is often called the lighthouse model  B/c some energy is released with every pulse, isolated pulsars slow down as they age

 Not composed solely of neutrons  Radius of 10 km, with a core of superconducting protons and superfluid neutrons  Superconductor: a material in which electricity and heat flow freely, without energy loss  Superfluid: a fluid that flows without any friction  Very large gravitational force: climbing a bump 1 mm high would take more energy than it takes to climb Mount Everest on Earth

 May have atmospheres  Crust may have events similar to earthquakes  When such an event happens, its rotation rate changes to conserve its angular momentum  Change creates a glitch in the rate at which the pulsar emits radiation  During collisions of two neutron stars (from a binary system), some heavier elements may be produced

 Protostar less than 0.08x M sun :  Protostar b/w 0.08 and 0.4x M sun :  Protostar b/w 0.4 and 8x M sun :  Protostar b/w 8 and 25x M sun :  Protostar more than 25x M sun :  brown dwarf  main-sequence star  red dwarf  main-sequence star  giant  red supergiant  planetary nebula + white dwarf  main-sequence star  giant  supergiant  supernova  neutron star  main-sequence star  supergiant  supernova  black hole