Neutron stars, discovered in 1967, are incredibly dense cosmic objects that emit regular radio pulses. These fascinating remnants of massive stars pack up to 3 solar masses into a sphere just 20 km wide, spinning rapidly with intense magnetic fields.

Pulsars, a type of neutron star, were initially mistaken for alien signals due to their precise timing. Their discovery revolutionized our understanding of stellar evolution and provided crucial evidence linking these compact objects to supernova explosions.

Discovery and Characteristics of Neutron Stars and Pulsars

Steps in neutron star discovery

1. In 1967, Jocelyn Bell Burnell and Antony Hewish detected regular radio pulses from a distant cosmic source

   • Pulses had a precise period of 1.33 seconds
   • Initially thought to be signals from an extraterrestrial civilization due to their regularity
   • Source named LGM-1 (Little Green Men 1) before being identified as a rapidly rotating neutron star

2. Neutron stars emit electromagnetic radiation, primarily radio waves, which allows for their detection despite their extreme distances

   • Closest known neutron star is about 400 light-years from Earth (PSR J0108-1431)

3. Radio telescopes are used to detect the faint, regular pulses emitted by neutron stars

   • Regularity of pulses distinguishes them from other radio sources (galaxies, quasars)

Characteristics for pulsar detection

• Neutron stars have extremely high densities, with 1.4 to 3 solar masses compressed into a sphere about 20 km in diameter
  • High density results from the collapse of a massive star's core during a supernova explosion
• Powerful magnetic fields, typically around $10^{12}$ gauss, about a trillion times stronger than Earth's magnetic field
  • Strong magnetic fields created during core collapse and conservation of magnetic flux
• Rapid rotation, with periods ranging from milliseconds to several seconds
  • Rapid rotation caused by conservation of angular momentum during core collapse
• Combination of strong magnetic fields and rapid rotation leads to emission of focused beams of electromagnetic radiation from the neutron star's magnetic poles
  • Beams sweeping across Earth as the neutron star rotates are observed as pulses, hence the term "pulsar"
• Regular pulsar emissions enable precise timing measurements, making them useful for studying astronomical phenomena and testing theories of gravity

Evidence linking pulsars to supernovae

• Crab Nebula, a supernova remnant from 1054 CE, contains the Crab Pulsar at its center
  • Provides direct evidence linking neutron stars to supernova explosions
• Other supernova remnants (Cassiopeia A, Vela) have been found to contain pulsars
  • Pulsars believed to be compact remnants of massive stars that exploded as supernovae
• Rapid rotation and strong magnetic fields of pulsars are consistent with properties expected from a collapsed stellar core following a supernova
• Theoretical models of core-collapse supernovae predict the formation of neutron stars
  • Models supported by observed association between pulsars and supernova remnants
• Distribution of pulsars in the Milky Way is consistent with their formation in supernova events
  • Pulsars more commonly found in the galactic plane, where most massive stars reside and end their lives as supernovae

Stellar Evolution and Compact Objects

• Stellar evolution describes the life cycle of stars, from birth to death
• Massive stars (>8 solar masses) end their lives in supernova explosions, potentially forming neutron stars
• Less massive stars may become white dwarfs, supported by electron degeneracy pressure
• The Chandrasekhar limit (~1.4 solar masses) is the maximum mass for a stable white dwarf
  • Stars exceeding this limit will collapse further, potentially becoming neutron stars