Astronomy 1021 Chapter Notes - Chapter 14: Accretion Disk, Crab Nebula, White Dwarf
29 May 2018
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Astronomy 1021
Chapter 14
Neutron Stars
•Neutron Star- ball of neutrons created by collapse of iron core in a massive star supernova,
remnants of Type 2 supernovae; 10km radius but more massive than the Sun; made entirely of
neutrons held tog. by gravity, very dense- a paperclip made out of neutron star material will
weigh as much as Mt. Everest
•Neutron star remains stable by neutron degeneracy pressure (like EDP but neutrons are packed
closely instead of electrons)
•Escape velocity of force of gravity at neutron star surface is half the speed of light
•Discovery: first observational evidence in 1967 by Jocelyn Bell who discovered pulsing radio
waves; pulses came exactly every 1.33 seconds, turned out to be pulsars
-End of 1968, astronomers found that pulsars say at centers of 2 supernova remnants (Vela and Crab
nebula); and pulsars are neutron stars left behind by supernova explosions
-Pulsations occur b/c rapid spin of neutron star as iron core collapses; this also bunches up magnetic
field lines in the core, the lines then direct beams of radiation along magnetic poles (stellar
lighthouses); we see a pulse of light each time one of those beams aligns with Earth
-Isolated Neutron Star- Continuous twirling of pulsar’s magnetic field causes electromagnetic
radiation, which takes away energy and angular momentum, causing neutron star’s rotation rate to
slow gradually, eventually it becomes so slow that we can’t detect it; neutron earthquake causes
pulsar to spin faster, so all pulsars are neutron stars but not all neutron stars are pulsars
-Pulsars must be neutron stars b/c no other massive object can spin that fast
Neutron Star in a Close Binary System
•Similar to white dwarf; can come back to life from gas overflowing from companionate star, but
accretion disk is much hotter and denser than accretion disk around white dwarf
-High temps in inner disk make it radiate powerful X-rays, close binaries that contain accreting
neutron stars often called X-ray binaries; pulsars accelerate
•X-Ray Bursts: H-rich material builds up on neutron star’s surface, pressure at bottom of H layer
are high enough for fusion, which produces layer of He beneath H, He fusion stars when temp in
that layer goes to 100 million K, He fuses to make C, generating a burst of energy that flows from
neutron star in form of X rays (X ray bursts)
-The flares occur every few hours to every few days
•Neutron Star Mergers: two close neutron stars emit gravitational waves, causes them to merge;
energy released during merger can be larger than a massive star supernova’s; mergers can produce
gold, platinum and rare Earth elements present in the Universe
•Pulsar Exoplanets: 1992- first extrasolar planets were found orbiting a pulsar, through wobble
detection method; planets probably formed after the supernova explosion
