Neutron stars Chapter 21 Neutron stars


НазваNeutron stars Chapter 21 Neutron stars
Дата конвертації19.04.2013
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Neutron stars - Chapter 21


Neutron stars

  • The remains of cores of some massive stars that have become supernovae.

  • Cores are a degenerate gas of mostly neutrons.

  • So much compression at each stage of core contraction that final radius is  10 km. Mass 1.4 – 3 M. Density  nuclear density. A sugar-cube volume on Earth would weigh as much as all of humanity.



More neutron star properties

  • Conservation of angular momentum: as a spinning object contracts, it speeds up –

L = IΩ α MR2Ω

is conserved = > the body spins very fast: periods msec – sec

(cf Sun: about 1 month).

Magnetic fields should be enormous too. Why?

  • Magnetic fields should be enormous too. Why?

  • The magnetic field of the star becomes concentrated as star collapses: magnetic flux (= magnetic field strength at surface x surface area) is conserved as star shrinks.

  • Surface area drops by ~1010, so magnetic field strength increases by factor of 1010! Should be 108 T (1012 G) or so.



Pulsars

  • Neutron stars were first proved to exist with the discovery of pulsars by Jocelyn Bell Burnell in 1967. She found some radio sources that “pulsed”. Her advisor, Tony Hewish, got Nobel Prize in 1974 (what?).

  • Now about 2000 pulsars known.



Lighthouse model for pulsars

  • Key point: (like planets) axis of rotation is NOT aligned with the magnetic axis. Strong electric fields are associated with strong magnetic fields and fast rotation. These pull charged particles off the surface at speeds close to c. Charged particles spiraling in magnetic field near poles emit synchrotron radiation.















Mass limit to neutron stars

  • Like white dwarfs and electron degenerate matter, neutron stars and neutron degenerate matter has an upper mass limit (~ 3 M, but not well understood).

  • When this is exceeded, the star collapses all the way to a black hole.





Novae

  • Novae are similar to X-ray bursters, but occur in a close binary system with a White Dwarf instead of a neutron star.

  • Accretion is weaker and will cause a hydrogen fusion outburst on surface of WD. Can repeat (years to decades). Eventually mass buildup may lead to carbon-detonation supernova.

  • 10-4 x fainter than

supernova (which

peaks at about 1036

W  109 L).



Pulsars

  • Neutron stars were first proved to exist with the discovery of pulsars by Jocelyn Bell Burnell in 1967. She found some radio sources that “pulsed”. Her advisor, Tony Hewish, got Nobel Prize in 1974 (what?).

  • Now about 2000 pulsars known.



Pulsars are primarily a radio astronomy phenomenon, but a few are also seen in the visible, UV, X-ray, and gamma ray.

Pulsars are primarily a radio astronomy phenomenon, but a few are also seen in the visible, UV, X-ray, and gamma ray.


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