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Dead stars · pulsars · magnetars

Neutron Stars

When a big star dies its core collapses to the size of a city, heavier than the Sun and dense as an atomic nucleus. Some spin hundreds of times a second.

Crushed to neutrons

A star shines by fusion; when the fuel runs out, gravity wins. Above about 1.4 solar masses (Chandrasekhar's limit) not even electrons can hold the core up: they are squeezed into protons, making neutrons, and the core falls in until neutrons pack shoulder to shoulder. The outside bounces off as a supernova. A teaspoon of what is left would weigh about as much as a mountain.

  • g: the pull at the surface: about 1 × 10¹² m/s², over 100 billion times Earth's
  • G: Newton's constant of gravity
  • M: the star's mass, typically 1.4 Suns
  • R: its radius, about 12 km

Lighthouses in space

The collapse keeps the old star's spin and magnetism, squeezed. Beams of radio waves stream from the magnetic poles; if the magnetic axis is tilted from the spin axis, the beams sweep round like a lighthouse, and Earth sees a tick on every pass. Pulsars keep time nearly as well as atomic clocks.

1.34 s

spins every

0.019 % c

surface speed

real time

shown slowed

The line at the bottom is what a radio telescope on Earth records: a tick each time a beam sweeps past. Fast pulsars are slowed down here so you can see them turn.

Magnetars are neutron stars with fields a thousand times stronger still, about 10¹¹ tesla. Their starquakes give out bursts of X-rays and gamma rays; one in 2004 lit up Earth's upper atmosphere from 30 000 light years away.

Timeline

  1. 1932

    James Chadwick discovers the neutron.

  2. 1934

    Walter Baade and Fritz Zwicky propose that supernovae leave behind stars made of neutrons.

  3. 1939

    Oppenheimer and Volkoff calculate the most a neutron star can weigh before it collapses.

  4. 28 Nov 1967

    Jocelyn Bell Burnell, a Cambridge PhD student, spots a radio source ticking every 1.337 seconds: the first pulsar.

  5. 1968

    The Crab pulsar is found at the heart of the supernova seen from Earth in 1054.

  6. 1974

    Hulse and Taylor find two neutron stars orbiting each other and losing energy to gravitational waves.

  7. 17 Aug 2017

    LIGO and Virgo hear two neutron stars merge; telescopes see the kilonova forge gold and platinum.

Sources

  • A. Hewish, S. J. Bell, et al., “Observation of a rapidly pulsating radio source”, Nature 217 (1968) 709
  • W. Baade, F. Zwicky, “On super-novae”, PNAS 20 (1934) 254
  • J. W. T. Hessels et al., “A radio pulsar spinning at 716 Hz”, Science 311 (2006) 1901
  • B. P. Abbott et al., “GW170817”, Phys. Rev. Lett. 119 (2017) 161101