23.4 Pulsars and the Discovery of Neutron Stars

Discovery and Characteristics of Neutron Stars and Pulsars

Neutron stars are the ultra-dense remnants left behind when massive stars explode as supernovae. They pack more mass than the Sun into a sphere roughly the size of a city, and their discovery confirmed key predictions about how stars die. The story of how they were found is one of the best in modern astronomy.

The Discovery of Pulsars

1. In 1967, graduate student Jocelyn Bell Burnell, working with her advisor Antony Hewish, detected regular radio pulses from a distant cosmic source using a newly built radio telescope.

   • The pulses had a remarkably precise period of 1.33 seconds.
   • The signal was so regular that the team half-seriously considered it might be from an extraterrestrial civilization. They nicknamed the source LGM-1 (Little Green Men 1).

2. Soon after, the team found more pulsing sources in different parts of the sky. Multiple sources scattered across the sky ruled out aliens and pointed to a natural phenomenon.

3. Theorists quickly proposed that these signals came from rapidly rotating neutron stars, objects that had been predicted decades earlier but never observed.

   • Thomas Gold argued in 1968 that a spinning neutron star with a strong magnetic field would produce exactly this kind of regular pulse.

Radio telescopes detect these faint, periodic signals. The regularity of the pulses is what distinguishes pulsars from other radio sources like distant galaxies or quasars.

• The closest known neutron star, PSR J0108-1431, is about 400 light-years from Earth.

Physical Characteristics of Neutron Stars

Neutron stars have extreme physical properties, all of which trace back to the violent collapse of a massive star's core during a supernova.

• Density: A neutron star crams roughly 1.4 to 3 solar masses into a sphere only about 20 km in diameter. A teaspoon of neutron star material would weigh about a billion tons on Earth.
• Magnetic fields: Typically around $10^{12}$ gauss, roughly a trillion times stronger than Earth's magnetic field. These intense fields result from the conservation of magnetic flux as the core collapses to a tiny size.
• Rapid rotation: Spin periods range from milliseconds to several seconds. This fast spin comes from conservation of angular momentum: as the core shrinks, it spins faster, just like a figure skater pulling in their arms.

How Pulsars Produce Pulses

The combination of a strong magnetic field and rapid rotation produces focused beams of electromagnetic radiation (mostly radio waves) that shoot out from the neutron star's magnetic poles. The magnetic poles don't line up perfectly with the rotation axis, so as the star spins, those beams sweep through space like a lighthouse.

If one of those beams happens to sweep across Earth, we detect a pulse with each rotation. That's why they're called pulsars (pulsating stars). The pulses are so regular that pulsars rival atomic clocks in their precision, making them useful tools for testing theories of gravity and detecting gravitational waves.

Evidence Linking Pulsars to Supernovae

The connection between pulsars and supernova explosions is supported by several lines of evidence:

• The Crab Nebula, a supernova remnant from an explosion recorded by Chinese astronomers in 1054 CE, contains the Crab Pulsar at its center. This is the most direct evidence linking neutron stars to supernovae.
• Other supernova remnants, including Vela and Cassiopeia A, also contain pulsars.
• The rapid rotation and intense magnetic fields of pulsars match what theoretical models predict for a collapsed stellar core after a supernova.
• Pulsars are found preferentially in the galactic plane, where massive stars live and die. This distribution is consistent with formation in supernova events.

Stellar Evolution and Compact Objects

Where a star ends up after it dies depends on its mass:

• Less massive stars (below about 8 solar masses) shed their outer layers and leave behind white dwarfs, which are supported against further collapse by electron degeneracy pressure.
• The Chandrasekhar limit ($\sim 1.4$ solar masses) is the maximum mass a white dwarf can have. Beyond this, electron degeneracy pressure can't hold the star up.
• Massive stars (above about 8 solar masses) undergo core-collapse supernovae. If the remaining core is below about 3 solar masses, it becomes a neutron star, supported by neutron degeneracy pressure. Above that threshold, even neutron degeneracy pressure fails, and the core collapses into a black hole.