Pulsars are both curious and extreme objects, compressed into one. Compression is the best way to describe the birth of a pulsar – the result of the imploding core, during the final moments of a massive star’s life.
When a massive star explodes as a supernova, a lot of the star is thrown off into space in a brilliantly bright detonation that can sometimes outshine entire host galaxies. However, the core of that star goes in the opposite direction – and implodes.
During this implosion, electrons (negative charges) and protons (positive charges) are forced together to form neutrons (no charge). The core of the dead star becomes a neutron star, being extremely dense and massive.
Most neutron stars have the equivalent of approx. 1.4 times the mass of the Sun squeezed into a sphere about 20km across. And here’s where things start to get very strange.
As the core collapses, it preserves the original rotational velocity of a star – but now over a much smaller area. To compensate, it spins up. Very rapidly. Think of this analogy as an example: when watching a figure skater spin on the ice with their hands extended, their velocity is impressively fast. What happens when the figure skater draws their arms in? They start to spin at an extremely fast velocity. This is exactly the same scenario.
Neutron stars and pulsars rotate on their axis hundreds of times per second. The fastest has been clocked at an astonishing 716 revolutions per second – which means its equator is moving at 24% the speed of light (that’s 72,000 km per second!)
Surely any object moving at these velocities would rip itself apart instantly? Normally, yes – this would happen. But because of the density of neutron stars and their extreme mass – they also contain extreme gravity. This assists in holding the star together. At the moment, we also don’t fully understand what the inside of neutron stars are made off (akin to Earth’s mantle) – so this must also assist in holding the star together at these extreme velocities.
In addition to the velocity being preserved during core collapse, the magnetic field of the original star also is preserved in the core – and becomes very strong. In some cases, billions of times stronger than the magnetic field of the Earth. The magnetic field of a neutron star, also like the Earth, has a north and south pole.
It is at these poles that enormous amounts of electromagnetic radiation is beamed and blasted away from the star. As the star rotates (rapidly) these beams sometimes sweep past the Earth and we see a ‘pulse’ of energy with every sweep. That’s what we call a pulsar – a star that is pulsating at us with its electromagnetic beams.
A good way to imagine this is to think of a lighthouse, set high atop a cliff. The light of the lighthouse beams away and out into across the ocean, as the lighthouse mirror rotates. When a beam passes by a ship, the ship ‘sees’ a pulse of light from the lighthouse. In our astronomical scenario, the Earth is the ship and the lighthouse is the pulsar – and the beams are the pulses.
Pulsars were discovered in 1967 by Dame Jocelyn Bell-Burnell.