Pulsars
Original post date: January 01, 2019
What does a motorbike and a dwarf star have in common? Technically speaking, not much. But when you hear the word "Pulsar", the first thing you picture is probably a bike. So why bring up a bike in a post about a star?
In space science, pulsars are bursts of radiation blasting out from a neutron star, created when ions meet the intense magnetic field near a neutron star's poles. Most people are familiar with the Aurora Borealis. Earth's magnetic field near its poles energizes radiation from the Sun, producing those spectacular colourful lights in the night sky. The same phenomenon occurs on other planets. A neutron star takes this to an extreme.
What is a neutron star? It is what remains after a larger star exhausts its fuel. When a star can no longer sustain its own gravitational pull, it undergoes a supernova and collapses under its own gravity. The result is a neutron star, one of the most bizarre objects in the known universe. The pressure inside is so extreme that electrons are forced to fuse into the nucleus, producing a frictionless fluid in constant motion within the star. To grasp the density: a single teaspoon of neutron star material weighs more than Mount Everest. That density also generates one of the strongest magnetic fields of any cosmic body we know of.
Then there is the spin. Consider a ballet dancer spinning with arms outstretched who suddenly pulls them in. The rotation accelerates sharply. Or a stone on a string: shorten the string and the stone spins faster. This is the conservation of angular momentum. In classical physics, angular momentum is proportional to the radius of the spinning body. When the radius decreases, rotational speed increases to compensate.
A neutron star goes through exactly this. A star that was millions of kilometres wide collapses to roughly 25 kilometres across. The original spin of the star increases exponentially as a result. The fastest recorded neutron star spin is nearly 700 rotations per second. A point on its surface travels at close to one fifth the speed of light.
So how do pulsars form? The combination of that intense magnetic field and the rapid spin produces beams of radiation from the star's poles. Because the rotational axis of a neutron star is not fixed and sweeps around like a lighthouse beacon, the radiation sweeps through space in a regular pattern. This is actually how neutron stars are detected. We look for regular dips in brightness, the signature of a pulsar sweeping past us. A neutron star is only detectable if its pulsar happens to be aimed in our direction.
Without pulsars, finding a 25-kilometre object somewhere in the vastness of space would be essentially impossible. Thanks to them, we have a window into the afterlife of stars and some of the most extreme physics the universe has to offer.
Originally posted on thedopplerdude.blogspot.com.