Pulsars

What does a motorbike and a dwarf star have in common? Technically speaking, not much. But when I say ‘Pulsar’, the first thing you picture is a bike. So why am I speaking about a bike and relating it to a star?

Pulsars, in space-science, are sparks of radiation blasting out from a neutron star. They are created when ions meet the high-density magnetic field of a neutron star. Most of you must be familiar with the beautiful Aurora Borealis. These are one of nature’s most spectacular phenomena. The earth’s magnetic field near the magnetic poles sparks up the radiation from our sun and thus, we see colourful lights in the night sky. This phenomenon is found on other planets as well.

How is this related to a neutron star? What is a neutron star? A neutron star is a cosmic body of very high density and high rotational speed. But this isn’t the exact definition of a neutron star. A neutron star is a remnant of a larger star which loses its fuel. When a star loses its fuel, it can no longer sustain its own gravitational pull. Thus, the star undergoes a supernova and collapses under its own gravity. The result of this is a neutron star, which has some of the most bizarre properties of cosmic bodies.

Since the star collapses into itself, the resulting neutron star is highly dense. In fact, the pressure is so much that electrons are forced to fuse into the nucleus, resulting in a frictionless fluid that is in constant motion inside the neutron star itself. For our understanding of the intense density, here’s an analogy – a teaspoon of neutron star weighs more than the Mount Everest! The high-density results in one of the strongest magnetic fields exhibited by any cosmic body in the known universe.

What else happens in a neutron star? Let’s talk about the spin. Let’s take some examples and relate them, shall we? When a ballet dancer spins on her toes with her arms stretched wide and suddenly pulls them towards her, she starts spinning fast! When you start spinning string with a stone tied to one end and keep reducing the length of the string, you’ll notice that the stone starts revolving faster. Why is this? It is the law of conservation of angular momentum. In classical physics, angular momentum is directly proportional to the radius of the spinning body. And when the radius of a spinning body reduces, the angular momentum increases in order to make up for the reduction of the radius. In other words, the body conserves its momentum by increasing the speed of rotation.

A neutron star experiences a similar phenomenon. A star which was millions of kilometres wide is now reduced to a mere 25 kilometre. So, the spin which the star carried increases exponentially. The fastest recorded spin of a neutron star is nearly 700 rotations per second! So, a point on the surface of this neutron star travels nearly at one fifth the speed of light!

Back to square one, how are pulsars formed? The intense magnetic field and the spin of the neutron star leads to a blast of visible radiation from its poles. This radiation is called a pulsar. The axis of spin of a neutron star is not always the same. It swivels and moves randomly, causing the pulsar to act like a lighthouse beacon in space. In fact, this is how neutron stars are detected! A neutron star is detected by the regular dips in the brightness of a neutron star. A neutron star can be detected only if the pulsar is pointing towards us.

Thanks to pulsars, we can get to know an amazing cosmic body and the afterlife of a star!

How on earth (literally), can anyone search for a ball 25 kilometres wide in space if it weren’t for the pulsars!?

- Sameer V Devipur