Neutron star variants - Pulsars

 What are Pulsars?

Pulsars are one of the most famous and observed classes of neutron stars in astronomy. As their name suggests, they are a type of neutron star that emits powerful radio pulses at regular intervals. A few such classes of neutron stars are based on the direction of their radiation emitted (as seen from earth), like magnetars, blazars, etc. But unlike magnetars and blazars (or even quasars), pulsars are easy to detect because the radiation emitted by a pulsar will always point to the earth.

How are Pulsars formed?

The initial stages of the formation of a pulsar are just like that of a neutron star when a star whose mass is more than 8 solar masses go supernova, and its core collapses all of its mass into a small core, only about 25 km wide (the size of a city). This easily makes neutron stars the second most dense objects in the universe, after black holes. 

Conservation of Angular Momentum in Neutron Stars:

But after it goes supernova, the only remnant that exists after the whole explosion would be the dense core of the star, which is what we call a neutron star. A neutron star will spin more rapidly than ever before, to conserve the angular momentum. This is because a bigger and wider object will have relatively low angular velocity, than when it's more compact in size (provided that the mass remains constant). 

This can be seen in various real-life instances, for example, while spinning in an office chair with hands held wide out versus spinning on the same chair while hands held back in, in a compact manner. We can see that the chair spins faster in the second case when the hands are held in so that more mass is concentrated at the centre of the spinning axis. 

This is the same mechanism that happens with neutron stars, when they collapse into a dense core, by reducing their huge size. They spin a hundred times faster when they're a neutron star than when they were a giant regular star. Along with their sin, their magnetic field substantially increases too. This is because as they become denser and more compact to form a neutron star, the existing magnetic field (from the star before undergoing supernova) will also undergo the same phenomenon, thereby increasing the density of the magnetic field lines per unit area. This greatly increases the magnetic field strength surrounding a neutron star and will contribute to the powerful radiations they emit.

Basically, a pulsar is just a rotating neutron star that emits powerful electromagnetic radiation like gamma rays and radio waves. This emission appears to blip in and out because of the neutron star's rotation in its axis. So when we view it from earth (in our telescopes) it'll appear as a star that glitches at specific time intervals.  

What causes Pulsars to emit radiation?

We have already seen how pulsars spin very rapidly (about 10-100 spins per second), and their extreme magnetic fields, which are of magnitudes billions of times more than that of the earth, and these two properties of pulsars are one of the main reasons why they emit such powerful radiation including gamma rays.

The phenomenon which takes place here is what we call synchrotron radiation. It occurs when charged particles are accelerated to speeds near to the speed of light, under the action of magnetic fields acting perpendicular to their velocity. Synchrotrons are often used as particle accelerators to study elementary particles by accelerating them near to the speed of light and then bombarding them with other particles. The radiation is given off by these fast-moving charges (like electrons) in a fast-spinning neutron star, by the presence of a strong magnetic field. The radiation is given off at both ends (poles) of its magnetic field axis.

Simulation of radiation given off by a Pulsar

While they seem like a deadly way for the earth to get killed from radiation, they're not at all dangerous, and not even a threat when compared to magnetars. 

Pulsars and Quasars are also known as cosmic particle accelerators, and they seem to share similar properties. The difference is that pulsars are a type of neutron stars and quasars are a type of black holes.

There are about 1,000 pulsars that we have found in our Milky Way galaxy alone, and some of them are spinning at ridiculously fast rates, over 150 spins in one second! Such pulsars are also called millisecond pulsars, Even so, this spinning of such neutron stars is believed to slow down over time, as they exhaust their rotational energy. 

 To check out the post on neutron stars, click here

 To check out the post on quasars, click here

 To check out the post on magnetars, click here