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Space Phenomena: Pulsars

Pulsars are one of my favorite astronomical phenomena.  They are an amazing consequence of some fascinating physics and one possible end-result of the largest known explosions in the universe.  So…what is a pulsar, you ask?

Pulsars are a type of neutron star; one of several possible end-of-life scenarios for stars that have about 4-8 times the mass of our own beloved Sun.  I should stop here for a second and say that there is some disagreement over whether or not pulsars are a special subclass of neutron star or simply a neutron star exhibiting a behavior that all neutron stars are capable of exhibiting…but that’s another discussion in its own right.  Anyway…

The onion-like layering of a dying red-giant star.  Image: NASA
The onion-like layering of a dying red-giant star. Image: NASA

When these large stars 4-8 times the mass of our sun reach the end stages of their life, the energy produced by the familiar stellar hydrogen fusion reaction becomes increasingly insufficient to counter the relentless pull of gravity which has been trying to collapse the star since it was born.  The subsequently intensifying heat and pressure of the condensing star makes it possible for other nuclear reactions to occur which consume helium and produce a variety of heavier elements all the way up to iron.  While these reactions may help keep gravity at bay for a while, the star is ultimately doomed to undergo an intense gravitational core collapse as its core becomes increasingly larger, more solid, and inert.

Image of the Vela pulsar captured by the Chandra Observatory
Image of the Vela pulsar captured by the Chandra Observatory. Photo: Chandra X-Ray Observatory/NASA

Here is where things get absolutely spectacular!

Once the equilibrium is broken between the inward pull of gravity and the outward push generated by released fusion reaction energy (and electron degeneracy pressure), the star undergoes cataclysmic implosion in a matter of seconds!  SECONDS!   A core around 2765000000000000000000000000000 kg in mass and around 8000 km in diameter suddenly compresses down to something with the same amount of mass, but now crammed in a volume only 20 km in diameter! As it does so, its temperature increases to a paltry 100 BILLION DEGREES (Kelvin)…and all this in about the time it takes you to sneeze!

This compression is so incomprehensibly intense that it starts to push electrons into nearby atomic nuclei where they react with protons to form the neutrons which can finally put a stop to these core collapsing shenanigans.

Just when gravity thought it had the upper hand and was all like, “Take that star!  I win!“, the neutrons formed by the intense heat and pressure step-in and are like, “Naw bro…sorry…“.

You see, a funny thing can happen when you squeeze neutrons into a tiny container; in this case the imploding core of a star.  The principles of quantum mechanics (like the Pauli Exclusion Principle) inform us that you can only compress neutrons so far.  Eventually they reach neutron degeneracy (here is an additional link for you physics buffs that want to know the math behind that) which essentially puts the breaks on any further collapse because those adorable little neutrons can’t get any closer together than they already are without some really exotic physics coming into play.

Filaments of the Vela Supernova Remnant.  Photo: Jade Scope Observatory
The Vela Supernova Remnant offered the first direct observational evidence that Type II supernovas form pulsars. The explosion occurred approximately 11,000 years ago and resulted in the Vela Pulsar.

Assuming that conditions are just right and those exotic relativistic physics don’t crash the party and crush this neutron-rich core into a black hole, the neutron degeneracy pressure abruptly stops the gravitational collapse dead in its tracks.  This causes a MASSIVE shock wave which is strong enough to propel the surrounding material out of gravity’s grasp…but you’ve already heard about this process.  It’s called a supernova.

What’s left behind after this explosion is an energized, super hot, super dense remnant known as a neutron star.  How dense you ask?  A sugar-cube’s worth would weigh about 1 billion tons…no big deal.

Check out this simulation of a supernova from JPL/CalTech which has recently changed the way that scientists (and by extension, the public) think of supernova dynamics.  It’s not a uniform ball!

So now we have our neutron star, but we want a pulsar.  How do we do get a hold of one of those?

Well, first, our neutron star needs to be spinning and lucky for us, even though the original star collapsed, much of its angular momentum was conserved.  So, just like a figure skater spinning like a top and pulling his/her arms closer to his/her body, the star began to spin faster as it reduced its diameter…and when I say fast, I mean really fast.  Some neutron stars can have a rotational period between 1 and 10 milliseconds!

Check out the neutron star in this animation by NASA which depicts a neutron star in a binary system sucking matter away from its neighbor and using the energy to spin-up to extremely high rotational speeds.  Also, note the “wobble” in the jets…it’s an important feature that we will discuss next.

This high speed rotation coupled with the fact that a neutron star still has some charged particles (protons and electrons), means that it can produce some extremely powerful magnetic fields which can subsequently produce focused astrophysical jets of energy which blast outward from the star’s magnetic poles.  The columnar shape of these jets is thought to be the result of warped magnetic fields which twist and direct the energy flow into focused beams.  (If you want to learn more, you can look into magnetohydrodynamic theory to explore the precise proposed mechanisms behind the field line distortion).  

These beams are responsible for the pulsar phenomenon.  Since the magnetic poles aren’t aligned with the true north and south poles along the star’s axis of rotation, they appear to “wobble” as the star rotates.  If one of these wobbly rotating beams happens to sweep across an observer, like an astronomer on Earth, then the star will appear to pulse like a strobe light with a regular frequency…hence pulsar.  So, there you have it!  Fascinating!

(On a funny side note, the first discovered pulsar was referred to as “Little Green Men”, because the regular periodicity of the flashing signal was hard to explain as a natural phenomenon and there was some half-humorous discussion over whether or not astronomers had discovered some sort of extraterrestrial beacon.)

Further Reading:

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