Let’s Talk About 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.

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

When these large stars reach the end stages of their life, the energy produced by their internal hydrogen fusion reactions becomes increasingly insufficient to counter the relentless pull of gravity. The subsequently intensifying heat and pressure of the condensing star make it possible for other nuclear reactions to occur in which helium is consumed to produce a variety of heavier elements up the Periodic Table to iron.  

While those reactions may keep the crushing pull of 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

A Suddent Collapse

Here is where things get absolutely spectacular.

Once the equilibrium is broken between the 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 a volume only 20 km in diameter! As it does so, its temperature increases to a paltry 100 billion degrees (Kelvin) – and all in about the time it takes you to sneeze.

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

You see, a funny thing can happen when you squeeze neutrons into a tiny container like the core of an imploding 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. At a certain point, the neutrons simply can’t get any closer together without some really exotic physics coming into play that I won’t even pretend to understand.

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.
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 that propels the surrounding material out of gravity’s grasp. That event is called a supernova.

What’s left behind after that 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.

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.

Spinning into a Pulsar

So now we have our neutron star, but we want a pulsar.

In order to get there, 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. When I say fast, I mean really, 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.

That high-speed rotation, coupled with the fact that a neutron star still has some charged particles (protons and electrons), produces some extremely powerful magnetic fields. Those fields 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 twisting 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.

(Funny side note: the first discovered pulsar was referred to as “Little Green Men” because the 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: