Pulsars as Neutron Stars

For those of you who missed my last couple of posts, allow me to introduce the neutron star: a stellar remnant similar to a white dwarf, but much denser, so dense that its protons and electrons have combined to form a neutron soup.

A neutron star forms from the collapsing core of a star between 10 and 20 M (solar masses). Its collapse produces powerful magnetic fields and extremely high temperatures, but because it becomes so small—less than the size of Los Angeles—it is very faint and radiates away its heat very slowly.

The exception to that rule comes in the form of two powerful beams of radiation that blast away from the object’s magnetic poles. As a neutron star spins—at around a hundred times per second—these radiation beams sweep across the sky like the the beams of a lighthouse.

If these beams happen to sweep over Earth, human observers see regular, rapid pulses of light. This visual phenomenon produced by neutron stars is called a pulsar.

Now that we have a basic understanding of neutron stars and pulsars, let’s explore some of the details of how these extreme objects work.

For one thing, neutron stars are not unchanging.

They mark the final state of the “lifespan” of a massive star, and once an object can be classified as a neutron star, it’s no longer a star. But neutron stars themselves evolve over time. One place where we can see this is the eventual slowing of their rotation and fading of their luminosity.

First, let’s take a closer look at why they have so much energy in the first place.

In this diagram, the dark grayish dot is the star’s core. Even after the shockwave that produces the supernova (shown here as an expanding red circle) rips outward through the star, the core has collapsed a great deal.

There’s a general rule in astronomy: when any object collapses, it generates energy, which goes into producing radiation and speeding up the object’s rotation. We see this, for example, in the collapse of an interstellar dust cloud to form a protostar.

In this case, as the dust cloud collapses, it spins faster and it also grows hotter, until the beginnings of a star—a protostar—form at its core.

A star in the main part of its life cycle—the main sequence—follows the same rule in regulating its own internal “homeostasis.” If the rate of nuclear reactions in its core ever drop too much, the core begins to collapse under the weight of the layers above, which then heats up the core and speeds up reactions again.

So…since a neutron star formed from the core of a massive star, it has a ton of energy. And because it’s so small (and a consequently small luminosity), it can’t get rid of that energy very fast.

What it can do, thanks to its powerful magnetic field, is blast radiation away at its magnetic poles. Since this energy is linked to its energy of rotation, neutron stars should gradually cool off and also slow down.

Now, remember that pulsars are specifically the visual phenomenon that happens whenever a neutron star’s beam sweeps over Earth. If it cools off to the point that it no longer produces detectable beams, we won’t see a pulsar even if its magnetic axis is pointed straight at us.

There’s still a neutron star there, though. So neutron stars can “live” longer than pulsars. Neutron stars older than about 10 million years old stop producing pulsars.

While they exist, though, pulsars are powerful. One in particular is our old friend, the Crab Nebula.

Right now, you’re looking at a visual-wavelength image of the Crab Nebula. That means you’re only seeing a tiny portion of the total light it emits.

For reference, here’s the electromagnetic spectrum:

Visible light is only a tiny portion of the light available to astronomers to study. Some of these wavelengths should be familiar to you, even if you don’t realize it—infrared is just a fancy word for heat, and ultraviolet is just a fancy word for the energy that produces sunburns.

No, heat doesn’t produce sunburns. If it did, you would feel yourself sunburning, and you’d have a tangible warning, making sunblock far less critical. Ultraviolet exposure is dangerous because you can’t feel its effects until it’s already damaged your skin (and possibly other parts of your body).

All of what you see in your daily life is just perceived with that tiny window of the electromagnetic spectrum called visible light. The rest of the spectrum actually tends to be far more valuable to astronomers. For example, only radio wavelengths can penetrate the densest interstellar dust clouds.

Anyway, the pulsar at the heart of the Crab Nebula emits photons all across the EM spectrum. That’s pretty impressive for an object that’s not producing its own energy anymore.

Here’s another thing. Remember those radiation beams that neutron stars produce? Well, would you be surprised to hear that they only account for a tiny fraction of the energy that neutron stars emit?

That’s right. Most of it—about 99.9%—is actually carried away in the form of a pulsar wind.

The pulsar wind can produce some really cool phenomena, to say the least.

Here, you can see the x-ray and infrared data paired with artist’s conceptions of what these phenomena might look like close-up.

The sphere at the center of the illustrations is the neutron star, and the two beams of radiation—especially clear in the illustration on the left—can be seen jetting away from the neutron star’s magnetic poles. The ring around the neutron star is an accretion disk, a disk of dust and gases that falls into orbit around an object.

Astronomers know that the pulsar wind is made up of high-speed atomic particles. We don’t know that these illustrations are accurate, but we do know that the pulsar wind produces small, high-energy nebulae near a young pulsar.

People…we’re talking about nebulae that emit photons like gamma rays.

Let me give you an idea of how crazy that is. Here is the interstellar medium.

You see the interstellar medium whenever you see a nebula. It’s the stuff between the stars. Generally, all nebulae are made out of the same stuff, whether they’re hazy-blue reflection nebulae, bright pink emission nebulae, shadowy dark nebulae, glowing round planetary nebulae, diverse supernova remnants, or…nebulae produced by pulsar winds.

Specifically, nebulae are a visual phenomenon—what you see when a specific portion of the interstellar medium is lit up.

This stuff isn’t naturally hot. Like I said, it’s between the stars. If it were as hot as stars, the whole night sky would glow at visible wavelengths, instead of just the stars twinkling on an otherwise dark canvas. Even when the interstellar medium emits its own light, as in the case of most nebulae, it’s usually in the infrared, radio, or visible wavelengths.

Gamma rays are ridiculously high-energy for a nebula. But, sure enough, that’s what we see where neutron stars are involved.

Here’s another tidbit for your reading pleasure…

Since all pulsars are neutron stars, and all neutron stars are the remains of stars that went supernova (and produced supernova remnants)…shouldn’t we expect to find all pulsars at the heart of supernova remnants? And, conversely, shouldn’t we expect to find a pulsar at the heart of every supernova remnant?

The weird fact is…it doesn’t actually work that way.

But why?

For one thing, not all neutron stars have radiation beams that sweep directly over Earth. These neutron stars are not called pulsars. So a neutron star found in the center of a supernova remnant wouldn’t necessarily be a pulsar.

For another thing, neutron stars don’t just spin fast, they travel fast. Supernova remnants don’t. So many neutron stars, no matter if they produced pulsars or not, leave their supernova remnant behind. We in the present day might observe a supernova remnant whose neutron star left it behind long ago.

There’s yet a third complication. Since pulsars are so small, they lose energy slowly, and they remain detectable for around 10 million years. Supernova remnants, on the other hand, tend to dissipate into the interstellar medium after around 50,000 years.

Last but not least…not all supernovas produce neutron stars.

So, the long story short? Most pulsars are actually found outside of supernova remnants, and most supernova remnants do not actually contain pulsars.

But…wait a second. How come not all supernovas produce neutron stars?

Never fear—we’ll get to that! But first, we’ll cover binary pulsars and some particularly unique neutron stars in my next couple of posts.

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