What is a Pulsar?

Imagine you’re observing the sky with a radio telescope. Observing the faintest, lowest-energy photons the universe has to offer is your specialty. You study interstellar dust clouds, protostars, and lots more.

One day, though, something interesting pops up in your data. You’re looking at raw data on a computer screen, not an eyepiece of a “typical” (optical) telescope—you get all your data from the giant dish above. Strangely enough, there’s a series of regular pulses.

At first, you think it’s just “noise” from sources on Earth—like static on your car radio. But then you see it, day after day, in the same place in the sky. It’s not static. It’s real.

You wonder if this is perhaps evidence of contact with a distant civilization. Personally, I’d hope for that one. Unfortunately, more research leads to the conclusion that it’s nothing of the sort—within weeks, you find that there are several other objects in completely different parts of the sky, all emitting similar (but different) pulses.

You’ve discovered a pulsar. But…what exactly is a pulsar?

In my last post, we explored the theoretical implications of neutron stars, the remains of stellar cores that are basically neutron soup. We wondered what evidence there is that neutron stars actually exist.

At the time pulsars were discovered, neutron stars had already been theorized, but it took some time and observations to connect the dots. Now, let’s follow the evidence that pulsars are actually neutron stars.

The first clue came in the form of the periods of the pulses, which ranged from 0.033 to 3.75 seconds. And each one was crazy exact—nearly as exact as an atomic clock. But here’s the even weirder part. The periods were also slowly lengthening by a few billionths of a second each day.

Honestly, just the precision of that measurement impresses me. But it’s not just the astronomers’ accuracy we should be noticing. Whatever produced these pulses produced this kind of on-point accuracy in the first place.

So, what could possibly produce such fast and extremely regular pulses?

Definitely not variable stars such as Cepheids—while these stars do pulsate, they don’t do so nearly fast enough to be a pulsar. And the answer can’t be a main-sequence or giant star because even if it had a hot spot on its surface that flashed bright on our instruments whenever it faced Earth, the star couldn’t rotate fast enough to produce a pulsar’s fast pulses.

The second clue was actually found in the pulses themselves.

Imagine that a normal star blinks on and then off in the space of 0.001 seconds. You won’t see a 0.001-second pulse. Why?

Remember that the objects we’re talking about are spherical. They have a single point on their surface that’s closest to an observer on Earth, and the rest is a bit farther away. It has a slightly longer distance to travel.

Even as the light from the point on the object closest to you changes brightness, the same fluctuation occurs for the light coming from the rest of the object. For most objects in the universe, by the time the light from the other side of the object reaches you, it has most definitely been longer than 0.001 seconds.

The result? You’ll essentially see the change in brightness smeared over a greater time interval than it actually took the object’s surface to fluctuate.

So what does that actually mean?

Well, if a pulse of light from an object only lasts 0.001 seconds, some math that I don’t understand yet says that the object can’t be larger than 300 km in diameter. The smallest objects we’ve talked about so far are white dwarfs, but even they tend to be around the size of the Earth—too big for a pulsar.

Neutron stars, on the other hand, tend to have diameters comparable to the distance across Los Angeles. They’re actually so tiny that they can’t pulsate slowly enough to match observations of pulsars.

Which…means there’s still a missing link here. And it comes in the form of one of my favorite supernova remnants, the Crab Nebula.

Because the Crab Nebula is a supernova remnant, we can theorize that a neutron star has been left behind at its center. And, lo and behold, a pulsar has been discovered blinking away right at the heart of the nebula.

Another hint that there is a neutron star at the heart of the Crab Nebula is actually the blue haze you see spread throughout the nebula.

That haze is produced by synchrotron radiation, which is not actually a type of radiation so much as a cause of radiation. Long story short, synchrotron radiation is electromagnetic energy radiated when high-speed electrons spiral through a magnetic field.

Here’s the thing. This energy can be spread across any part of the electromagnetic spectrum. So, because we’re seeing a hazy glow at the relatively short wavelengths of visible light, that means there must be a source of high energy at the center of the nebula.

Meaning…a neutron star.

So…why do pulsars blink on and off, anyway?

The Lighthouse model of a pulsar

For our purposes, I’m going to describe pulsars as slightly different from neutron stars. Pulsars are the visual phenomena we observe—the pulses of light. Neutron stars are the physical phenomena we theorize—the cause of the pulses of light.

Essentially, a model of a pulsar consists of a neutron star at the center of a powerful magnetic field. Beams of radiation jet from the poles of the magnetic axis, where “hot spots” occur on the surface of the neutron star.

Notice that the neutron star’s axis of rotation and magnetic axis do not align; they’re actually closer to being perpendicular. This means we can think of a neutron star as like a lighthouse.

A pulsar’s magnetic field has a dipole structure, just like Earth. Radiation is emitted mostly along the magnetic poles.

As the neutron star rotates, its beams—which are locked to its magnetic poles—sweep across the sky. If one happens to cross over Earth’s surface, observers on our remote little planet see a pulse of light…that is, a pulsar.

That means that all pulsars are neutron stars, since the pulsation phenomenon we see is caused by a neutron star. But not all neutron stars produce pulsars because not every neutron star has radiation beams that sweep directly over Earth.

Notice the implications for other potential civilizations. A neutron star that we humans don’t see as a pulsar might sweep its beams directly over another planet out there, and to any creatures living on that planet’s surface, the neutron star would appear as a pulsar.

So, here’s the million-dollar question. What the heck produces these beams of radiation, anyway?

It’s not well understood, and I’m probably going to botch up the explanation enough to appall experts on the subject. But I think my understanding is enough to form a general mental model of what goes on, if not to apply to the intricacies of how the radiation beams are formed.

Essentially, we know that the magnetic field generates “hot spots” on the star at the poles of the magnetic axis. As I understand it, it seems that the magnetic field also funnels radiation out from those hot spots like wind through a tunnel. It’s a product of the magnetic field being so insanely powerful.

Now we’ve covered a number of aspects of neutron stars, from the theoretical to the observational. Next up, we’ll combine theory and observation to follow a neutron star’s evolution.

Questions? Or just want to talk?

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