Neutron stars—the compact remains of massive stars that have gone supernova—are some of the most extreme objects in the universe, narrowly beaten by black holes (and, as we’ll talk about in future posts, active galaxies and such).
Dense balls of pure neutron material with diameters barely larger than Los Angeles, neutron stars have strong magnetic fields that produce beams of radiation at the magnetic poles. Their speedy rotation makes these beams sweep across the sky like a lighthouse.
When one of their beams crosses directly over Earth, human astronomers observe rapid pulses of light called pulsars.
These objects are whacky, to say the least. And there’s more…
Way back when we spent a number of posts surveying the stars, we covered binary systems. These are star systems that contain multiple stars. Imagine if our sun had a companion, and two stars rose and set in our sky over the cycle of day and night.
It might surprise you that the majority of stars in the universe are actually in binary systems. Our solar system seems to be an outlier in that regard. Most stars have a companion or two or six…
…and so do some neutron stars.
Remember that neutron stars are the collapsed remnants of massive stars that have gone supernova. If most stars are part of binary systems, then naturally, some of these stars will evolve into neutron stars and still be part of their birth system.
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.
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?