In 1967, S. Jocelyn Bell Burnell was a graduate student at Cambridge University. She and her thesis advisor, Anthony Hewish, were using a custom-built radio telescope to study radio signals from outer space.

To their surprise, they saw a series of blips coming from a specific point in the sky. It looked like a star -- but it was blinking. The blinks came like clockwork, every 1.34 seconds. It was so regular that it seemed like it couldn’t be a natural signal. At first, they thought they might have detected a beacon of some kind... an alien beacon.

What they’d found wasn’t aliens, but it was just as strange: a star, denser than liquid mercury, revolving completely in just over a second, and made of a material totally unlike anything you’d find on Earth.

Radio telescopes like the one Burnell and Hewish were using can be very sensitive to outside interference. With all the AM and FM broadcasts around, they first had to eliminate any Earthly source for the mysterious blinks. But it was clear the signal was coming from outer space. As a joke, Burnell and Hewish called the signal “LGM-1”, for Little Green Men.

They knew it was unlikely that the signal was actually coming from an alien civilization, but they didn’t want to publish their discovery until they had a better idea of what it was. The last thing they wanted was to announce that they’d found extraterrestrial life, only to have it turn out they that were wrong.

Then they found a second set of blips, which made it pretty clear that this wasn’t something artificial. Because what were the odds that two sets of little green men happened to be pointing their beacons at planet Earth?

What they’d found was a pulsing star, or pulsar, specifically PSR B1919+21. The catchy name comes from its celestial coordinates. But it was another astronomer, Thomas Gold, who first proposed that pulsars are neutron stars.

A neutron star is formed when a very, very massive star — at least eight times the size of our Sun — explodes in a supernova at the end of its life. All that’s left is the star’s iron core. Gravity wants to pull the core into a smaller sphere.

It pulls so hard, it starts to affect the atoms in the core itself. The electrons in the core are actually squeezed into the nuclei of their atoms, where they combine with protons to form neutrons.

Neutrons actually make up almost all the matter in the stellar core — that’s why it’s called a neutron star. And this strange form of matter is unbelievably dense. A neutron star might have 1-3 times our Sun’s mass, all smushed into a sphere just a few kilometers in diameter. Astronomers Walter Baade and Fritz Zwicky first predicted neutron stars existed in the 1930s. But no one had ever observed any kind of neutron star until Burnell and Hewish found their pulsar almost 40 years later.

So where do a pulsar’s pulses come from? Pulsars have extremely powerful magnetic fields. This makes them emit beams of electromagnetic radiation, often in the form of radio waves, from their magnetic poles. What we see as pulses are the beams of radiation sweeping across Earth as the neutron star spins, like the beams of a lighthouse. And they’re spinning faster than you might expect.

Normally, adult stars do rotate — but at a nice, leisurely pace. As a star’s core collapses after a supernova, though, it starts to spin much faster. It’s like an ice skater. As she pulls her arms in closer to her body, she spins faster. The same thing happens with a neutron star. As it shrinks, the core spins faster.

And because neutron stars are so small, they can end up spinning really quickly. The fastest pulsar we’ve ever found is spinning 716 times per second! Astronomers think the pulsars with the fastest spins -- called millisecond pulsars because their spins can be measured in milliseconds -- started their lives in binary pairs, pairs of stars orbiting each other.

After the pulsar’s core collapses, its super strong gravitational pull sucks down material from the surface of its companion star. As the neutron star’s mass increases, the star spins even faster. These whirling bodies are sometimes called recycled pulsars, since they re-use material from other stars.

Because a pulsar’s rotation is so fast and so regular, any changes to it are really significant. The first exoplanets ever discovered were actually orbiting a pulsar. They were found because the planets’ gravity affected the pulsar’s motion, causing changes in the pattern of pulses.

But what’s probably the strangest pulsar is part of the only binary pulsar system we’ve ever discovered. That’s two pulsars orbiting each other. The smaller of the two pulsars is the weird one: it doesn’t seem to have exploded in an enormous supernova, like a regular neutron star. It had already lost a lot of its mass to its companion, which became a pulsar first. Astronomers think it must have exploded at some point, but in an explosion that was smaller than a supernova. So they’re not quite sure the star was like before it became a pulsar or how exactly it turned into one. It’s just one more unexplained pulsing signal teasing astronomers.

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