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You can go to ground.news/scishow or click the link in the description to get 30% off the Vantage level subscription. They may be star corpses, but neutron stars are still intense.

Most are more massive than the Sun, but smaller than New York City. Their surfaces can get as hot as a million degrees Celsius. And some also spin almost a thousand times a second, turning themselves into interstellar radio wave beacons.

But some neutron stars are so extreme they seem to break the laws of physics. They’ve got too much mass and they spin too fast. Astronomers have spent decades working on the puzzle, and they’ve found a very important piece that could solve the whole thing.

They’re called black widow pulsars, and they can turn stars into planets. [intro jingle] The classic Cliffs Notes for neutron star formation goes like this: A star that’s a lot more massive than our Sun runs out of usable fuel in its core. The nuclear furnace turns off, and the entire star quickly collapses under its own gravity. Along the way, particles in the core get squeezed so close together they turn into neutrons… the only kind of matter that can resist such absurd pressure.

Then, the rest of the star’s collapsing gas bounces off this neutron ball and blasts out into space as a supernova. This dramatic explosion carves some absolutely gorgeous structures into the surrounding interstellar gas and dust. And nestled at the heart is that leftover neutron core, which astronomers call a neutron star.

But that’s not the end of the story. Like a skater pulling in their outstretched arms, the star’s material spins faster as it collapses, so neutron stars spin much faster than conventional stars do. Then, as they wobble and spin, their twisted magnetic fields grab nearby particles and whip them around, sending out pulses of radio waves that we can detect from here on Earth if we’re in the beam’s path.

As a result, many of the neutron stars we know about are called pulsars. But as you might expect, this Cliffs Notes summary misses some critical details. For one thing, nobody quite agrees on how those beams of radio waves actually happen.

Magnetic fields are almost certainly involved somehow. But that’s not all they do. They also help regulate how fast the neutron star is spinning.

Without those fields, a collapsing star could easily spin so fast that it would tear itself apart. So about a year after its supernova, astronomers expect to find a neutron star spinning between once and a hundred times a second. And they’ve definitely found plenty of young pulsars spinning at those rates… but they’ve also found a lot of pulsars that spin faster.

And I mean Way faster. In fact, impossibly faster, given what we know about how they form. Neutron stars just shouldn’t be able to reach six or seven hundred rotations per second on their own.

At that spin rate, if you could somehow survive hanging out at the equator, you’d be moving at around 20 percent of the speed of light. And to get going so quickly, astronomers think these outliers have to act a little … arachnoid. Many spider species engage in what’s called “sexual cannibalism”, where females eat their mates once they’re done with them.

And while it might not seem like it from our one-star solar system, many stars have partners. And just like spiders, sometimes those partners are radically different sizes. Now if you’re a star, how much mass you’re rocking dictates how long you live.

So if you’ve got one of these spider-esque setups, one star can keep humming along while the other explodes and collapses into a neutron star. And once that drama’s over, the neutron star may still be close enough that it can gravitationally siphon off the outer bits of its partner. And since those outer bits come with a little momentum of their own, the neutron star doesn’t just gain mass.

It also starts to spin faster. So to astronomers, it’s no surprise that many of the fastest pulsars have nearby stellar companions. They’re known as redback pulsars, named after a species of Australian spider.

But I did say “many”. Some loner pulsars are also out there spinning hundreds of times a second. Which they shouldn’t be able to do without a companion.

So how did they get so fast? Well, the better question to ask is how lonely are they, really? Back in 1988, a team of astronomers reported on a pulsar whose light flicked on and off 430 times each second… which told them how fast it was spinning.

But once every nine hours or so, it also seemed to turn off for about 50 minutes. The light was being blocked by something orbiting the pulsar that made no visible light of its own. They concluded that this pulsar was the first example of what would be known as a black widow.

It didn’t just steal some gas from its companion star. It stole its life. A star can only be a star if it has enough mass to keep its core hot enough for at least your standard hydrogen fusion.

The minimum mass is about 8% that of our Sun’s, or about 80 times the mass of Jupiter. So basically, this black widow pulsar did start off with a companion star, but it siphoned off so much gas that the star shut off. It became a gas giant planet.

Over the decades, astronomers have found a bunch more black widows as well as redbacks. But finding these things isn’t just about bookkeeping. Astronomers have a good idea of what a neutron star’s minimum mass is.

Because if there’s not enough gravity, then everything can’t be squeezed into neutrons. But predicting the maximum mass is trickier. If a neutron star is too massive, it’ll collapse into a black hole.

But where that limit is depends on a lot of physics that nobody quite knows yet… from the tiniest details of gravity to the way matter acts under the incredible pressures at the center of a neutron star. Since the most massive neutron stars appear to be black widows, they’re a great place to look for answers. Back in 2022, one team reported the most massive “well measured” neutron star to date: And it was 2.35 times the mass of the Sun.

But according to some scientists, that’s too massive for our current laws of physics to account for. And it’s not the only black widow that seems to break the law. Something is keeping them from collapsing into black holes, but no one knows exactly what that something is.

One popular idea is that the core of a neutron star is filled with an ocean of free-floating quarks that adds some extra structural support. What we do know is that neutron stars are incredibly weird. They challenge our assumptions about how matter works, and the study of black widows only adds to that mystique.

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