Astronomers recently discovered the remains of a dead star that definitely set a record, even if they’re not sure which one.

At somewhere between 2.09 and 2.71 times the mass of our Sun, the recently discovered object could be the most massive neutron star ever discovered; then again, it could also be the tiniest black hole. Either way, it could shed light on the details of a giant star’s final moments, when the star’s burned-out core collapses into a densely-packed ball of extreme physics. And this particular object has a wild, gloriously morbid backstory that may be the key to the whole cosmic mystery.

Astrophysicist Ewan Barr, of the Max Planck Institute for Radioastronomy, and his colleagues, published their findings in the journal Science.

Unidentified Astronomical Object

Barr and his colleagues recently used the MeerKAT radio telescope array in South Africa (which may well have one of the cutest names humanity has ever bestowed on a telescope) to precisely measure the timing of a pulsar 39,500 light years away. Every now and then, when a massive star dies, its burned-out core ends up spinning wildly — hundreds of times a second — and blasting radio beams outward from its poles like a cosmic lighthouse. By measuring the timing of those pulses, Barr and his colleagues could calculate exactly how fast the pulsar was spinning.

This particular pulsar, called PSR J0514-4002E, orbits a shared center of gravity with a companion, unseen but detectable by its gravitational pull on the pulsar. It doesn’t shine, so it can’t be a star; it must be a stellar corpse, but what kind? Barr and his colleagues used their MeerKAT data to work out how long it took the pulsar to orbit its companion and what the pair’s combined mass was. From there, they managed to calculate how much of that mass belonged to pulsar 4002E and how much belonged to its companion.

The companion turned out to be about 2.35 times the mass of our Sun. That makes it slightly more massive than the heaviest neutron stars astronomers have found so far but much lighter than the smallest known black holes, which weigh in at around 5 times our Sun’s mass. Since astronomers usually tell black holes and neutron stars apart by their mass, that makes 4002E’s companion an unidentified astronomical object.

When a giant star dies, if the core left behind is less than two or three times our Sun’s mass, it collapses into what’s called a neutron star, which is exactly what it says on the tin: a ball of neutrons that used to be a star but has now collapsed so hard under its own gravity that all its atoms have been crushed into neutrons. Some of those neutrons may even have been crushed into even smaller particles called quarks, forming a type of matter that behaves in ways conventional physics can’t properly describe.

But if the core left over after the supernova is more than two or three times more massive than our Sun, it keeps collapsing. Atoms squash down into neutrons, which squash down into quarks, which squash into something “too dense to be described by anything other than gravity,” as University of Toronto astrophysicist Maya Fishbach put it in a recent paper commenting on Barr and his colleagues’ study. All this happens in a matter of catastrophic instants, and the result is a black hole.

The deciding factor in the fate of a star’s burned-out core is its mass. But physicists aren’t sure exactly where that line is because there’s a gap between the heaviest known neutron stars (2.2 to 2.5 solar masses) and the lightest known black holes (about 5 solar masses). This object fits neatly into that gap.

A Glittering Danse Macabre

It’s not the first object astronomers have found in the so-called “mass gap” between 2.5 and 5 solar masses. They’ve spotted a few more over the years — all of them in mergers between neutron stars, which produced ripples in space-time called gravitational waves. In fact, without gravitational wave observatories, we wouldn’t know of any objects in the mass gap.

The gap’s existence has puzzled physicists for years because they can’t come up with a reason there shouldn’t be any objects with masses between 2.5 and 5 solar masses. Some have suggested that something in the extreme physics of supernovae may somehow avoid forming black holes in this mass range. But at this point, we just don’t know because we don’t know enough about the detailed physics of those intense moments when a stellar core collapses. We know the broad strokes, not the tiny subatomic details that could answer that question.

But the fact that the only mass-gap objects known (until this one) have been spotted in the gravitational waves of neutron star mergers suggests an interesting possibility.

“Even if stars do not collapse into black holes in the mass gap, such objects are produced in the merger of two neutron stars,” writes Fishbach. In other words, maybe objects like this one only form when two super-dense stellar corpses collide and merge.

That could make this particular stellar corpse’s wild, morbid history a little more understandable.

We know that this object, whatever it turns out to be, is orbiting another dead star, pulsar 4002E. Based on the pulsar’s extremely fast spin, and the fact that the pair orbit each other on a weirdly lopsided path, [authors] say this object may not have been pulsar 4002E’s original partner. The pair live (well, un-live) in a densely-packed region of space called a globular cluster: a bright, crowded ball of stars all perilously close to each other.

“Stars at the center of a globular cluster constantly tug on each other gravitationally,” writes Fishbach, “breaking up weaker binaries and assembling new, tighter binaries.” That may be how pulsar 4002E and its mysterious partner got together amid the throng of whirling stellar pairs at the heart of the cluster. And based on the mystery object’s mass, Barr and his colleagues say it may actually have formed in a collision between two neutron stars before caught up in its current danse macabre.

Space is bizarre, and it only gets weirder the more we learn.

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