And she tried to fancy what the flame of a candle is like after the candle is blown out.
—LEWIS CARROLL, ALICE’S ADVENTURES IN WONDERLAND
On my side of the planet, it was nearing nightfall, a steady rain ushering in the darkness a bit early. For Nikhil Sarin, though, it was a sunny Stockholm morning at NORDITA (Nordic Institute for Theoretical Physics) when we began our video call. If anyone could explain the fate of a binary neutron star system, I figured that he could, since he had recently defended his award-winning dissertation on the topic. We fought through some technical difficulties so that Sarin could share his computer display with me, and after a few keystrokes, he excitedly presented a screen filled with—I squinted—graphs?
It wasn’t quite the swirling simulation of merging neutron stars that I had been hoping for, but I was game to understand the data nonetheless.
“These plots show you how the luminosity in hard X-rays and also gamma rays changes over time,” he explained. The objects emitting the most energetic types of light are over a billion light-years away. That anything about them could be observed was astonishing.
“So take a look here.” He waved the cursor over a place where the intensity of the X-ray emission had initially dropped a bit on the graph, but then leveled off. “The X-ray plateau lasted, in this case, at least a million seconds. We believe that this is a neutron star that survived a binary collision and was actually ultimately stable.”
“What if it doesn’t survive? What happens to it?” I asked.
Sarin made cursor circles around another graph. “Look at this one in the middle. There are some X-rays lasting up to 100 seconds, and then all of the sudden, they just seem to switch off.” He seemed particularly enthusiastic about this event, but it was impossible to tell why.
“What does that mean?” I asked.
“That could be the collapse,” he replied.
A black hole.
Sarin had dozens of graphs to choose from, each telling the story of the last step of a binary neutron star dance billions of years ago. Some showed a lasting, stable emission of high-energy light for the duration of the observations. Survivors. Others seemed to hold on for up to a couple of minutes and then drop off. Some of them came together before Earth was even formed, their light finally arriving just as we developed the tools and talent to understand what we are seeing.
Nine decades had passed since scientists had ascertained that there was only so much mass that can be piled onto the corpse of a low-mass star before the outward pressure of the electrons no longer supports the white dwarf’s weight. Once it hits the Chandrasekhar limit and its electrons find refuge within its protons, the white dwarf collapses into a denser, smaller neutron star. It’s the rare white dwarf that can actually achieve the Chandrasekhar limit, though, as they have a habit of blowing themselves to smithereens just before reaching that point. But the makeup of the core of a massive star does allow for this fate when it gets to the end of its last reserve fusion tank. The iron core begins collapsing into a neutron star when the Chandrasekhar limit is reached, the electrons unable to stand any further pressure. It’s a feature that yields a fair degree of uniformity in the masses of neutron stars.
It’s natural to then wonder: if there is a limit to how much pressure electrons can withstand, is there also a limit to how much pressure neutrons can cope with before they, too, buckle?
The answer is, apparently, yes. What is less certain is what the limiting mass is before a gravitational bottomless pit opens up in the fabric of spacetime.
“If you look back in the 1980s, people said the maximum mass of a neutron star should be maybe 1.8 or 1.9 solar masses,” Sarin explained. “And everyone was happy. Then—whoops!—astronomers observed one with 1.98 times the mass of the Sun. So the nuclear physicists said, ‘Let’s try something new.’ ” The “something new” would have to allow neutrons to withstand more than twice the mass of the Sun squeezed into a city-sized ball.
Then a slightly more massive neutron star was discovered, and theorists were once again back at the drawing board. Astrophysicists at that point afforded the neutron star lots of wiggle room. Three times the mass of the Sun, they figured, was definitely, positively, absolutely the maximum mass that a neutron star could withstand before fully collapsing into something that creates such a steep dent in spacetime that even light can’t race up its falling sides and escape. As a result, that limit—three solar masses—shows up in some basic astronomy textbooks when the discussion turns to the fate of the most-massive stars. If the collapsing core has more mass, a black hole forms. Less than that? Well, let’s just say that there’s not a hard and fast limit like there is with a white dwarf.
So far, we have largely ignored stars capable of leaving behind black holes, but their story is essentially the same as their slightly lower-mass cousins that pack 9 to 25 times the Sun’s mass. They will gorge themselves on hydrogen, fusing madly and producing the light of hundreds of thousands of Suns for a cosmic blink of an eye. This energy pushes outward, the atoms themselves acting as miniature sails catching the light and racing away from the gravitational grip of the star. A relentless light-driven wind moving at speeds of thousands of kilometers per hour gradually strips off the outer layers of these stars. The most-massive stars produce so much energy in their cores that they actually tear themselves apart, often expelling much of their outer layer of hydrogen in the process. The relatively lightweight Zeta Puppis, a blue supergiant with 25 times the Sun’s mass, is currently losing about 1/500,000 of the mass of the Sun each year to these stellar zephyrs. But even when the stars tear themselves apart, the feast must go on. When the hydrogen is exhausted, the stars turn their ravenous appetites to helium, carbon, oxygen, and on down the line.
Most likely, such a star—at least to begin with—is in a multiple system, where it might merge with a companion as Eta Carinae did, a disruptive process that also helps the star to shed some unwanted pounds. Or perhaps the stars in a multiple system have enough distance between them that they each live out their lives held together by only a tenuous bond. No matter a massive star’s path, its days are numbered, and once it fuses iron in its core, the party is over.
Does it go out with a bang? Possibly. The more massive member of Eta Carinae, with over 100 times the mass of the Sun, could ultimately wind up being something called a “superluminous supernova,” a pinpoint of light that despite its enormous distance, would be brighter than the full Moon. It would be a formidable contender for the title held by the supernova of 1006, visible even during the day. When the dust ultimately cleared, the remaining object would not be a neutron star. The crushing pressure inside the heavyweight star of Eta Carinae would certainly breach the three-Sun limit in its core, so it would leave behind a black hole.
There’s another way its future could play out, though, a scenario just as dramatic but significantly less flashy. The collapse that begins in the core simply might not end. For stars with less than about 25 times the mass of the Sun, the creation of neutrons provides a barrier for the inward rush of material trying to race down the gravitational drain. But Eta Carinae is far too massive. It’s possible that the failure of electrons to support the iron core will be followed almost instantaneously by a similar failure of neutrons to support the crushing pressure. Anything falling inward would simply keep falling.
“In that case,” Eta Carinae researcher Nathan Smith said, “you might just look out one day and say, ‘Hey, Eta Carinae looks much fainter today’ because you’d only be seeing light from the remaining companion star.”
He continued: “The problem is that we can’t really predict the end fate of some stars because we don’t understand the physics well enough to say which stars of a certain mass will explode and which ones will collapse. But I’m hoping one of those things will happen to Eta Carinae right around the time I retire.”
The lack of understanding is a problem. In fact, there isn’t even a firm consensus on the lowest mass required for a star to leave behind a black hole. Some researchers push that figure as far down as 17 times the mass of the Sun, which would mean Betelgeuse, the star occupying Orion’s armpit, might simply vanish as a black hole in the not-so-distant-but-probably-not-in-our-lifetime-and-certainly-not-before-Smith’s-retirement future. So much depends on what is going on under the star’s surface, including how quickly its various layers are rotating, the star’s exact composition, and how much churning goes on inside as the star tries to pull energy out of its core with giant conveyor belts of unfathomably hot and violent plasma.
Even with all the uncertainties, astronomers think they have caught stars in the act of disappearing. For instance, in 2015, a red supergiant star in the galaxy NGC 6946, also known as the Fireworks Galaxy for its enthusiasm for producing supernovae, simply . . . vanished. Whether this disappearance was due to a complete collapse of the star or to the stellar equivalent of hiding behind a smoke screen is unclear. The Fireworks Galaxy is 25 million light-years away, so astronomers can’t exactly jet over there to check on it. But the fact that such a collapse isn’t out of the question reveals how far astronomers have come in accepting some of the completely bizarre things the universe conjures up.
The universe doesn’t care how a neutron star’s weight limit is surpassed. All that matters is that it is surpassed—whether by the collapse of a too-massive core of a single star or by the merging of objects that individually are well below the maximum mass allowed for a neutron star. In the case of the Hulse-Taylor binary, which will come together in an upcoming geologic era (stay tuned!), the neutron stars weigh in at 1.44 and 1.39 times the mass of the Sun, each essentially at the Chandrasekhar limit. For now, these two are fine, but in 300 million years, they will merge to create something well over 2.5 times the mass of the Sun. When they do, the resulting object will likely be a black hole, but only after a fierce battle. As it turns out, it’s one thing for a stellar core to implode under its own weight, but it’s another thing entirely when two neutron stars experience a close gravitational embrace.
“It isn’t a nice friendly handshake upon coming together. It’s more of a ripping-apart explosion,” astronomer Jeff Cooke explained animatedly, telling me this story as a prelude to another, seemingly unrelated one. “By the end, the intense gravity starts shredding them apart, and they start to get really stretched out, spilling neutrons everywhere. And that’s messy, because if you have a neutron that’s not inside an atom or in a neutron star, it only lasts for about 15 minutes.”
That fact feels like it should get much higher billing in high school chemistry classes. Protons, the positively charged heavyweight cousins of neutrons, apparently have no expiration date, even when left out in the open. At least, that’s what decades of underground Cherenkov detectors like Kamiokande II and its global siblings have told scientists. But making up a large percentage of the familiar material inventory of the universe are particles that, left to their own devices, will self-destruct within a lunch break. When two neutron stars come together, torrents of these particles are unleashed, and the neutron clocks start ticking. Once their internal timers run out, the neutrons spray the area with protons, electrons, and, to satisfy the universal accountant, the antimatter version of a neutrino.
Forcefully shredded from the gravitational prison of the neutron stars, the newly converted protons and escaped neutrons fuse to form atomic nuclei, the very things that the massive star’s core squashed out of existence when it collapsed. In the ensuing mosh pit of protons, neutrons, and atomic nuclei, heavier and heavier elements are forged. It is all in a day’s work for this stellar alchemist to whip up the Earth’s weight in gold, and along with that comes a flood of radioactive elements that will gradually shed bits and pieces of themselves, along with gamma rays, for up to thousands of years.
The electromagnetic energy released from the initial flurry of destruction is considerable, perhaps a tenth of a foe, but this figure pales in comparison to the full foe that two white dwarfs conjure up upon colliding. But unlike a Type Ia supernova, which obliterates two white dwarfs in a stellar-mass fusion bomb, a merger like this blasts away only about 1% of the combined mass of the neutron stars. That’s still a few thousand times the mass of Earth, but nothing compared to what is ejected in a supernova. As for what happens to the remaining 99% of the matter, well, that’s the story that Sarin’s graphs tell astronomers.
What his graphs don’t reveal is their half-century-long backstory.