Heat not a furnace for your foe so hot that it do singe yourself.
—WILLIAM SHAKESPEARE, HENRY VIII
Two years before the appearance of S Andromedae, the event that made astronomers rethink what the universe was capable of, our planet experienced its own fabled cataclysm: the eruption of Krakatoa. The entire island was—and the operative word is “was”—only about 45 square kilometers in area, situated inconveniently at the boundary between the Indo-Australian tectonic plate and the Eurasian plate. As it had done for millennia, the Indo-Australian tectonic plate crept along imperceptibly, moving scant centimeters each year as it rode northward on a current of magma driven by forces still deeper inside Earth’s swirling interior. Off the shore of Sumatra, it hit the Eurasian plate. Unable to resist the push of the churning interior, but equally unable to persuade the stubborn Eurasian plate to move out of the way, the Indo-Australian plate took a dive. Pulled beneath the barrier by a planetary conveyor belt, it grasped at its opponent. There was a moment of tension, just a blink of an eye on the geologic time scale, and then . . .
Release.
It was just a bit of a slip, globally speaking. A readjustment to ease the strain. But to the bipedal life forms scrambling around on Earth’s surface, it was legendary. One warm-up eruption on 26 August 1883 was followed the next day by four intense explosions that were quite literally heard around the world. The force of the blasts shot ash and pumice miles into the air, and the debris rained back down in a muddy rock-storm. The smoke and ash darkened even the noonday skies and triggered eerie, almost supernatural lightning.
The shock waves from the explosions were so intense that anyone within 160 kilometers of ground zero was rendered permanently deaf, assuming they were lucky enough to survive. Two-thirds of the island almost immediately dropped off the map as though swallowed by a city-sized sinkhole. Tsunami after tsunami of biblical proportions—some estimate that the waves were up to 30 meters in height—swept over the nearby islands, killing tens of thousands. The weary remnants of those waves reached as far as Hawaii 11,000 kilometers (7,000 miles) to the east. Mysterious booms reminiscent of distant cannons were reported as far away as Perth, Australia, 4,500 kilometers (2,800 miles) southward. Current estimates suggest that Krakatoa’s explosive power was equivalent to 200 megatons—that’s 200 million tons—of TNT.
More subtly, barometers around the globe responded to the sudden, but imperceptible changes in air pressure as the dissipating sound waves whispered secrets about their cataclysmic origins. For days, these inaudible ripples circled the planet, nudging the readings on barometer after barometer in what was, in hindsight, a completely predictable pattern. Had meteorologists of the time been better networked, they could have even divined something of the nature of the disaster from those weak signals. But those puzzle pieces would have to wait to fall into place.
On a planet like Earth, Krakatoas are an inevitable rite of passage. The interior is a torrent of enormous convection cells that, upon reaching the surface, push and pull thin crustal fragments. Here, a rift is opened; there, a mountain range appears. And at the border of the Indo-Australian and Eurasian plates, solid crust is pulled down to a planetary forge. But fast-forward billions of years into the future, and our crust will become too thick, the convection in the mantle no match for the stubbornness of the surface.
All Krakatoas stop eventually.
Despite its fame, Krakatoa was not the most powerful event to rock our world. Not by a long shot. The dinosaur-killing asteroid impact of 65 million years ago upended the Yucatan with the strength of half a million Krakatoas, changing the course of evolutionary history in our favor. Tsunamis the height of skyscrapers would have raced across the Gulf of Mexico, chasing after winds that would make a Category 5 hurricane seem like a gentle breeze.
But as intense as these events were, they were not truly Earth-shattering.
Certainly not star-shattering.
To measure what it takes to be a star shatterer, we need numbers beyond megatons of TNT, beyond Krakatoas, and even beyond the dinosaur killer. The Sun, a modest star, spends its life internalizing the equivalent of more than a billion Krakatoa eruptions each second in its core as it fuses hydrogen nuclei into helium nuclei, releasing energy in the process. Yet on a stellar level, this violence is easily contained, kept in check by the suffocating embrace of its 700,000-kilometer-thick (435,000-mile-thick) blanket, which pushes inward against the explosive energy with the pressure of hundreds of billions of atmospheres.
The constant distillation of energy from mass in the Sun’s core, a power source that was understood only after Albert Einstein’s “very interesting conclusion” in 1905 that mass and energy are equivalent, destroys nearly 4 million tons of matter every second. Taking the place of that 4 million tons per second are approximately 380 trillion trillion watts of power. Remarkably, the Sun has the capacity to eat mass at this rate and to shine with this wattage (give or take) for about 10 billion years.
The Sun’s longevity can be attributed to its colossal mass. To express the Sun’s heft in kilograms requires 30 zeroes, which is why astronomers almost never do that.* Instead, we simply say that it contains one solar mass. Whether reported in kilograms or solar masses, our nearby star contains more than 300,000 times as much material as Earth. The history-altering asteroid impact of 65 million years ago would have been less dramatic than a fly hitting the Sun’s enormous, blazing windshield.
Thus, while consuming 4 million tons of matter per second might seem like stellar gluttony to those of us impressed by a single Krakatoa, the Sun’s appetite is, cosmically speaking, more like that of a slow-nibbling picky eater. After all is said and done, the Sun will have converted just over 3% of 1% (0.034%) of its entire mass to energy. In the process, it will have released a grand total of about 120 billion quadrillion quintillion joules of energy (that’s 1.2 × 1044 joules, or 12 with 43 zeroes after it). This is assuredly an enormous amount of energy, but its release is diluted over billions of years.
As you can see, attempting to describe the power source of the Sun in joules or Krakatoas becomes an exercise in keeping track of all the zeroes. But in the cosmos, these big numbers are commonplace. Baade and Zwicky first hinted at the enormity of the energies that the universe is capable of generating, and subsequent generations have refined their figures. Enormous energies are so common, in fact, that in the late 1970s, a relatively obscure unit of measure was invented almost jokingly: the foe.
As legions of astronomers had done before them, Hans Bethe and Gerald Brown were wrestling with the physics of exploding stars and their leftovers. In their calculations, Bethe and Brown frequently arrived at the staggeringly enormous 1051 ergs, a number similar to the original supernova calculations of Baade and Zwicky, and so they created a catchy acronym: foe = fifty-one ergs. The unit never really caught on, unfortunately, possibly because it fails to capture the fact that 51 is not the number of ergs, but the power to which ten has been raised. As everyone knows, there is a vast difference between 51 mosquitoes and 1051 of them.*
The erg is another relatively obscure unit, and a minuscule one at that. Officially equivalent to one ten-millionth of a joule, an erg is approximately the energy required for a mosquito to take flight. If 1051 mosquitoes were to abruptly and simultaneously take flight (an impossibility even in the swampiest of climates), they would release star-shattering energy. A foe.
Why astrophysicists, who study the largest, most powerful things ever, adopted a standard unit of energy so tiny has been a perennial puzzle. It has been suggested that using the centimeter-gram-second (CGS) system of units, rather than the more familiar meter-kilogram-second (MKS, or standard metric) system simplifies some of the astrophysical equations. On the other hand, the results in either convention still become ten to the (incredibly large) power, so it really matters little whether “incredibly large” is 44 or 51.
Modern supernova researchers still agree that the quantity 1051 ergs (1044 joules) needs its own unit, though, and in those closed circles, it is known as the bethe. Truth be told, I think the foe sounds more appropriately menacing. The existence of these units illustrates what one of my graduate professors once said: The answer to every problem in astronomy is one, as long as you adopt the appropriate unit.*
The bethe (née foe) does make an excellent shorthand for swift comparisons. On that scale, Krakatoa becomes quite friendly, having exploded with only about ten trillionths of a quadrillionth of a foe, or 0.00000000000000000000000001 (10−26) foes (or bethes) of energy. The Sun, meanwhile, will steadily generate a grand total of approximately 1.2 bethes of energy over its 10-billion-year lifetime, some tiny scrap of which will have been harnessed by the life forms on its third-nearest neighbor.
That’s what a fathomable, well-behaved star does, after all.
Despite Zwicky’s and Baade’s early assertions that all stars will eventually explode, the reality—disappointing to many—is that the Sun is never going to blow up. It has neither the mass nor the companionship to pull off this feat. While it’s true that our star is fully capable of unleashing life-altering fury on us, a supernova Sun is simply not in the cards.
What is in the cards for the Sun and for countless stars containing about the same mass, give or take a factor of a few, is a lengthy run where the size and energy output remain mostly constant, at least cosmically speaking. The rest of its life will unfold fairly predictably. It will run out of fuel in its core, at which point the core, no longer experiencing the support of its energy source, will succumb to the inexorable squeeze of gravity. The core will then shrink and heat.
And shrink.
And heat.
As hydrogen pours into the reserve tank vacated by the retreating core, a new round of hydrogen fusion will kick in. This new energy, coupled with the energy released by the heating and shrinking core, will upset the balance that the Sun has enjoyed for 10 billion years. The outer layers, once equally attracted by gravity and held at bay by the churning, hot plasma inside, will feel a greater outward push than inward pull. The edge of the Sun will swell, and it will become a red giant, incinerating the inner planets as it does so.
It is somewhat unfair of me to characterize the Sun’s life as uneventful. Near the end, it will enjoy one brief, incredibly energetic phase, but only once the shrinking core hits the admirably hot temperature of 100 million degrees Celsius. At this temperature, the helium nuclei, each a collection of two protons and two neutrons, will find that their strong mutual repulsion is not enough to keep them from fusing into carbon nuclei, each containing six protons and six neutrons. An additional helium nucleus occasionally will find its way into one of these, and oxygen, with eight protons and eight neutrons, will be born.
Getting three of anything to come together simultaneously is challenging, but the fact that like charges strongly repel at all but the shortest of distances makes matters even worse. Overcoming this repulsion and thrusting the protons and neutrons together into a tightly bound carbon nucleus is a bit like trying to roll a ball up a steep hill with a deep gorge on the other side. The repulsion of the positive charges creates the hill, but there is a counterintuitive strong attraction between protons and neutrons when they get within about a proton’s width of each other, hence the gorge. If you can get the particles up the hill and to the edge of the gorge, they will naturally plummet in.
The scorching hot temperatures found in the helium core of such a star are enough to race the subatomic balls up the repulsing hills—and even burrow through the hills. They will then collect at the bottom of the gorge as freshly made carbon nuclei.
At this point, the star experiences a bit of a crisis. Dropping a ball into a gorge releases energy, and so does combining helium nuclei into carbon. This energy finds itself trapped in the dense core, and no matter how hard it pushes on the material, it won’t budge. In the everyday life of an Earthling, if something heats up, it expands. Warmer air is less dense than cooler air, a feature that allows for hot air balloons. And in the everyday life of the Sun, the same is true. But in the near-death existence of the innermost heart of the Sun, adding energy fails to expand the matter in the core. It merely heats it.
Unfortunately, heating the core means the helium nuclei are coming together even faster to form carbon as more balls are rolled up the hill and plunged into the gorge even faster. And more fusion means more heat, which means more fusion, which means more heat . . .
Which means that within a minute, the core of the Sun will experience something called a helium flash, fusing nearly half its helium to carbon and releasing as much energy in 60 seconds as it currently releases in 10 million years. Or, if you like, about 0.003 foes of energy.
It would seem that something this energetic would be enough to blow the star apart, but the process is anything but flashy, at least to an outside observer. That energy doesn’t create an obvious transient event, but instead mostly goes into rearranging the core structure enough so that the helium fusion is no longer a runaway process. The dying core then settles back down, this bit of indigestion suitably soothed, and proceeds to quietly fuse the rest of its helium into carbon and oxygen. The outer portion of the star, kicked off by the initial energy imbalance, even settles back down a bit temporarily. Ultimately, though, the fragile relationship between core and envelope ends, and the outer portion of the star drifts away into space, leaving the inner core of carbon and oxygen behind.
This is no ordinary ball of carbon and oxygen, though. Gravity’s unrelenting squeeze has forged a material so dense that, with its present particle residents, it can be no denser. Resisting the crush of gravity are electrons obeying the bizarre rules of the very small. On these scales, things like electrons are not really objects. An electron is happy to coexist in the exact same location as another electron, but not if the other electron is doing the exact same dance. On a subatomic, quantum level, the dances of electrons are marked by their energies, and in the dying star, gravity has compressed the matter to the point that there is no more energy for the electrons to give and still exist.
In this state, the network of carbon nuclei, oxygen nuclei, and electrons will be so dense that a teaspoon of it would weigh as much as a car. The Sun’s remains will contain quite a lot of teaspoons of material: enough to fill the volume of Earth but dense enough that it will hold nearly half the mass of the Sun. Having just been immersed in a stellar forge for billions of years, the resulting object will also be white-hot. A millionth of the Sun’s current volume, this dense stellar end point is known as a “white dwarf.” Meanwhile, the sloughed-off outer shell will become something known as a “planetary nebula,” briefly glowing—here, “briefly” means about 10,000 years—with the intense, high-energy light of the exposed heart.
No explosion. Just a dead, cooling core and a gradually exiting envelope of rarefied hot gas.
The end.
(Or is it?)
Because of its mass, the Sun’s fate might not be particularly exciting, but plenty of stars do explode. To create the sort of supernova that Zwicky and Baade envisioned, you need to start with a star whose mass is between 9 and 25 times that of the Sun. Those stars are not easy to come by. Less than one in 100,000 stars are born with such heft, and those that are die in a flash. If the Sun’s entire 10-billion-year life were compressed into a day, a star with 10 times its mass would be gone in about three minutes. A star with 25 times its mass would be gone in less than a minute. Of the millions of stars within 1,000 light-years of Earth, there is only one monstrous 25-solar-mass cosmic mayfly—Zeta Puppis, also known as Naos—and even it is likely a hair farther than 1,000 light-years.
In the simplest explanation, the life of a star is dictated by how rapidly it uses up its own fuel stores, and this pace is determined by the unforgiving laws of physics. The nuclei of four hydrogen atoms can fuse into the nucleus of a single helium atom while converting some of the original mass to energy only in environments of extreme temperatures and pressures. The most-massive stars have such extreme environments in spades, and as a consequence they burn through their hydrogen at a rate tens of thousands times that of the Sun. If the Sun swaddles a billion Krakatoas each second in its core, these stars cradle tens of trillions. The end result is the same, though. Eventually both gluttons and dainty eaters will consume all the hydrogen on their plate (in their core), and this is where a star’s mass makes all the difference.
There is a poster in nearly every Astronomy 101 classroom that illustrates the seemingly unremarkable track that the Sun and its ilk will take from hydrogen fusion to giant to planetary nebula to white dwarf. The same poster reveals the slightly more exciting fate of the one-in-a-million stars with significantly higher masses. The extreme environment that allowed for hydrogen fusion shrinks, forcing helium nuclei to join to make carbon, oxygen, neon, magnesium, sulfur, and ever heavier atomic nuclei. Each new fusion channel is shorter and shorter in duration as the star’s core desperately tries to squeeze another bit of life from the nuclear mass. All the while, the dying star’s outer layers are swelling, and the star morphs into a supergiant.
When the core fuses its contents into iron, the star is done. Unable to produce further energy, but equally unable to efficiently shed the energy it has created, the heart of this seething monster hits temperatures of several billion degrees Celsius and densities billions of times that of water. Although the star has spent its entire life working to create its iron core, the high-energy light trapped within now destroys it, ripping apart the iron nuclei.
It might not be immediately obvious why this should be a problem for the star, but pulling so much light energy out of the core to disintegrate the iron nuclei is like pulling out the first of many support blocks. The balance of light and particles and gravity was already a precarious one, with gravity held at bay largely by the outward push of electrons in the core. That balance is tipped ever so slightly by the removal of light energy and the rearrangement of the core’s particles. The core begins to collapse, and as it collapses, it becomes hotter and denser. Soon, protons and electrons, typically holding each other at arm’s length by the rules of subatomic particles, join to become neutrons. Taking all that like-charge repulsion out of the picture is like removing the last support block. The core has nothing left to hold itself up until the nuclear forces between the neutrons put a halt to the madness.
All of this plays out in less than a second. In the time between the tick and the tock, the core has compressed almost to the point of vanishing, becoming even more intensely hot and dense in the process. Now 100 billion degrees Celsius and 100 trillion times as dense as water (about 100 million times as dense as a white dwarf), this least extreme forge of a massive star crafts a newly minted neutron star. The energy generated in this final dramatic act of the stellar core blows the rest of the star to kingdom come in a heartbeat.
And that’s what a not-so-well-behaved star can do.
This story makes sense at the most qualitative level. You can practically hear astronomy students muttering “iron core . . . core collapse . . . boom!” as they study for their exams. But the ellipsis between “core collapse” and “boom!” has historically been an enormous source of frustration for supernova theorists. Clearly stars went boom! Computer models, however, have struggled to create anything that isn’t a dud.
One problem is that we simply haven’t had an abundance of nearby core-collapse supernovae to scrutinize. Even by the mid-1960s, just as astronomers were getting a handle on the processes that trigger these explosions, only about 250 supernovae of any kind had ever been observed. Perhaps half of these had been the final acts of massive stars. Perhaps.
Things have been no easier on the theoretical side. A dying massive star is both enormous and enormously complicated. If placed where the Sun is, its outer envelope would engulf the inner planets, its interior churning with complex and dynamic interactions even before core collapse. As David Branch and J. Craig Wheeler point out in their 2017 book, Supernova Explosions, theorists have to contend with “hydrodynamics and turbulence, magnetic fields and rotation, weak and strong particle interactions, neutrino transport, hot and catalyzed matter at nuclear densities and above, condensed matter and many-body physics, and the potential, at least, to produce exotic ‘strange’ particles, never mind black holes.”
It’s hardly a wonder that the recipes we humans program into computers so often fail to reproduce what the universe cooks up.
The initial supernova models in the 1960s and 1970s were idealizations that by necessity ignored all but the most fundamental properties. Stars were treated as one-dimensional objects, which is to say that their internal conditions were dependent only on the distance from the center of the star. There were no asymmetries, no rotation, no magnetic fields, and certainly no hope of adequately representing reality. They are to modern supernova models what the pixelated 1972 Atari game Pong is to immersive virtual reality: a necessary start. You have to admit that Pong, for all its shortcomings, paved the way to more richly rendered environments.
For a moment, though, let’s forget about all these complexities and focus on the core. The core of a massive star contains 1057 protons, give or take, along with an equivalent number of neutrons and electrons. Just before “the end,” those core protons and electrons are forced by the extreme conditions to become neutrons. But the universal accounting system cannot let this particular union happen without an additional payment. Protons are a type of subatomic particle known as a “baryon,” a relative heavyweight consisting of even tinier subatomic particles known as quarks. Electrons, on the other hand, are part of a different class known as “leptons,” lightweight particles that have distinctly different interactions with the rest of the universe.
In any particle interaction, particles themselves might come and go, but some things are fixed. The universe will not allow the number of baryons to change, nor will it allow the number of leptons to change. It also strictly requires the incoming and outgoing charges to be the same. The universal ledger is extraordinarily unforgiving. What this means is that when the proton and the electron become a neutron, there must be another particle formed, another lightweight one. Furthermore, because the initial proton-electron pair had no net charge, the neutron and the newly created lepton must not have any charge. A neutral lepton must exist.
This is what physicist Wolfgang Pauli concluded in 1930, to his immediate regret. “I’ve done a terrible thing today, something which no theoretical physicist should ever do,” he declared. Then he confessed the mortal sin of science: “I have suggested something that can never be verified experimentally.”
Pauli understood that a neutral lepton is necessarily an elusive beast. It almost never features in high school chemistry classes alongside its well-known cousin, the electron, for the simple reason that it doesn’t interact with much of anything. Ever. Chemistry is all about electrons changing their allegiances, and even the proton and neutron play secondary roles. As a result, many people are under the false impression that the material universe consists solely of protons, neutrons, and electrons. But by the time you finish reading this sentence, tens of billions of vanishingly tiny neutral leptons from deep in the heart of the Sun will have raced at nearly the speed of light through your left thumbnail. And your right knee. And your eyelids. Turn your face to the warm springtime Sun, and trillions will pass unimpeded through your visage for every beat of your heart.
Unlike the photons of sunshine warming your skin, these particles will likely never interact with any part of you (although, strictly speaking, you have about a 25% chance that one will affect one of the trillions of quadrillions of particles that comprise you at some point in your decades-long life). In fact, the entirety of Earth doesn’t alter their determined straight-line trek from the Sun’s core to the edge of the Galaxy and beyond, and a trillion kilometers of lead is almost as transparent to them as a perfect vacuum. Almost.
And Pauli was almost correct. The neutrino (“little neutral one” in Italian) was ultimately experimentally verified, but only because scientists devised the most enormous and bizarre traps just to catch a glimpse of a few of them.
It’s hard to fathom that such a particle could play much of a role in anything noticeable. And yet, as improbable as it sounds, neutrinos are the driving force behind the explosions of massive stars. They even act as an early warning system, alerting us to the explosive brilliance that is to come, if only we can notice them in time.
A show as destructive as an exploding star should, it seems, leave something obvious in its wake, but unfortunately for supernova surveyors, there is no hope of seeing such a thing from millions of light-years away. The concept of an object that blew itself to smithereens did seem to account for at least one much closer enigma, though: the Crab Nebula. Astronomer Nicholas Mayall waxed eloquent in a 1939 leaflet. “In the year of our Lord 1054,” he wrote with a flourish, “when Omar Khayyam was a small boy, and the Battle of Hastings was still twelve years in the future, an unknown Chinese astronomer, perhaps weary and sleepy after working all night, was astonished to see a strange and brilliant new star appear.”
For nine centuries, the written record of this apparition was unknown in the West, but it surfaced in the 1920s. By then the location of the new star had been well observed, but not because it was associated with those accounts of a new star. Instead, this patch of sky had been the first entry in a catalog of objects whose common bond is their nebulous appearance. Compiled by French astronomer Charles Messier in the 1700s, the Messier Catalog is a list of 110 comet impostors, objects that have the wispy look of comets but, frustratingly to comet hunters like Messier, aren’t. What he cataloged ranged from galaxies to stellar birthplaces to, in the case of Messier 1 (aka the Crab Nebula, aka NGC 1952), remnants of stellar deaths.
Technically situated in the constellation Taurus but appearing to hover over the head of Orion, the Crab Nebula is a relatively easy target for amateur astronomers with even modest telescopes. Unfortunately, through small telescopes, the object is an unremarkable fuzzy patch, which is how virtually everything in the Messier Catalog appears through such telescopes. Fuzziness was, after all, pretty much a prerequisite for inclusion. As telescopes grew, it became apparent that there were two major categories of fuzzy patches: those that looked that way because they harbored countless stars and those that were bona fide gaseous nebulae with wispy or bubble-like appearances.
As the years passed, the Crab Nebula showed itself to be unlike anything else astronomers had seen. In short, it is a mess, a great cosmic sneeze. Irregular tendrils wind their way through a hazy, slightly elongated blob. One of the original sketches that captured anything beyond a foggy oval had the appearance of an insect, and so—inexplicably—it became known as the Crab Nebula.
What’s more, astronomers noticed it was growing.
Using observations spanning decades, astronomers were able to measure this growth. By imagining the clock running in reverse, they estimated when the entire nebula was at ground zero. Edwin Hubble, best known for his work on the expansion of the entire universe, declared in 1928, “the nebula is expanding rapidly and at such a rate that it must have required about 900 years to reach its present dimensions.” It seemed like far more than a coincidence that a bright new object had appeared in the same patch of sky nine centuries before.
Emboldened by this victory in the early twentieth century, astronomers scrambled to find the smoking gun remnants of other naked-eye supernovae, but those searches failed to produce anything as unambiguous as the Crab Nebula. It wasn’t the scientists’ fault. They just didn’t have the right eyes to see them.