Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)

Chapter 3. The Stellar Fury of Supernovae

THE FIRST ONES TO NOTICE ARE PROFESSIONAL astronomers.

Researchers at the Super-Kamiokande neutrino observatory in Japan are shocked when their detectors light up like Christmas trees. Such unprecedented readings prompt them to look for malfunctions in their hardware, because surely no astronomical event could generate so many of the ghostly subatomic particles—even the Sun, the brightest object in the sky, barely produces enough neutrinos to be picked up by their instruments. There would have to have been millions of neutrinos detected to register so strongly! Poring over their instruments, it takes them nearly two hours to figure out that the flood of neutrinos was indeed real, which was far too late . . . not that advance knowledge would have helped.

Within minutes of the event, automated observatories orbiting the Earth perk up. Astronomical satellites designed to observe high-energy light such as X-rays and gamma rays see a rise in detections. One by one, as they slew over to focus on the source of the particles, their detectors saturate with photons as the fierce light intensifies. Within minutes the satellites are blinded, overwhelmed with light, and lose track of the target.

On the ground, across the night sky of Earth, thousands of amateur astronomers, truckers, police officers, and general night owls notice the light in the sky. It’s getting brighter by the minute. Some of the amateur astronomers momentarily think it’s an airplane, or the glint of reflected sunlight off an orbiting satellite. But many immediately realize what’s happening, and start taking data. Others send out e-mails, alerting astronomers all over the world. Get out your scopes! There’s a new supernova!

But the e-mails are unnecessary. Within minutes, the “new” star is so bright that other stars in the sky can’t compete. Like the sunrise or the full Moon, the supernova is washing out the sky around it.

Astronomers are beside themselves with glee. It’s been over four hundred years since the last naked-eye supernova in our galaxy, and this one will no doubt be a record breaker.

But their joy is short-lived. In the middle of their observations, all their machines suddenly lose power. The images and data are all lost when the computers controlling the telescopes die. And before they can properly assess the problem, all the power goes out. One astronomer ventures outside to see what’s going on and realizes that the glow of the nearby city is gone. Normally, the combined luminescence of thousands of streetlights, buildings, spotlights, car dealerships, and house lights drowns out the fainter stars in the sky. In a huge ironic twist, the power is out everywhere and the sky is truly dark for the first time in years, yet she cannot observe because her power is off too. Her telescope is useless.

She stares upward at the stars and, after a few minutes, realizes the sky isn’t as dark as it was earlier: the fierce eye of the supernova is glaring down on her, and the sky around it is blue. No other nearby star could possibly compete.

Her attention is diverted when she sees another bright light in the sky, this one moving slowly across the ever-bluer sky. She realizes it’s the International Space Station. She laughs, glad to see something normal for a moment.

What she doesn’t realize is that the astronauts on board are dead. Had she known, she certainly wouldn’t have smiled. But then, in a few years, everyone on Earth will be dead too. Humans were doomed from the instant the first rays of light from the supernova touched the atmosphere.

Gamma rays from the supernova destroy vast amounts of ozone, which is quickly reduced to half its normal amount. When the Sun rises in the morning, its ultraviolet light will stream all the way through the atmosphere nearly unabated. Severe sunburn will be the least of the problems faced on Earth as the UV radiation kills off the ocean’s phytoplankton, which make up the base of the food chain. Animals that feed off phytoplankton find their food source dwindling and eventually disappearing in mere days, and animals that feed off those animals face the same dire problem a few days later. This die-off marches up the food chain, and it won’t stop until it reaches the top: us.

It’s been a long time since an astronomical event touched off a mass extinction. But now, another one is under way.

A STAR IS BORN

If you go outside on a dark, clear night, far from city lights, you can see thousands of stars sprinkled across the sky. They may seem unchanging, fixed—some people even refer to the night sky as the “starry vault,” implying a strength and permanence. Sure, the stars rise and set, but that is a reflection of the Earth’s motion, not theirs. They twinkle too, but again the fault lies in ourselves and not the stars—they flicker because the ocean of air above our heads blurs their light.

Even if you go out night after night, week after week, you may not see any changes in the stars. A sharp-eyed observer may note that some stars subtly and periodically change their brightness; these so-called variable stars wax and wane over days and weeks. But the stars themselves neither appear nor disappear, and do very much seem as permanent as the night sky itself.

But the Universe is deceiving. Things do change, and sometimes that change can be dramatic. On July 4, 1054, a new star appeared in the sky in the constellation of Taurus. Chinese astronomers recorded this “guest star,” noting that it appeared to be even brighter than the planet Venus, which is third only to the Sun and the Moon in our sky. There are records scattered throughout the world of the appearance of this new object, though they are spotty and not without some controversy, but there is no doubt about the reality of the event.

Today, a thousand years later, if you use a pair of binoculars to scan the sky in the constellation of Taurus between the horns of the bull, you might note a faint fuzzy blob that is clearly not a star. A small telescope can back up this observation. A big telescope—especially one equipped with a camera capable of taking long time exposures—reveals this object to be a gaseous and filamentary cloud. It looks like the aftermath of an explosion. In fact, images taken many years apart reveal the gas cloud (called a nebula by astronomers, from the Latin word for “cloud”) to be expanding; filaments and knots in the cloud have obviously moved outward in the intervening time. Backtracking the expansion shows that all the gas originally came from one point in the sky, the position of which is marked by a star very near the center of the cloud, indicative of a single explosive event. By measuring the speed of the expansion, the date of this event can be estimated. Remarkably, that date is the mid-eleventh century, suspiciously close to when the Chinese guest star was observed. Today, no astronomer on the planet doubts that the two events are the very same thing.

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The Crab Nebula is the expanding debris from a massive star that exploded in July 1054. It is perhaps the best-studied object in the sky, and one of the most beautiful.

NASA, ESA, J. HESTER AND A . LOLL (ARIZONA STATE UNIVERSITY)

What the Chinese saw was one of the largest and scariest events in astronomy: a supernova. It might not have seemed all that scary at the time—after all, it was just a light in the sky! But upon closer examination the magnitude and scale of the event are revealed.

The gas cloud—called the Crab Nebula because of its current dubious resemblance in a small telescope to a crustacean—is the expanding remnant of this stellar explosion. In the subsequent millennium since its creation the cloud has grown to trillions of miles in diameter. The gas is still ferociously hot, heated to thousands of degrees by the shock waves generated as it expands supersonically and rams into the cooler gas surrounding it. Energy also continues to be poured into the cloud by the emanations from the central star, the cinder left over from the explosion.

The distance to the Crab is an estimated 6,500 light-years, or about 40 quadrillion (40,000,000,000,000,000) miles, and yet even at such a distance, and after ten centuries, it is one of the brightest nebulae in the sky. At the time, the supernova event itself was bright enough to be seen in full daylight, indicating that in just a few weeks the explosion released awesome amounts of energy into space—as much as the Sun will put out over its entire lifetime of 12 billion years. In fact, a typical supernova can easily outshine the combined light from all the hundreds of billions of stars in an entire galaxy, and persist that way for weeks.

Our eyes can only see visible light, a very narrow slice of the energy band of light called the electromagnetic spectrum, which includes radio waves, infrared, ultraviolet, X-rays, and super-high-energy gamma rays. If you had X-ray eyes, the Crab would be one of the brightest objects in the sky. The same is true of radio waves, and if you could see in gamma rays, the Crab would be the single brightest persistent object in the sky.

I’ll gently remind you that the Crab is 400 million times farther away than the Sun.

Clearly, supernovae are awesome events capable of wreaking destruction on a vast scale. At its remote distance, the explosion that generated the Crab Nebula was little more than a pretty light in the sky, but not all supernovae are so far away. In fact, the Earth has had close shaves with exploding stars in the past, and there will certainly be more in the future.

But how close is too close? To understand just what a supernova can do to its environment and just how these events can be a danger to us on Earth, we’ll have to understand what would make an otherwise stable star explode.

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Hubble snapped this picture of Supernova 1994D, the fourth exploding star seen in 1994. The host galaxy is called NGC 4603, and is located a very safe 100 million light-years from Earth. The supernova was about as bright as the entire galaxy.

NASA AND J. NEWMAN (UC BERKELEY)

THE LIFE OF A STAR

While ancient astronomers were baffled by the stars in the night sky—were they holes in the vault of the sky, letting the radiance of the Sun through?—we have a pretty good understanding of them now.

Stars are not just points of light—each is a sun in its own right, most smaller but some fantastically larger and more luminous than our Sun. What a revelation that must have been, the first time someone realized that stars are suns, but terribly far away!

As astronomers studied stars, slowly and inevitably they learned more about them. Some stars are red, and some are blue (you can see this with your own eyes by examining a handful of the brightest ones), which indicates that they have different temperatures: red stars are cooler, blue stars hotter. Many stars are not individuals, but instead are pairs of stars orbiting each other in what are called binary systems, only masquerading as single stars because of their remote distance. Using laws of physics established by the astronomer and mathematician Johannes Kepler in the seventeenth century, astronomers could determine the masses of the stars in binaries, opening the door to an understanding of the physical processes inside them.

At the most basic level, a star is a big ball of gas, and so its behavior is in many ways simple. As a gas is compressed its temperature will rise. A ball of gas with the mass of the Sun will compress under its own gravity, heat up, and shine brightly, but it will have a limited lifetime—without an internal source of heat it will cool in about a million years or so.

By the nineteenth century there was mounting evidence the Earth was at least millions of years old, and perhaps even older. And surely the Sun was older than the Earth! Then, in the 1930s, scientists realized that a star is a nuclear furnace, like a vast H-bomb, whose explosion is held in check by the star’s own gravity. Nuclear fusion could support the Sun’s energy output for not just millions but billions of years, solving its age crisis. In an ironic twist, objects as huge as stars are powered by the tiniest of objects: atomic nuclei.

A typical atom is composed of a dense nucleus in its center surrounded by a cloud of negatively charged electrons. The nucleus contains electrically neutral neutrons, and positively charged protons. The number of protons is what gives the atom its characteristic properties: for example, hydrogen has one proton in its nucleus, helium has two, oxygen eight, and iron twenty-six. Under some circumstances (intense heat or absorption of ultraviolet light, for example) electrons can be removed from an atom, but it’s the number of protons in the nucleus that defines the atom.

As you might remember from grade school science, like charges repel. If you try to squeeze two atomic nuclei together, their mutual positive charges resist it. But in the cores of stars, temperatures are in the millions of degrees—meaning the atomic nuclei are zipping along very quickly, making collisions between them frequent and violent—and pressures are so high that the nuclei are squeezed together very hard indeed. If that electrostatic repulsion can be overcome, other nuclear forces take over that merge the nuclei, fusing them together.

This nuclear fusion does two things. First, it creates a new type of atom, since the new nucleus has more protons than either of the two nuclei before the merger. In general, four hydrogen atoms fuse together to make helium (two of the hydrogen protons become neutrons in the new helium nucleus), three heliums fuse to make carbon, and so on. The actual process is far more complicated than this, but that’s the basic idea.

Just as important, nuclear fusion releases energy. When you look at the overall process of fusing nuclei, you would expect that the total mass of the atom created by fusion would equal the sum of the masses of the atoms going into the process—a lump of clay created by smacking together two smaller lumps of clay would have the same mass as the sum of the two lumps, of course. But nuclear physics is different from what we see in the everyday macroscopic world: atoms are ruled by quantum mechanics, with its weird properties and common-sense-defying behaviors.

In the process of nuclear fusion, a small amount of the mass is converted into energy. The energy produced is enormous compared to the mass; it follows Einstein’s famous equation E = mc2, where the energy produced is equal to the mass times the speed of light squared—and the speed of light is a very big number. Even so, the mass converted is so tiny per atom that the energy released is incredibly small—it would take a million hydrogen atoms fusing into helium to equal the energy released when a flea jumps.

But stars are vast repositories of hydrogen. As we saw in chapter 2, in the core of the Sun, 700 million tons of hydrogen are fused into 695 million tons of helium every second! The missing 5 million tons are converted to energy, and that is enough to power the star, letting it give off the heat and light we need to survive. In fact, the heat released is what holds the star up from its own gravity: the pressure to expand outward from the energy release balances the gravity trying to crush the star. An equilibrium is maintained as long as the gravity and energy release remain constant.

As stars go, the Sun is on the big end of the scale (most stars are much less massive, less energetic, and less luminous); however, far larger and more massive stars exist. The nuclear fusion in a stellar core depends very strongly on the mass of the star, with the rate increasing rapidly with mass. A star with twice the mass of the Sun fuses hydrogen into helium in its core more than ten times faster than the Sun does, and is therefore ten times as luminous. A star with twenty times the mass of the Sun—and many such stars exist—“burns” its nuclear fuel over 36,000 times faster than the Sun. Even though such stars have more fuel, they go through it so much more quickly that their lifetimes are significantly shorter; the Sun will fuse hydrogen steadily for billions of years, while a 20-solar-mass star might live only a few million.

They say that even the brightest star won’t shine forever. But in fact, the brightest star would live the shortest amount of time. Feel free to extract whatever life lesson you want from that.

What happens when the hydrogen runs out? It should be noted that a star like the Sun never really runs out of hydrogen; most of the mass of the star, in fact, is hydrogen! But fusion only occurs in the core, where the pressure and temperature are highest. In the outer layers of a star it is much cooler (tens of thousands of degrees as opposed to millions), so fusion cannot take place. This gas isn’t available to the core anyway, so it can’t be fused. It’s like having a gas can in the backseat of your car. It’s there, but it doesn’t do much good while you’re driving.

But in the core, eventually, the available hydrogen runs low. As the process of converting hydrogen into helium goes on, the helium nuclei build up in the very center of the star. Because helium has two protons, its nuclei resist coming together even more than hydrogen nuclei do, so it takes higher temperatures and pressures to fuse them. For stars with half the mass of the Sun or less, these conditions are never met. Eventually the star runs out of available fuel, and energy generation ceases.

But for more massive stars the helium “ash” can continue to build up. The core gets more massive, its own gravity crushes it more and more, and eventually the conditions for helium fusion are met. In a flash, helium nuclei smash together to form both carbon and oxygen nuclei. This process releases even more energy than hydrogen fusion, so the star becomes more luminous—it literally gets brighter. All the extra heat from the core is dumped into the surrounding envelope of hydrogen. This throws off the balance of pressure outward versus gravity inward, so the star responds as any gas does when heated: it expands. The star swells in size to epic proportions.

Ironically, though, the outer layers of the star cool off! While the total energy emitted by the surface of the star increases, the surface area increases even more. Each square inch of star emits less energy; it’s just that there are a whole lot more square inches than before. Even though the star gets more luminous, it cools off, becoming red. Because of its color and size, the star is called a red giant.

This is the eventual fate of the Sun. Eventually carbon and oxygen will build up in its core, and just as before, it takes more heat and pressure to fuse them than helium. The Sun doesn’t have what it takes to fuse carbon or oxygen, and the process ends there.21

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Just before a massive star explodes as a supernova, elements are piled up in its core like the layers of an onion. Iron sits at the very center, surrounded by shells of silicon, oxygen, neon, carbon, helium, and hydrogen. When a star gets to this stage, it doesn’t have very long to live.

AURORE SIMONNET AND THE SONOMA STATE UNIVERSITY EDUCATION AND PUBLIC OUTREACH GROUP

Stars with more than about twice the Sun’s mass do have what it takes to get to this third round of nuclear fusion. In their cores carbon can fuse into neon, releasing even more energy. But it takes even more massive stars to get neon to fuse into magnesium and oxygen, and more massive stars yet to get oxygen to fuse to silicon.22

Silicon will fuse into iron, but it takes a vast amount of pressure and heat, and that can only come from stars with a mass more than twenty times that of the Sun. All of those steps get their turn in such a star, one after another. Each of the steps in the chain, though, takes less and less time, since the temperatures and therefore the fusion reaction rates increase hugely with each process. A 20-solar-mass star will fuse hydrogen for many millions of years, helium for one million years, carbon for a millennium, and neon for just one short year (those steps happen even more rapidly for stars with more mass).

Eventually, the massive star’s core is layered like an onion: hydrogen lies in a shell on the outside, surrounding a shell of helium, surrounding a shell of carbon, then neon, then oxygen, then silicon. Finally, at the deepest part of the core is a sphere of white-hot iron. To be sure, there is some mixing going on, but in general the layers are fairly well separated. But this is just the core: the outer layers of the star up to the surface are still almost entirely nonfusing hydrogen. These layers absorb all this heat being created in the core, and, as in their less massive cousins, this gas swells out, becoming grossly extended. Stars in this mass range, though, get far larger than red giants. They can swell to diameters reaching many hundreds of millions of miles, and so we call these bloated beasts red supergiants.

In such a massive star, after millions of years, the fusion cycle is nearing its end. Iron is different from other elements. Unlike hydrogen, helium, and the others, iron resists fusion under almost any circumstances. No normal star in the Universe can produce the temperature and pressure needed to fuse it. At the very heart of the star, deep inside its core, a ball of inert iron just a few thousand miles across sits there ticking like a time bomb. And when enough of it builds up from silicon fusion, the bomb goes off.

RAGE, RAGE INTO THE NIGHT

And now, finally, we have come to the moment of truth. For a year the iron has been accumulating in the massive star’s core, and all that time has been writing the star’s death sentence.

Until this point in the star’s life the core has been generating energy; now this has stopped. Remember, the heat from nuclear fusion is one factor that supports the star against its own crushing gravity.

A second source of support against gravity is the tremendous sea of electrons in the core. In a normal atom, electrons stay connected to the nucleus. However, in the core of a star the conditions are so extreme that electrons are stripped off their atoms. Anytime an electron tries to attach itself to a nucleus, the intense heat and pressure rip it off again.

In the core, electrons are squeezed together very tightly, and weird quantum mechanical effects become important. One of them is called degeneracy, which is similar to electromagnetic repulsion: if you try to squeeze too many of the same kinds of particles together (regardless of charge), they resist it. This resistance is a major source of support for the core. Degeneracy, together with the raw heat from nuclear fusion, keeps the star’s core from collapsing under its own gravity.

The problem is, degeneracy pressure can only withstand so much gravity. As the iron piles up, the core gets more and more massive, and its gravity gets larger and larger. There is a moment when the iron core reaches its tipping point, when its mass is about 1.4 times the Sun’s. At that point, degeneracy loses. It simply cannot hold back all that mass. Previously, when the star was fusing other, lighter elements, this point was never reached; the next element up the chain would start fusing and the star’s core was saved.

But iron won’t fuse, and degeneracy is no longer enough. The core cannot withstand its own titanic gravity, and its support mechanism fails. Catastrophically. The core collapses . . . but this is no gradual deflation, like a balloon losing its air. When the core of a massive star collapses, it collapses. And all hell breaks loose.

The collapse is incredibly fast: in a thousandth of a second—literally, faster than the blink of an eye—the tremendous gravity of the core shrinks it down from thousands of miles across to a ball of ultracompressed matter just a few miles in diameter. The speed of the collapse is breathtaking: the matter falls at speeds upward of 45,000 miles per second. The core heats up almost beyond belief, to a billion degrees. High-energy gamma rays are produced, and these vicious photons are so energetic they can actually destroy atomic nuclei when they collide with them. This process, called photodissociation, rapidly starts destroying the iron nuclei in the core, blasting them into bits of helium nuclei and free neutrons. This actually makes things worse (if you can imagine), since these can absorb even more energy, accelerating the collapse.

The events in the core reverberate throughout the star. The core was supporting the outer layers of the star, and when the core collapses, for them it’s a real-life Wile E. Coyote moment: just as when the cartoon character suddenly realizes he is no longer over solid ground and starts to fall, the gas from the star’s outer layers suddenly finds itself hovering over a vacuum and comes crashing down. The incredible gravity of the core accelerates the gas hugely, and it slams into the compressed core at a significant fraction of the speed of light.

This creates a huge rebound effect that reverses the direction of the inbound gas and starts to blow it back out. This rebound, as vast as it is, is amazingly not enough on its own to blow up the star; the explosion would stall, and the outer layers would begin to fall once again onto the core. But the star has one more surprise up its sleeve.

Even after the initial collapse, the core is still loaded with electrons. The tremendous heat and pressure from the collapse applies a huge force on these electrons, squeezing them together into the protons in the core. When this happens, the electrons plus protons create more neutrons. But they also create ghostly subatomic particles called neutrinos, and these are what spell disaster for the star.23

Neutrinos are extremely tenuous particles, able to penetrate huge amounts of material without getting absorbed; to them even the densest material is nearly transparent. They blast out of the core, carrying away vast amounts of energy from the collapse. The energy they carry out is nothing short of staggering: it can equal the Sun’s entire lifetime output of energy! In fact, the solid majority of the energy released in a supernova event is in the form of neutrinos; the visible light we see, blinding though it is, only adds up to a paltry 1 percent of the released energy.

The core generates neutrinos in unbelievably prodigious quantities: some 1058 (that’s a 1 followed by 58 zeros, folks) of the particles scream out of the core over the course of about ten seconds. This is just around the same time that the outer layers of the star fall onto the core and begin their failed rebound. Just as the gigantic bounce fails, and all that material is about to fall back on the core, all those countless neutrinos slam into the gas.

Even though neutrinos tend to pass right through normal matter, the stellar gas is incredibly dense. Plus, there are just simply so many neutrinos that some fraction of them get absorbed no matter what—it’s like driving through a swarm of bugs in your car; no matter how much they avoid you, you’re still going to get some goo on your windshield.

Only a tiny fraction, maybe 1 percent, of the neutrinos get absorbed by the gas, but it’s still an epic event: the total energy dumped into the gas is huge.

This, this is what destroys the star.

It’s like setting off a bomb in a fireworks factory. The energy of a hundred billion billion Suns rips into the star’s outer layers, reversing their course, literally exploding them outward. Octillions of tons of doomed star tear outward at speeds of many thousands of miles per second. The event is so titanic that even the tiny fraction of it that is converted into light can be seen clear across the visible Universe.

And that’s just visible light. Other forms of light—X-rays, gamma rays, and ultraviolet light—also pour out of the newly formed supernova. As the shock wave of the explosion tears through the outer layers of the star, pressures and temperatures get so high that nuclear fusion can be triggered. In fact, elements heavier than iron can finally be created in this way, since the conditions in the blast wave are, incredibly, actually more violent than in the core of a star. Radioactive versions of elements like cobalt, aluminum, and titanium are created in the expanding debris, and they emit gamma rays when they decay. The gas, already hellishly hot, absorbs this energy and becomes even hotter, heated to millions of degrees. It glows in X-rays and ultraviolet light. Also, these explosions are rarely perfectly smooth. Some materials will be accelerated faster than others, and the inevitable collisions between them generate even more tremendous shock waves, similar to sonic booms inside the expanding material. This can also generate X-rays.

All in all, a supernova is a seething cauldron of power, chaos, and violence. It is one of the most terrifying events in the visible Universe.

ROUGH NEIGHBORHOOD

Needless to say, anything close to the exploding star is facing upwind in a flaming hurricane. Any planet orbiting the nascent supernova is a goner: having your primary star explode in a billion-degree conflagration can end in only one way, and it’s not pretty. The planets will be torched, sterilized, and any air or water is stripped away by the sheer energy of the explosion.

The sudden decrease in mass of the star weakens its gravity severely, thus ejecting any planets from the system. It’s possible that there are thousands or even millions of scorched rogue planets wandering the Milky Way, their birth stars long since dead. Space is so vast, however, that we may never find such planets even if the galaxy is loaded with them.

Clearly, supernovae are dangerous. Your best bet is to stay as far away from them as possible. But how far away? If a star in our galaxy explodes, how close is too close?

In the appendix is a table that lists all the known stars within 1,000 light-years that have the potential to go supernova. The closest, Spica, a blue giant in Virgo, is about 260 light-years away, and most of the others are considerably farther off. While we can’t give the specific date any one of these stars will explode, it is a dead cold fact that they all will blow up, and some in the next few thousand years.

How much should we worry about this?

It depends on what it is we should be worried about, actually. At first glance, you might think that just the sheer enormity of the event is all you need to consider. An entire star just exploded! But in fact there are many weapons in a supernova’s arsenal. Some are not cause for concern. But others . . .

Kinetic impact

If you’re standing near an explosion, the most obvious danger is from debris. That’s bad enough if you’re near, say, a grenade, but a supernova takes this quite a bit further: the launch of a few octillion tons of gas into space at a significant fraction of the speed of light sounds more than a little dangerous. And it is! But only if you’re relatively close by. A planet circling the doomed star is itself doomed, of course, but what if you’re watching from the cheap seats, around another star?

To simplify the situation somewhat, let’s imagine that all that matter is ejected from the supernova in one instant. We’d see a thin shell of gas expanding outward, its diameter increasing with time. Almost all the mass of the original star is in that shell (the outer layers that explode outward may outweigh the core by several times). As it expands, the area of the shell increases, and so the amount of mass in a given area decreases—it’s very much like light emitted from a lightbulb; the farther you are from it, the more the light gets spread out and the dimmer it appears.

The debris from a supernova spreads out too. If you are on a planet near the explosion, more matter will slam into you than if you’re farther away. In this case, the amount of impacting material will drop with the square of your distance: if you double your distance, you’ll get one-quarter as much material hitting you. But how far away is far away enough?

Just to assume a worst-case scenario, let’s take an improbably close distance of 10 light-years for the supernova. That means it would be about 60 trillion miles from the Earth.24 Let’s further assume the total ejected mass is 20 times the mass of the Sun, about typical for your run-of-the-mill supernova. In this case, the amount of matter that would hit the Earth is about 40 million tons.

Yikes! Duck!

But just how much is that really?

That sounds like a lot of material, but it really isn’t; a small hill about 1,200 feet tall would have about that much mass. If that hit all at once it would be bad—chapter 1 made that very clear—but remember, this would be spread out over the surface area of the entire Earth. In fact, it’s far less than an ounce per square foot over the whole Earth: once spread out, it’s more like a single raindrop falling in your backyard.

And we know it wouldn’t be an extinction-level event, since we’ve survived asteroid impacts of this size and larger before. We might notice a slight diminution of sunlight, but no real long-term effects.

We have a more realistic situation, with the explosion of the star in 1054 that formed the Crab Nebula. At 6,500 light-years away, how much debris will impact the Earth? It turns out to be about 100 tons.25 And again, while 100 tons sounds like a lot, the Earth gets hit by 20 to 40 tons of meteoric material a day. Debris from the Crab is just a bump on top of our normal daily influx. But you needn’t worry anyway: at typical ejection speeds of one-twentieth to one-tenth the speed of light, it will take 100,000 years for that material to hit us—and the event was only 1,000 years ago. Not only that, but the material will certainly never reach us anyway: gas and dust between the stars will slow down and stop the Crab ejecta before it ever gets close.

Optical light

Another obvious feature of supernovae is that they’re bright. The Crab was about as bright as the planet Venus, even from 6,500 light-years away. How close would a supernova have to be for the light to be too bright?

We have to think for a moment about what “too bright” means. Some animals, for example, time their cycles to the Moon. Breeding, feeding, hunting, and so on are timed or at least aided by lunar light. Having a supernova as bright as the Moon hanging in the sky day and night could in theory affect some species.

For a supernova to get that bright, it would have to be at a distance of about 500 light-years. There are in fact one or two stars that close that could explode, notably again the blue giant Spica in Virgo. If it blew up, it would be easily visible in broad daylight, and at night would rival the Moon in the sky, bright enough to read by and to cast sharp shadows! But this extra light would be more of an annoyance than anything else. Bright as it is, the supernova would still just be a point of light in the sky, difficult to look at directly without making your eyes water. However, there wouldn’t be any actual physiological damage to your eyes. You’d just learn to avoid looking at it (or wear sunglasses at night).

There would be no added heat from this new source of light; the supernova would still be too far away to actually warm us up. Think of it this way: the Moon doesn’t heat the Earth noticeably, so a supernova as bright as the Moon wouldn’t either.

One possible problem would be the disruption of some animal cycles, but the effects of this are hard to determine. They might very well be minimal, since even the fury of a supernova dies down with time. Within a few months the explosion will have faded to more tolerable levels. Animal cycles timed with the Moon may be disturbed, but likely would recover.

It’s worth noting that the closer a supernova is, the brighter it is. To get as bright as the Sun, it would have to be much closer: about a light-year. Not only are there no stars that close capable of exploding, there are no stars that close to us at all (except, of course, the Sun itself).

Neutrinos

What about all those neutrinos, created when electrons in the core of the star merged with protons to form neutrons? The total energy emitted is huge. Are we in danger from that?

The answer is a little bit difficult to ascertain, actually. Physically, the direct absorption of the energy of a neutrino by a human cell is not terribly worrisome. Neutrinos are incredibly slippery; in fact, just while you are reading this sentence several trillion neutrinos have passed right through your body, and odds are very high that not a single one was absorbed by you. A supernova would have to be impossibly close—as close as the Sun is to the Earth—to be able to directly kill a human being through neutrino absorption.

But before you sigh in relief, there’s more to consider. Neutrinos can bounce off the nuclei in atoms, and deposit their energy that way, rather like hitting a bell with a hammer. It turns out that this method of depositing energy is more efficient—that is, more likely to have an effect. If a neutrino did this, a cell nucleus (specifically the DNA there) could be damaged, potentially leading to the development of cancer.

Once again, the exact danger from this is difficult to calculate, but mathematical simulations have shown that a supernova would have to be improbably close to do any damage in this manner. The effects are minimal for a supernova farther away than about 30 light-years, and again it’s worth noting that there are no potential supernovae this close to Earth. Your DNA is safe.

Direct exposure to gamma rays and X-rays

Things get stickier when we consider other forms of light. You’re almost certainly familiar with X-rays from visits to the dentist’s office, or if you’ve ever broken a bone. Medically, X-rays are wonderful because they can penetrate the soft tissue of our skin and muscles; as far as an X-ray photon is concerned, those cells are transparent. But bones are denser, and more likely to absorb the X-ray. If you put film underneath an arm, X-rays will pass right through soft tissue and expose the film, while bones absorb the X-rays, leaving only a shadow on the film.

However, soft tissue does absorb some X-rays, and that’s part of the danger. If a cell absorbs the high-energy X-ray, it’s like shooting a bullet into an egg. The energy released as the tissue absorbs the energy can destroy the cell. Low-energy X-rays can also damage DNA, potentially causing a cell to become cancerous. While this sounds alarming, it should be noted that a typical medical X-ray procedure is quite safe—Space Shuttle astronauts, for example, who stay in space for two weeks receive a dose of radiation from the Sun equivalent to about fifty medical X-rays with no ill effects. Digital technology has made it possible to lower the dose even more, since digital detectors are far more sensitive to X-rays than film.

Supernovae are a bit brighter than the dentist’s X-ray machine, though. However, the X-rays from an exploding star can only hurt you if they can reach you. As it happens, we have a built-in shield.

You’re sitting in it.

The Earth’s atmosphere is very good at absorbing these types of light. Many astronomical sources emit X-rays, but astronomers didn’t even know about them until the 1960s because of the Earth’s atmospheric absorption. X-rays are blocked while still high in the atmosphere, so they never reach the ground, and even mountaintop telescopes can’t detect them. It wasn’t until the advent of the Space Age that it was found that stars, galaxies, and other objects emit X-rays.

So we here on Earth are pretty safe from exposure. X-rays, even from a nearby supernova, are absorbed by our atmosphere, posing little threat. But what about any humans above the atmosphere? Astronauts orbiting the Earth in the International Space Station are in fact at risk. Given typical X-ray emission from a supernova explosion, the astronauts will receive a lethal dose if the star is closer than about 3,000 light-years or so. That’s quite a long way! There are many stars capable of exploding within that distance of us. Astronauts are clearly our most serious casualties from the prompt (that is, immediate) radiation from a supernova.

Gamma rays, which are higher-energy than X-rays, have pretty much the same story. They are absorbed by our atmosphere, and pose little threat to human tissue for landlubbers. However, they actually make things worse for our spacebound crew. The absorption of the gamma ray by a piece of metal—say, the hull of a space station—can lead to the metal emitting many X-rays in response; it’s like electromagnetic shrapnel. A solar flare (as discussed in chapter 2) can generate enough gamma rays to do serious harm, and a supernova within a few thousand light-years can still generate enough gamma rays to equal or surpass the amount created in a big solar flare. Direct exposure to these gamma rays can be lethal. The “secondary radiation” from metal absorption can also be very high, lethal in its own right to unprotected astronauts.

Don’t forget that our satellites are also sensitive to this event (see chapter 2). Not only that, but the flash of gamma rays and X-rays from a nearby supernova would ionize the upper atmosphere, creating a cascade of subatomic particles. This would create a strong pulse of magnetic energy that can damage our power grid in the same way a solar coronal mass ejection can (see chapter 2 for details on this kind of event). Communications, television, global positioning, high-flying aircraft, and even the supply of electricity by power lines could be severely damaged by this pulse of supernova radiation.

Again, there are several stars ready to pop within that distance. The odds of any one blowing in the near future are incredibly low, but we are now a spacefaring race, and highly dependent on our orbiting infrastructure. The good news is that if governments take the threat from solar outbursts seriously and fortify our infrastructure against that, we’ll be safe from supernovae as well.

At least, safe from that particular threat. We’re not done touring the arsenal quite yet.

Gamma and X-rays, redux

Before you start to breathe too easily, sitting under this ocean of air, you should realize we’re forgetting something. It’s true that we ground-based humans are safe from direct exposure to high-energy radiation because the atmosphere absorbs this radiation. But then it’s fair to ask, how does this affect the atmosphere itself?

This is potentially the greatest threat a supernova poses.

Our atmosphere is a many-layered thing. We sit at the bottom, where there’s plenty of oxygen mixed in with nitrogen, as well as traces of other gases like carbon dioxide and argon. But up higher, things are different.

As covered in chapter 2, between heights of about 10 to 30 miles above the Earth’s surface sits a layer of ozone, which absorbs dangerous ultraviolet (UV) radiation from the Sun. Unimpeded, this UV light would reach the ground and do all sorts of damage, including causing sunburn and skin cancer in humans. Moreover, many protozoa and bacteria, the basis of the food chain on the planet, are very sensitive to UV.

Obviously, the ozone layer is critically important to life on Earth, and as far as a supernova is concerned, it has a big fat bull’s-eye painted on it.

When the X-rays and gamma rays from a supernova hit the Earth’s atmosphere, they can destroy ozone molecules, leading to the cascading series of events described at the beginning of this chapter. The critical factor, as it has been all along, is distance. How close can a supernova be before it damages the ozone layer enough to affect life on the surface?

This is an important issue, and many scientists have taken it very seriously indeed. Some have set up computer simulations to see how much damage a nearby supernova can inflict on our atmosphere. They used a mathematical model of the atmosphere, which includes such effects as the height of the supernova over the horizon, the time of year, the distance, and so on.

Different models yield different answers, but the result seems to be good for us: a supernova would have to be at most 100 light-years away before there would be enough damage to the ozone layer to kill off the base of the food chain. Some models indicate it would have to be even closer, perhaps 25 light-years.

There are no massive stars ready to explode that are that close, so we once again appear to be safe . . . or do we?

SIRIUS DANGER?

I have some more bad news: massive stars are not the only kind that can explode. Low-mass stars like the Sun lack the mass to create the conditions needed for a core collapse, but it turns out core collapse is not the only way to blow up a star.

In a massive star, helium piles up in the core and eventually will fuse into carbon and oxygen. But in a low-mass star, that doesn’t happen: there just isn’t enough pressure from the weight of the overlying layers in the star to get the helium nuclei to fuse. Instead, helium just accumulates in the very center of the star, forming a dense ball. This helium sphere is degenerate; degeneracy is that weird quantum mechanical state discussed earlier that occurs when too many particles—in this case, electrons—of one type are squeezed together very tightly. As more helium piles on, the degeneracy increases, and the temperature soars (though in this case still not enough to actually fuse the helium into carbon and oxygen).

As we also saw earlier, the low-mass star expands and cools, becoming a red giant. If it’s massive enough it might yet fuse helium into carbon, with carbon eventually building up and the cycle repeating. If the star doesn’t have the mass to fuse carbon, the fusion process ends there.

But the red-giant star’s life is not quite over just yet. While all this is going on deep in the core, at its surface the situation is different. The star’s vastly increased size means that gravity at the surface is much lower; the gas there is not held on as tightly as it was before. Remember too that the star’s brightness has increased greatly. Any gas particle at the surface is bombarded with light coming up from below. The gas absorbs this light, which gives it a kick upward. That kick can easily overcome the weakened gravity, giving the gas enough momentum to break free of the surface and be launched out into space.

A dense stream of material is emitted from the star. Astronomers call this a stellar wind, like a solar wind on steroids. Red-giant winds can be very dense, blowing off thousands of times as much gas as the star did before its core became degenerate; the stream can be so thick that the star’s outer layers can be entirely blown off in just a few thousand years. In just a short time compared to the star’s life span, it loses as much as half its mass.

When this happens, the degenerate core is eventually exposed to space, and is called a white dwarf. Although it can contain the mass of an entire star, it is so dense that it’s no bigger than the Earth. The surface gravity is unimaginable, hundreds of thousands of times stronger than the Earth’s. A cubic inch of white-dwarf material would have a mass of several tons, like compressing dozens of cars into the size of a sugarcube. It’s also very hot, glowing at a temperature of over 100,000 degrees Fahrenheit.

After the outer layers are shed in the stellar wind, this ball of ultra-compressed superhot material is now sitting in the center of an expanding cloud of gas. The white dwarf is so hot that it emits a flood of ultraviolet light that energizes the gas in the expanding wind, setting it aglow. Seen from Earth, these gas clouds look like pale, ghostly disks, glowing a characteristic green color due to oxygen in the gas. Astronomers named them planetary nebulaebecause of their resemblance to distant planets seen through the eyepiece, but that’s a misnomer: they are the dying gasps of medium-mass stars, and someday the Sun will go through this stage as well (making life here very uncomfortable, so you just know there’s a whole chapter later on devoted to this).

From there on out, though, the star’s life is rather boring. Eventually the gas expands away, dissipating entirely and mixing with the lonely cold gas that exists between stars. Over billions of years the white dwarf cools, dims, and simply fades away.

But for some white dwarfs, the story does not end there.

Something like half of all the stars in the sky are a part of binary or multiple-star systems: stars that orbit each other because of their mutual gravity. Imagine now such a binary star, with two stars in mutual orbit. Both have roughly the mass of the Sun. One ages somewhat faster than the other; perhaps it is slightly more massive than its companion, and so fusion progresses a bit more quickly. It becomes a red giant, blows off its outer layers, and becomes a dense helium white dwarf.

Eventually, the other star begins to go through the same process. But when it expands into a red giant, its partner star is already a white dwarf, with its commensurate strong gravity. If the dwarf is close enough to this new red giant, its intense gravitational pull can essentially draw material off the other star, literally feeding off it. This gas, which is almost entirely hydrogen, then falls on the surface of the white dwarf and accumulates like snow on the ground.

Things get dicey from there. The gravity of the white dwarf is incredibly strong, squeezing the mass accumulating on its surface immensely. If the mass is raining down too quickly, it piles up on one spot, and the pressure builds there beyond the breaking point. The hydrogen in the pile fuses in a flash, detonating like a thermonuclear bomb—except one with 100,000 times the energy output of the entire Sun.

There is an immense flash, and the accumulated matter blows off the surface of the star despite the intense gravity. Like belching after eating too much food too quickly, this takes the pressure off the white dwarf, and after things settle down, the matter begins to accumulate again, resetting the cycle.

The energy released is gigantic on a human scale, but still much smaller than a supernova, and this event is called simply a nova. The white dwarf is largely unaffected by the blast—the amount of matter blown off in the event is only a few hundred times the mass of the Earth,26 which is far, far less than the mass of the star—and therefore the cycle can repeat as long as the red giant feeds the white dwarf.

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A white-dwarf star greedily sucks down a stream of matter from its companion, a normal star. When enough matter piles up the white dwarf will either erupt as a nova or detonate utterly as a Type I supernova.

DAVID A. HARDY (WWW.ASTROART.ORG) & STFC

However, if the red-giant matter stream is on the slower side, things are very different. The gas won’t pile up as quickly and explode in one spot. Instead, it will get spread out over the entire surface of the white dwarf, forming a shell of inert hydrogen all around it. But this time there is no pressure release, no ability to burp. Since the matter is spread out, the pressure is lower than in the earlier case, and the material continues to build up, getting thicker and thicker everywhere on the white dwarf’s surface. Eventually, however, when enough matter piles up, it will still reach that fusion flashpoint.

In this case, it’s not just the hydrogen in one small spot that fuses in a thermonuclear flash; it’s all the matter over the entire surface of the star. The explosive energy released is much, much larger, and eats its way down into the white dwarf as well as up into space. The energy release is so titanic that it can disrupt the structure of the star itself, causing a catastrophe on an epic scale. The entire star explodes like one enormous thermonuclear bomb the size of Planet Earth. It is literally a disaster: the star goes supernova.

By a cosmic coincidence, the total energy released in this event (called a Type I supernova) is very similar to the energy emitted by a massive star going supernova (called a Type II), even though the two physical processes are completely different. In fact they look so similar that it took astronomers quite some time to figure out that the two events were actually entirely separate. But both release huge amounts of energy, and both are very dangerous if they happen too close to us.

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The brightest star in the night sky is Sirius, which is less than nine light-years away. Sirius is a binary, consisting of a normal star like the Sun (but more massive) plus a white dwarf. In this X-ray image, the white dwarf is brighter because it is far hotter than its normal companion. In optical light it is far fainter than the normal star.

NASA/CXC/SAO

One category in which the two events are quite different is their emission of high-energy light: a Type I gives off far more X-rays and gamma rays than a Type II. This means it can be farther away and still hurt us. We know there are no nearby Type II candidates. What about Type I?

Happily, no, none are nearby. However—and there’s always a “however”—there is a binary star with a white dwarf that is extremely close to the Earth: Sirius, the brightest star in the night sky. It’s a mere nine light-years from Earth, which in cosmic terms means it is practically sitting in our lap.

Sirius A, the primary star, is a normal star (that is, fusing hydrogen to helium in its core as the Sun does) with about twice the Sun’s mass. In orbit around it is Sirius B, a white dwarf with roughly the same mass as the Sun. Someday, Sirius A will become a red giant, and Sirius B will feed off it . . . but as far as we can tell, Sirius B is way too far from A to be able to feed at the right rate to explode. The white dwarf will indeed accrete matter, and this will cause it to become brighter as the matter heats up and impacts the degenerate star’s surface, but that’s probably not enough to affect us here on Earth. Also, Sirius A is most likely tens or hundreds of millions of years away from becoming a red giant. As far as we know, there are no other Type I candidates anywhere near us.

So once again, you can breathe a sigh of relief. We appear to be safe from this kind of supernova as well.

COSMIC-RAY GUN

There is one last weapon available to both types of supernovae to consider, and it may be the most destructive yet.

Interstellar space is filled with subatomic particles—protons, neutrons, even whole helium nuclei—moving at high speeds, sometimes within a whisper of the speed of light. Called cosmic rays (or just CRs), they were discovered by a scientist named Viktor Hess in 1912. He lofted a balloon with a simple apparatus that detected ionizing radiation, subatomic particles capable of smacking into normal atoms and stripping them of their electrons. It was thought that most of this radiation would be near the ground (because of natural radioactive elements in the Earth), but as the balloon got higher the radiation level increased. That means a lot of that radiation must come from space.

What could accelerate particles to such high speeds? Why, it would take the energy of an exploding star . . . oh, right.

As mentioned before, when a star explodes, massive shock waves bounce around in the ejected material. A shock wave can dump a lot of energy into these particles, accelerating them. In the turbulent chaos of the expanding gas, a particle can get tossed around many times by shock waves, giving it a terrifying amount of velocity. When it finally escapes, it can be shot out at 99.9999 percent of the speed of light.

It’s essentially a subatomic bullet, and supernovae make them by the gigaton. And it turns out that they are very dangerous indeed, because there are several ways they can hurt us here on Earth.

When CRs slam into our atmosphere, they can ionize the molecules in it and even disrupt them. Ozone, for example, is destroyed when a CR hits it. Models of nearby supernovae show that the effects from cosmic rays damaging the ozone layer are similar to those from gamma rays. Remember, anything more than about 25 light-years from a supernova is safe from its gamma rays, so we can assume the ozone will survive a cosmic-ray onslaught from such an event farther away than that.

However, when a CR hits a molecule in our atmosphere, it can create lots of dangerous high-speed secondary particles as well. These spread out like shrapnel, distributing the destruction over a larger area. These secondary particles, called muons, can shower down all the way to the surface of the Earth. This can be extraordinarily dangerous: muons will slam into tissue, destroying cells and DNA willy-nilly. A big enough wave of cosmic rays hitting the Earth’s atmosphere could radiate muons all over the planet, killing vast numbers of plants and animals.

This type of interaction is very difficult to model. Cosmic rays are affected by magnetic fields, for example, which can alter their trajectory and speed. The galaxy has very complicated magnetic fields, and it’s unknown precisely how this will affect us. The Sun’s and even the Earth’s magnetic fields also play into this, making it an incredibly complicated game. Still, scientists have tried to assess the situation, and because of all the uncertainties the numbers have a pretty wide range: some models show a supernova would have to be only a few light-years away to hurt us via cosmic-ray assault, while others put the distance closer to 1,000 light-years. I won’t lie to you: that’s not terribly reassuring, since there are plenty of such stars within that distance that can explode (as the table in the appendix indicates).

However, we can look to history for some reassurance. The sheer amount of radiation predicted by the most dire models would practically wipe out all life on Earth; muons are incredibly penetrating, so digging deep into the ground or going deep underwater to hide out doesn’t help that much. Since we’re here, that’s pretty good evidence that the milder models are more accurate.

However, there are other effects of CRs we need to consider. As mentioned in chapter 2, when ozone is broken up by incoming cosmic rays, it can form nitrogen dioxide, which turns into nitric acid. Even a relatively mild cosmic-ray event from a supernova could increase the amount of acid rain that falls. However, if the numbers for muon events are rough, the models for acid rain from a supernova are even more poorly determined. Odds are, a supernova would still have to be pretty close to inflict this damage on us, but just how close is still a matter for discussion.

BLAST FROM THE PAST

Finally, let us consider one more thing. Although right now there are no potential supernovae of either type close enough to kill us, that doesn’t preclude any having been too close in the past. The Earth is about 4.6 billion years old, and stars change their distances from each other as they orbit the galaxy like cars on a highway. Could there have been a nearby supernova sometime in the distant past that had some impact on the Earth?

Statistically, it’s almost a dead certainty. Depending on the distance (the closer they are, the rarer they would be), it’s possible that the Earth has had several front-row-seat views of exploding stars. One model predicts that the Earth has seen at least three within a distance of 25 light-years, close enough to severely damage our ozone layer or irradiate us with muons.

But we have more than just math to go by. We have geology.

In 2004, the scientific community received a jolt when it was announced that a team of scientists had found an anomalously high amount of the radioactive isotope 60Fe (iron 60) in a sample of the seabed taken in the Pacific Ocean. The isotope is exceptionally rare on Earth, and no known terrestrial process can make it in detectable amounts.

However, this isotope is produced in a supernova when explosive fusion occurs in the expanding debris. It seems likely that the 60Fe found in the Pacific sample was created by a supernova, and deposited when the debris swept over the Earth.

What’s so very interesting about this is that 60Fe has a relatively short half-life. Radioactive elements decay, producing “daughter” elements. Over time, all of the original element is gone. The half-life is the statistical time it takes for half a sample to decay, and is different for different isotopes. For 60Fe, the half-life is only about 1.5 million years. By measuring how much 60Fe there is compared to other elements found in the sample, the age of the sample can be determined. In this case, the 60Fe sank to the bottom of the Pacific just 2.8 million years ago.

This means that a nearby supernova went off in relatively recent times, geologically speaking. Given the amount of 60Fe in the sample, the supernova couldn’t have been very far away either: perhaps as close as 50 light-years. Maybe closer.

In fact, the possible birthplace of the supernova has been found: a loosely knit cluster of massive stars—the kind that explode as Type II— called the Scorpius-Centaurus Association. This grouping of stars is currently about 400 to 500 light-years away, but it was closer to Earth three million years ago—just about 100 light-years away, putting it suspiciously near the right spot for a supernova to inject 60Fe onto the Earth.

Moreover, it’s known that the Sun sits in a region of space called the Local Bubble: a cavity in the usual fog of gas and dust permeating the galaxy. Bubbles like this can be carved out by exploding stars; the expanding gas pushes open the cavity like a snowplow. The age of the Local Bubble, curiously enough, is less than 10 million years. The Sco-Cen Association looks pretty guilty here as well.

There are no known mass extinctions that occurred at the time the 60Fe drifted onto the Earth, which is reassuring: even a supernova 50 to 100 light-years away doesn’t appear to pose much of a threat.

But the statistical evidence is still interesting. Life on Earth has existed for more than 3 billion years, and multicellular life for the past 600 million or so. Was there some cosmic event sometime in that period that rocked the world?

THE CYCLE OF LIFE

But speaking of life on Earth and supernovae, there’s an important point that I think we shouldn’t overlook.

When the Universe began, a lot of complicated stuff happened.27 At first it was too hot for even normal matter to exist; it was a soup of exotic subatomic particles. But after a short period—literally, three minutes after the Big Bang—it had cooled enough for normal matter to settle out. The early conditions were such that the only elements created at that moment were hydrogen, helium, and just a dash of lithium.

That’s it. No carbon. No iron, no molybdenum, nothing but those three lightest elements. After a few hundred million years, stars formed. These were supermassive stars, a hundred or more times the mass of the Sun, and they were made of just these three elements; in fact they were about 75 percent hydrogen and 25 percent helium, with lithium barely even registering.

They did the usual (for today, that is) cycle of creating heavier elements out of lighter ones, all the way to iron. Then they exploded, of course, and when they did, they scattered all those heavy elements out into space. That debris slammed into nearby gas clouds, compressing them. The clouds formed the next generation of stars. These stars were different, though: they started out with some extra heavy elements in them. Some of these stars too were massive, and exploded, seeding space again with iron, carbon, calcium . . .

Eventually, the Sun was born. The Universe was already over nine billion years old at that point. Several generations of stars had polluted interstellar space with those heavy elements, so when the Sun coalesced it already had a pantry full of the periodic table. In fact, the disk from which it formed was loaded with such things as iron, silicon, and oxygen. When the planets formed from that disk, they got their share too. So the Earth is chock-full of iron, nickel, zinc, calcium, and all the rest.

But those materials didn’t exist when the Universe began! It took those supermassive stars to create them. These stars were the alchemists of their day, transforming simple chemicals into more complicated ones: hydrogen became helium, became carbon, became oxygen. All the way up to iron and beyond.

When you cut your finger and a thin rivulet of blood seeps up into the slice, the red color you see is due to hemoglobin, and the key factor in that molecule is iron. That iron was forged in the heart of a supernova. There is enough iron created in a supernova to make well over five thousand Earths.

The calcium in your bones was most likely created in a Type I supernova, which tends to make more of that element than a Type II does. In fact, a typical Type I supernova makes enough calcium to create about 6 × 1028 gallons (that’s 60 octillion gallons) of milk.

Yeah, we’ve got milk.

The gold in your wedding ring? Supernova. The lead in your fishing weight? Supernova. The aluminum in your foil? Well, that was probably from a red giant (they create aluminum in their cores and blow it into space in their stellar winds), but supernovae make aluminum as well.

A nearby supernova could cause destruction on an unimaginable scale . . . but without supernovae, there would be no life in the Universe at all. We owe our very existence to a chain of unnamed and unobservable supernovae, massive stars that died long before the Sun was more than a wisp of vapor.

It’s okay to be a little scared of supernovae. But it’s also okay to appreciate them. If supernovae didn’t happen, who would be around to understand them?