Death From the Skies!: These Are the Ways the World Will End...

by Philip C. Plait

  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.

  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.

  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 mat
ter 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?

  CHAPTER 4

  Cosmic Blowtorches: Gamma-Ray Bursts

  THE BEAM CAME WITHOUT WARNING.

  There could be no warning: the wave front was moving at the velocity of light, the ultimate speed limit in the Universe. Nothing can move faster, so the wave of death brought its own announcement.

  Across the Earth’s southern hemisphere, people were having a normal day: shopping, working, playing, walking, hunting. When the beam reached Earth, that all changed instantly. The sky looked perfectly normal for one second, and then literally in the next it suddenly lit up, like a switch had been flipped. An intensely bright spot flashed in the sky, so bright that anyone looking at it instinctively looked away, eyes watering from the onslaught.

  The new star in the sky was so fantastically bright it could outshine the full Moon, but didn’t last long. It started to fade after less than half a minute, and was bearable to the eye after a few minutes. People stood in the streets, in the deserts, in the Antarctic plains, on ships at sea in the South Pacific and Indian oceans, and boggled at the incredibly bright but rapidly dimming new star in the sky.

  But their amazement soon faded, and they began to go about the
daily business of their lives.

  Most people had already put the event out of their minds when, hours later, a flood of subatomic particles from the fading star slammed into the Earth’s atmosphere. Invisibly, these particles rained down out of the sky, covering the Earth from the south pole to 30 degrees north of the equator. Australia, New Zealand, South America, essentially all of Africa and India, and half of China were blanketed with a lethal dose of radiation. It didn’t matter if people were inside their houses, or outside under a clear sky: all of them were exposed.

  Across two-thirds of the Earth, people started dying.

  North America, Europe, and much of Asia were spared the immediate effects, but it hardly mattered. With most of the human population dying, the impact on the globe was overwhelming. And those who weren’t killed outright by the burst of radiation were doomed anyway: the Earth’s ozone layer disintegrated in the onslaught, dropping to half its usual strength. Ultraviolet light from the Sun was able to penetrate almost freely to the Earth’s surface, killing off the base of the food chain.

  The final blow was yet to come. Spawned by the wave of subatomic particles, a thick layer of smog began to form in the air, and within days the sky was a dank reddish-brown color over the whole planet. Any hardy plants that had managed to stay alive thus far suddenly found the sunlight and temperature dropping . . . which was bad enough, until the acid rain began.

  And that was short-lived as well. Within weeks, the Earth’s temperature had dropped enough that a new ice age was triggered. It wasn’t long before the glaciers started their march from both poles.

  The people who had survived the initial months of the event learned that they had witnessed the death of the supermassive star Eta Carinae, but that knowledge didn’t help them. The mass extinction the star triggered would be the worst the Earth had ever seen, and when it was finally over, there were no humans left to wonder at how a single star trillions of miles away could destroy all of history in less than a minute.