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

by Philip C. Plait

  Eta Carinae is the Milky Way’s scariest star. It may be a binary, with one star 100 times more massive than the Sun. In 1843, it underwent a titanic convulsion, almost as powerful as a supernova, that ejected the two lobes of matter on either side. When Eta finally blows, it may explode as a hypernova and a GRB.

  JON MORSE (UNIVERSITY OF COLORADO), KRIS DAVIDSON (UNIVERSITY OF MINNESOTA), AND NASA/ESA

  Eta has all the markings of a GRB in the making. While it will certainly explode as a supernova, it isn’t known if it will be a hypernova-type GRB or not. It should also be noted that if it does explode and becomes a GRB, the orientation of the system is such that the beam will almost certainly miss the Earth. We can tell this from the geometry of the gas ejected in the 1843 paroxysm: the lobes of expanding gas are tilted with respect to us by about 45 degrees, and any GRB beaming would be along that axis. To make this more clear: we are in no danger from a GRB, Eta or otherwise, in the near or even mid-term future.

  But still, it’s fun to speculate. What if Eta were aimed at us, and it went hypernova? What would happen?

  Again, bad things. While it wouldn’t get anywhere near as bright as the Sun, it would certainly be as bright or possibly even ten times brighter than the full Moon. While this is bright enough to make you squint, it would only last a few seconds or minutes, and so would probably do no long-term damage to any flora’s or fauna’s life cycles.

  The levels of influx of ultraviolet light would be intense, but brief. People outside would be mildly sunburned, but in all likelihood there would not be a statistically measurable increase in skin cancer rates down the line.

  But the situation is very different when you look at gamma rays and X-rays. These would be absorbed by the Earth’s atmosphere, and the effects would be far worse than for a nearby supernova.

  The most immediate effect would be a strong electromagnetic pulse, far stronger than the one experienced in Hawaii during the Starfish Prime nuclear test. In this case, the EMP would immediately wipe out every unshielded electronic device on the hemisphere of the Earth facing the burst. Computers, phones, airplanes, cars, anything with electronic circuitry would be fried. This also includes power grids; a huge current would be induced in the transmission lines, overloading them. People would be without power, and without any means of long-distance communication (satellites would have all been fried by the gamma rays anyway). This would be more than an inconvenience, as it means hospitals, fire stations, and other emergency personnel would be without power as well.

  But as you’ll see in a moment, we may not have much use for emergency services . . .

  The effects on the Earth’s atmosphere would be severe. This situation has been extensively studied by scientists. Using the models described in chapter 3, and assuming a GRB at Eta’s distance, they have determined the effects. They are not pretty.

  The ozone layer would take a huge hit. The gamma rays from the GRB would blast apart ozone molecules wholesale. Globally, the ozone layer would be reduced by an average of 35 percent, with smaller localized regions being depleted by more than 50 percent. This all by itself is incredibly damaging—mind you, the ozone troubles we have currently are due to a relatively slight dip of just 3 percent or so.

  The effects of this are very long-lasting, and could persist for years—even five years later there could be as much as a 10 percent ozone depletion. During that time, ultraviolet light from the Sun would be more intense on the Earth’s surface. Microorganisms that form the base of the food chain are highly susceptible to UV radiation, and would be killed in vast numbers, leading to a possible extinction-level event that would work its way up that chain.

  To make matters worse, the amount of reddish-brown nitrogen dioxide formed (see chapters 2 and 3) in an Eta Carinae GRB event would actually reduce the amount of sunlight reaching the Earth by a significant amount.

  The exact effect of this is hard to determine, but it seems likely that even a few percent drop in sunlight over the entire Earth (the nitrogen dioxide would spread all over the atmosphere) would cool the Earth considerably, and could conceivably start an ice age.

  On top of this, enough nitric acid would be generated in the chemical mix that there would be acid rain, which would also potentially have devastating environmental effects.

  Then there’s the issue of subatomic particles (cosmic rays) from the burst. It’s unclear just how damaging these would be from a GRB. But, as discussed in chapters 2 and 3, high-energy particles may have all sorts of effects on the Earth. A GRB from 7,500 light-years away would inject a vast number of subatomic particles into our atmosphere, and they would be moving at just a shade under the speed of light. Within hours of the initial burst they would slam into the air, creating a shower of muons. We see muons coming from the sky all the time, but in small amounts. From a nearby GRB, the number of muons generated would be huge. One team of astronomers calculated that as many as 300 billion per square inch could hit the Earth’s surface all over the hemisphere facing the blast.35 If that sounds like a lot, well, it is. These particles would cascade down from the sky and be absorbed by anything out in the open. Given how well human flesh can absorb muons, the astronomers who did the calculation found that the energy absorbed by an unprotected human would be ten times the lethal dose. Hiding won’t help much; muons can penetrate water to depths of more than a mile and also go right into rock down to depths of half a mile! This would therefore affect nearly all life on Earth.

  So in reality, ozone depletion wouldn’t be that big a deal. By the time that really became a problem, most of the animals and plants on Earth would be long dead anyway.

  That is the nightmare scenario depicted at the beginning of this chapter. However, before you panic, remember: Eta Carinae is almost certainly pointing in the wrong direction. But while we’re on the topic, there is another possible GRB progenitor to consider. Called WR 104, it’s coincidentally about the same distance from us as Eta. It’s a binary star, and one of the stars is a bloated, massive beast near the end of its life. It may blow up as a GRB, and it may be pointed more or less at us, but those are both pretty iffy. The odds are that we’re safe from this monster as well, but it’s worth mentioning.

  THIS IS NOW, THAT WAS THEN

  So we seem to be pretty safe at the moment, which is a good thing. The odds of a nearby GRB at any given time are extremely low . . . but the Earth is old. Is it possible that we got zapped by a GRB in the past?

  Statistically speaking, it’s actually quite probable that the Earth was hit by a relatively close GRB beam at some point in the past. While supernovae are common enough, they have to be close to hurt us. GRBs are far rarer, but are damaging at much greater distances. Some studies have shown that one should be near enough to do some ecological damage to the Earth every few hundred million years or so.

  It turns out that there may even be evidence for one such event in the Earth’s past. The end of the dinosaurs may be the most famous mass extinction event in history, but it was not the largest. The Ordovician era ended about 440 million years ago, when as much as half of all genera of life on Earth were wiped out. It happened rapidly, and appears to have had two separate extinction events separated by perhaps a million years. The cause has mystified scientists for many years.

  Could a GRB have pulled the trigger on this extinction event? There are many tantalizing clues. In a GRB event, you’d expect incoming UV radiation would more profoundly affect animals and plants that lived near the surface of the oceans than it would affect deep-sea creatures, and there is evidence in the fossil record that that is what happened. Trilobites, those curious crablike animals that dominated the oceans of the time, had a larval stage. It appears that at the time of the extinction event, larvae that lived near the surface of the water were more affected than those that lived in deeper water, indicating that whatever caused the sudden die-off may have come from above, from the sky. Moreover, animals that had longer larval periods in their life cycle were also more likely to go
extinct than those with shorter larval stages. These are both consistent with a sudden increase in UV radiation that could affect shallow-water regions, but not deep-water. Animals with longer larval stages would absorb more dangerous UV radiation, which would preferentially kill them off.

  Interestingly, such trends are not seen in other mass extinctions, indicating that the Ordovician extinction had an unusual cause. GRBs are many things, but “unusual” would be high on that list.

  The second Ordovician extinction event has been associated with rapid cooling of the Earth, followed by glaciation. This is also consistent with the effects of a nearby GRB; the cosmic-ray shower and subsequent increase in atmospheric nitrogen dioxide would contribute to a possible global cooling. In fact, some researchers have found that at this time on Earth, a global glaciation could not have occurred without some sort of “forcing event”—that is, some outside mechanism to kick-start it. Perhaps that force came from a GRB.

  This evidence is interesting, perhaps even persuasive, but it is not conclusive. More research, as usual, is needed. But it does give one pause to think that an event that occurred thousands of years earlier and trillions of miles away could so profoundly affect life on Earth.

  BEAMING WITH CONFIDENCE

  Are GRBs worth worrying about?

  One answer is no, because if one goes off there’s nothing we can do about it. And since gamma rays travel at the speed of light—they are light—we will get literally no warning if one is headed our way. So why worry?

  On the other hand, it’s quite possible that there is nothing to worry about anyway.

  Almost every gamma-ray burst ever seen has come from an incredibly distant galaxy. But in astronomy, distance is the same thing as time: the farther away you look, the farther back in time you are seeing. When we see a GRB explode in a galaxy nine billion light-years away, we’re seeing that galaxy as it was nine billion years ago. GRBs were common in the past, and became less frequent as the Universe aged.

  This is significant because galaxies change over time. Early in their lives, they had fewer heavy elements like calcium, iron, and oxygen in them; these elements are created and distributed into the galaxies by supernovae, and that takes time. It turns out that it’s easier for stars with fewer heavy elements to turn into GRBs when they die. Since most massive stars currently being formed have lots of heavy elements in them, thanks to previous generations of supernovae, they are less likely to go GRB.

  Furthermore, stars that explode as GRBs need to be rotating rapidly before they collapse, or else the accretion disk that feeds the beams may not form. It turns out that stars with higher abundances of heavy elements tend not to spin so quickly. It’s not because the elements are more massive, however! Heavier elements are better than lighter ones at absorbing the light coming up from the star’s interior. This makes a star with lots of heavy elements in its gas hotter and brighter than a star with fewer heavy elements in it. Because of this, particles on the surface of the star are more easily blown away in a stellar wind—the equivalent of the solar wind, but from a star other than the Sun.

  As the particles leave the star, they are swept up by the rotating magnetic field of the star. This acts like a parachute, slowing the star’s spin in turn: imagine holding a plastic bag open and spinning around; as the bag catches the air, your spinning would slow because of the drag. The same thing happens to stars; their spin slows over time as their magnetic field drags through the stellar wind. In fact, this is why the Sun rotates only once a month. It probably spun much faster when it was young, but over billions of years the solar wind dragging through the magnetic field has slowed its rotation.

  So stars that have more heavy elements have a stronger stellar wind, and tend to spin more slowly. The converse—stars with fewer heavier elements tend to rotate more rapidly—means that stars that were born earlier in the life of the Universe will make more GRBs than stars born more recently. The upshot of all this is that GRBs from hypernovae—from massive stars exploding—will be more rare today than they were in the distant past.

  In other words, you really don’t have to worry too much about them.

  SHORT, BUT NOT SWEET

  So are we safe from this form of destruction, sitting comfortably in our twelve-billion-year-old galaxy with its heavy elements and slowly spinning massive stars?

  Maybe. But maybe not. If you recall, there appear to be two different kinds of GRBs, ones that last longer than two seconds, and ones that are shorter. The kind generated in the collapse of a massive star’s core is the long kind of GRB. But what of the short ones?

  Two NASA satellites were critical for understanding the short bursts. The High-Energy Transient Explorer-2 (HETE-2) and Swift missions detected dozens of short GRBs. Using these observations, astronomers were able to craft the idea that a short GRB can occur when two dense neutron stars merge. A neutron star forms when the core of a star going supernova is not quite massive enough to form a black hole. In many cases, massive stars form in pairs, with the two stars orbiting one another, and many such high-mass star pairs are seen in our galaxy. Over time, the more massive star will explode, leaving behind a neutron star. Some time later, the other star explodes, also leaving behind a neutron star.

  Through many forces, over billions of years, the orbits of the two stars will shrink. The two ultradense objects spiral closer and closer together . . . and then, finally, they will get so close that they literally merge. Their combined mass may be enough to form a black hole, and if enough matter is left over it will form an accretion disk around the hole. At this point, events are similar to what happens in the core of the massive star when it explodes: the accretion disk, tremendous magnetic fields, and powerful gravity of the black hole focus twin beams that explode outward.

  Models of these events indicate that the burst of gamma rays would be much shorter in duration than the massive star type of GRB, and would produce higher-energy gamma rays. Both of these predictions fit the observations. There are other models that also fit the observations (such as a black hole-neutron star binary, with similar results), but this is the leading theory.

  One major difference between the merging neutron star GRBs and the massive star hypernova GRBs is the time it takes before one can go off: while in modern times we expect to see few if any massive star GRBs, we expect to see plenty of neutron star mergers. It takes billions of years for the orbits of the two neutron stars to decay and cause the stars to merge, and so they should be able to occur today. This may very well be true, but in raw numbers they are less common than their more massive counterparts. This may be due to their uncommon origin—there are plenty more single massive stars that can explode than there are binary massive stars—so it’s difficult to get a handle on how many potential short GRBs there are in our galaxy. There are many neutron star binaries known, all of which could become short, hard GRBs . . . in a few more billion years. None are known that could go off in a century or millennium, or even in the next million years. But unlike massive stars, which are incredibly bright and obvious, binary neutron stars give off very little light and are difficult to detect.

  Two neutron stars finally succumb to their mutual gravity after billions of years of orbiting each other. Torn apart, they merge and collapse into a black hole, which announces its birth with a GRB.

  DANA BERRY, SKYWORKS DIGITAL INC.

  It’s unlikely in the extreme that there are any close enough to do us any harm. But it’s not possible to entirely rule them out either.

  THE FUTURE IS BRIGHT

  What we need, as always, are more observations. As the biggest explosions we know of—and probably the biggest explosions the Universe can make—GRBs are of great scientific interest. They tell us so much about how matter and energy act at the extreme limits of physics, how black holes are born and behave, and also about the environment around them. There’s still a lot we don’t understand about GRBs, of course. We’ve come a long way since the Vela satellites; in 2004, NASA la
unched the Swift satellite—so critical in understanding the origin of the short, hard bursts—which has observed hundreds of GRBs, including the most distant one ever seen at 12.8 billion light-years away. Swift’s observations have allowed keen insight into both long and short bursts, adding much-needed data to the theoretical models.

  As we learn more about GRBs, we’ll be better able to assess the danger from them, including how they may have affected life on Earth in the past. While there’s probably nothing we can do if one goes off—and it’s incredibly unlikely it will happen at all—it’s always better to have a good handle on the situation.

  So should you worry? I’m asked this all the time, and I have a simple answer: I have known people who’ve been killed in all sorts of unlikely ways, including car crashes and one who was hit by lightning. How many people do you know who have been killed by a gamma-ray burst?

  CHAPTER 5

  The Bottomless Pits of Black Holes

  EVER VIGILANT, IT’S AN AMATEUR ASTRONOMER WHO catches the first whiff of trouble.

  He had hopes of doing some imaging of Uranus using his automated telescope, but the computer consistently points the telescope in the wrong direction. After going to manual, he eventually finds the giant planet several arc minutes from its calculated position. Puzzled, he calls a friend who quickly confirms that he has the same problem. An Internet search on a few astronomy bulletin boards reveals many such events from astronomers all over the world.

  As days go by, things get worse. Jupiter seems to be off-kilter as well. Saturn, however, located on the other side of the Sun, appears unaffected. Rumors start to spread.

  Then the situation gets really weird. The Solar and Heliospheric Observatory, parked in an orbit where the Earth’s gravity and the Sun’s gravity are in balance, starts to drift. Engineers are puzzled, but soon have other problems with which they must contend. Now Mars is in the wrong place. NASA has a probe on its way to the Red Planet; will it miss? But soon that point becomes moot, as the spacecraft is drifting too. After a few days it’s clear the probe is lost . . . and that the probe is the least of our worries.