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

by Philip C. Plait

  Heading into such a gas cloud might almost be worth it. What a view!

  But then again, a nice dark sky with all those nebulae at a safe distance of a few thousand light-years away sounds pretty good too.

  PLANE FLIGHT

  As mentioned above, the stars in the disk of the Milky Way orbit the galaxy’s center similarly to the way the planets orbit the Sun. However, there are some important differences. On the scale of the solar system (many billions of miles across), the Sun is small (less than one million miles across). As far as the planets can tell, all the gravity in the solar system is concentrated in one spot.104 Because of this centrally located source of gravity, the orbit of a planet can only have a certain kind of shape, called a conic section. This includes circles, ellipses, parabolas, and hyperbolas. All of these shapes are planar; that is, they are flat. If you smack a planet hard enough the orbit will change shape, or it might change the tilt of the orbit, but the orbit itself will still be a conic section, will still be flat.

  But the situation is different for stars orbiting the center of the Milky Way, because the mass is spread out, distributed around the disk. A star orbiting in that disk feels gravity from masses all around it, and not just from a single point in the galactic center. Orbits of stars can therefore have all sorts of weird shapes. Let’s say you have a star that orbits the galaxy in a perfect circle that is exactly in the midplane of the disk. If you were to give the star a little bit of vertical velocity—perpendicular to the disk—the star would bob up and down relative to the disk, like a cork floating on water (while still circling the center).

  It’s a little like throwing a rock up in the air; gravity slows it and it falls back down. The vertical velocity of the star propels it above the plane of the disk, but the disk’s gravity pulls it back down. The disk, though, isn’t solid; it’s made up of stars that are separated by large distances. There is nothing to stop our star, so it passes right through the plane, and heads down, below it. Again, the gravity slows it to a stop, and the star reverses course. The cycle will repeat forever if the circumstances are right. When you couple this with the star’s circular orbit, you get a shape like a sine curve wrapped into a circle.

  There are many ways a star could get started on an excursion like this. It could pass by another star, and the gravitational interaction could kick the star upward or downward—but as we saw before, stellar encounters are extremely rare, so this is unlikely. On the other hand, stars form in clusters (see below), where they are much closer together and gravitational interactions are more common. A massive star in the cluster passing close to a less massive one could easily toss the smaller star right out of the cluster, and also impart a bobbing motion.

  Unlike planets orbiting the Sun, the Sun itself bobs up and down as it circles the center of the Milky Way. It pokes up above the disk about every 64 million years, making roughly four cycles every time it orbits the galaxy once. The vertical scale has been exaggerated here; the amplitude of the Sun’s motion is really only a few hundred light-years up and down.

  NASA/JPL-CALTECH/R. HURT (SSC) AND CHRIS SETTER, B.I.L .

  Another way is for the star to pass near a giant cloud of gas and dust. We saw above that a direct collision with a nebula has some deleterious effects, but another is that the mass of the cloud can warp the orbit of the star, giving it a vertical velocity and forcing that bobbing oscillation.

  It turns out that a star very near and dear to us exhibits just this sort of motion: the Sun! Careful measurements of the Sun’s velocity relative to the stars around it show that the Sun is in fact oscillating above and below the galactic plane. The excursion isn’t huge: maybe 200 light-years or so at maximum compared with the disk’s diameter of 100,000 light-years. The disk is also about 1,000 light-years thick, so the Sun still stays within the bulk of the material of the disk as well.

  The period of the Sun’s oscillation—from maximum height above the disk, diving down through it to the maximum depth below the plane, then back up to maximum height—is about 64 million years.

  Well, that sounds cool: we get a free ride to a (slightly) better view of the galaxy over a few million years, and no harm done, right?

  Right?

  Maybe not. But to see why, instead of looking up, we have to look down, into the layers of sediment on Earth.

  For many years, it’s been suspected that the fossil record of life on Earth has shown a periodicity in mass extinction events, as if life on Earth is following some sort of schedule for huge die-offs followed by a rediversification of species. Not all of these events follow such a schedule, and for many of them a smoking gun has been found; the most famous is the end of the dinosaurs, and we have pretty good evidence that an asteroid impact was behind it. But for others (with the exception of perhaps the Ordovician extinction event; see chapter 4) the causes aren’t so clear.

  A periodicity to mass extinctions implies some sort of cyclical cause, of course. While it’s impossible to rule out things like episodic super-volcano eruptions or some other internal cause, cycles on really long time scales imply extraterrestrial forces.

  Until recently, this cyclical large-scale grim reaping has only been suspected; the fossil record wasn’t all that clear. But new research has strengthened the supposition considerably. By mathematically analyzing the fossil record, researchers have discovered a very strong signal of periodicity in the mass extinction history. They examined diversity of species—literally, how many species of life existed at different points in the fossil record—and have found that the number of different species appears to rise up and down with a distinct period.

  That period, they determined, is about 62 million years.

  Uh-oh.

  Is it merely a coincidence that cycles of extinctions match up closely with the period of the Sun’s oscillation into and out of the Milky Way’s disk? There are ways to check, statistical methods to try to match up two different cycles and see if they might be correlated. Another group of researchers, Mikhail Medvedev and Adrian Melott at the University of Kansas, carefully performed this analysis, and their answer is “maybe.”

  Well, that’s not terribly reassuring. But this is a new field of research, and we’re just getting started looking into it. The data are sparse, and the results so new it’s hard to say how firmly based the conclusions are.

  But they are certainly provocative.105

  In this case, the culprit may be our old friend the cosmic ray. As you might remember from previous chapters, these little guys are subatomic particles accelerated to enormous velocities in outer space. When they impact the Earth’s atmosphere, there are a number of effects. For one, when a cosmic ray smacks into a molecule of air at nearly the speed of light, it shatters into a shower of smaller subatomic particles called muons. These scream down from the sky, and if they hit a DNA molecule in a cell they can alter or destroy it. This actually happens all the time, but in general living tissue can repair or reject the damage. But if enough muons rain down, there could be slow but long-term effects on life—a mass extinction, for example. As noted earlier, 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.

  Cosmic rays have other effects as well. They can destroy ozone molecules in the upper atmosphere, exposing life below to dangerous levels of ultraviolet light from the Sun. They can also create nitrogen dioxide, which can form acid rain. Over years, this can destroy plant life, and this effect would work its way up the food chain.

  Finally—and perhaps less well established—cosmic rays can seed cloud formation, so an increase in cosmic-ray influx may increase the amount of cloud coverage on Earth, forcing climate change as more sunlight is reflected into space. While it may not necessarily incite a full-blown ice age, even a temperature drop of a few degrees can be devastating to the biosphere.

  But where do these cosmic rays come from? And how is this tied into the Sun’s bobbing m
otion as it circles the galaxy? If such a connection does in fact exist, Medvedev and Melott may have found it.

  Most cosmic rays come from supernova explosions and pulsar winds; the material moving outward from those sources can slam into slower material and generate fierce shock waves that accelerate subatomic particles like protons and electrons to within a razor’s edge of the speed of light. Because they originate from events happening inside the Milky Way, they are called galactic cosmic rays.

  But there are cosmic rays that come from outside the galaxy as well. The Milky Way is part of a small cluster of galaxies called the Local Group, which consists of our galaxy, the Andromeda galaxy (a massive spiral similar in size to ours), and a handful of smaller galaxies. The Local Group is on the outskirts of the much larger and far more massive Virgo Cluster, which contains thousands of galaxies—we’re like the suburbs of a vast metropolis. The Virgo Cluster’s gravity is not to be trifled with: we (and the other Local Group galaxies) are locked in its grip, and being pulled toward the cluster at the astonishing speed of 160 miles per second.

  And we’re not moving in a vacuum. Remember the intergalactic medium ? The Milky Way slams into this rarefied stuff at high speed, creating a shock wave almost beyond imagining: it’s hundreds of thousands of light-years across, and generates huge amounts of energy. The energies are so vast that they create cosmic rays, but in this case they come from outside the galaxy, so they are intergalactic cosmic rays. The cosmic rays scream away from the shock front, and many of them are aimed our way, back into the galaxy.

  The galaxy, like the Sun, has a magnetic field. Also like the Sun’s, the galactic magnetic field is a mess of twisted, coiled loops. They are strongest right in the middle, the midplane of the disk, where the magnetic field does an excellent job of deflecting incoming galactic cosmic rays. However, their strength dims rapidly with height above or below the plane. If a star stays near the plane, it is protected from these high-energy particles. If it strays too far, the star gets exposed to them.

  And this is where the oscillating Sun comes in. Bobbing up and down, above and below the plane as it orbits the center of the Milky Way, the Sun finds itself high above the plane and its protective magnetic fields every 64 million years. This is toward the direction of the cosmic shock wave, where the Sun is relatively unprotected from the incoming cosmic rays. It’s like facing upwind while a tornado flings gravel at you. Medvedev and Melott found that the number of intergalactic cosmic rays that can reach the Sun during these periods can increase by a factor of five over quieter periods when the Sun is far below the galactic plane (which also has the shielding effect of putting the bulk of the galaxy between us and the incoming cosmic rays).

  The number of intergalactic cosmic rays that can reach the Sun thus goes up and down significantly over time with that 64-million-year cycle. The scientists then used the Sun’s predicted motion to make a model of the number of cosmic rays reaching us here on Earth, and plotted it against the graph of the fossil diversity going back in time. They found that the maxima from the first plot overlaid the minima from the second plot every time!

  In other words, whenever the Sun was high above the plane, and the number of incoming cosmic rays was at its peak, the number of species of life on Earth decreased. Every single time, back over the past nine cycles, over half a billion years.

  Let’s be clear: this is not direct evidence that the Sun’s motion causes mass extinctions. But it’s very compelling. When the researchers accounted for asteroid impacts and other non-cosmic-ray events that cause mass extinctions, the correlation between the Sun’s motion and those massive die-offs got even better. Incidentally, the research doesn’t indicate precisely what it is about cosmic rays that delivers the blow. There is some evidence that ice ages are also correlated with these periods, so perhaps cloud cover and climate change are behind it all. There’s interesting research connecting cosmic rays with the triggering of lightning on Earth too. It’s not clear which of the ways outlined above (muons, ozone depletion, smog generation, or cloud seeding) does the dirty deed, or if it’s a mix and match of any or all of them, or maybe something we haven’t even guessed at yet. But there is mounting evidence that cosmic rays do have an effect on life on Earth.

  This raises the obvious question: where are we now in the cycle? The Sun is currently on its way up, above the disk. We are only 25 or so light-years above midplane, well protected by the galactic magnetic fields, so we have a ways yet to go before we’re in the danger zone. Our descendants 20 or 30 million years from now, however, may have cause for concern; they’ll be watching their neighborhood deteriorate. If they can avoid the ever-heating Sun, supernovae, and the odd gamma-ray burst or two, they may still have intergalactic cosmic rays to deal with. To avoid them, they’ll have to find some Sunlike star with habitable planets in the galactic midplane (or below it) and move there. There are probably lots of potential colony sites . . . if the stars are right.

  MONSTER IN THE MIDDLE

  Our great-great-great (etc.) grandchildren may have another problem besides intergalactic cosmic rays. This one is slightly closer to home—just downtown, as a matter of fact. To understand it, though, we’ll need to take a small step back in time, and a giant leap back in space.

  In 1963, astronomers had an enigma on their hands. Radio astronomers had discovered an object that was pretty bright in the radio part of the spectrum, which is always nice. The problem was, the technology of the time wasn’t up to nailing down the object’s position very well— similar to the problem gamma-ray-burst astronomers would face a few years later.

  A cosmic coincidence saved the day: the object—called 3C273—is in a location in the sky that happens to overlap the apparent position of the Moon as it orbits the Earth. This means that every now and again, the Moon appears to pass right over 3C273, blocking it from our view. By timing precisely when the radio waves from the object are blocked by the Moon’s sharp edge, and knowing the exact position of the Moon, they were able to determine the object’s location with high accuracy . . . and when they trained their optical telescopes on that position, all they saw was a faint blue star. This was quite shocking—how could such a feeble emitter of visible light be so luminous in radio?

  Things got even more perplexing when the distance to the object was found to be a staggering one billion light-years. Far from being an innocuous faint blue star, 3C273 must be the most luminous known object in the Universe.

  Soon more objects like this were found, and they were dubbed quasars, for “quasi-stellar objects.” Other, similar objects were found as well, and sported names like blazars and Seyferts. They all emit light across the spectrum, and some are true monsters, emitting many trillions of times the Sun’s energy, hundreds of times the total energy output of our entire galaxy!

  Over time, it became clear that these objects were galaxies similar to ours, except that energy blazed forth from their cores, making them incredibly luminous. What could do that? Whatever the source of that energy was, it had to be small,106 and it had to produce radio, optical, and X-ray light on vast scales.

  M87, a giant elliptical galaxy in the Virgo Cluster, is the nearest active galaxy. The supermassive black hole lurking in its core emits a giant jet of energy and matter moving at nearly the speed of light.

  NASA AND HUBBLE HERITAGE TEAM (STSCI/AURA)

  Only one object astronomers knew of fit all these characteristics: a black hole.

  But even stellar mass black holes couldn’t put out that kind of power. Astronomers came to grips with the fact that there must be a different kind of black hole, a far scarier kind: a supermassive black hole (SMBH).

  In fact, over time it was found that every large galaxy in the Universe has an SMBH at its core. Even our Milky Way does—it’s called Sagittarius A* (pronounced “Sagittarius A star”), or Sgr A* for short—tipping the cosmic scales at 4 million times the Sun’s mass.

  And it’s considered a lightweight. The central black hole in the gi
ant elliptical galaxy M87, which at 60 million light-years distant is much closer than 3C273 (though that’s still a long walk), has one of the most massive SMBHs ever seen, weighing in at one billion solar masses.107 These superluminous objects—now collectively called active galaxies—are so bright because the black holes in their cores are actively feeding. Material, gas, dust, and even stars are falling into the gaping maws of these monsters. As the matter falls in (similar to when a black hole forms in a gamma-ray burst) it forms a flattened accretion disk. Friction and magnetic force heat the disk to millions of degrees, and matter that hot gets very, very bright (see chapter 5). It will emit numbing amounts of light, dwarfing the combined light from the rest of the galaxy. It will also emit X-rays and even gamma rays, the highest-energy form of light.

  As we saw in chapter 4, a black hole with a disk can also form jets of matter and energy, and supermassive ones can do this as well. Not all active galaxies’ SMBHs have jets, but many do. It’s like a GRB on a galactic scale, but instead of a few-seconds-long flare of energy, the jets are stable, constant sources of power, lasting for millions of years or longer. Active galaxies are the largest reservoirs of energy in the Universe.

  The environment inside one of these active galaxies must be interesting, if by “interesting” you mean “terrifyingly scary beyond belief.” Even without a jet, the core of these galaxies would be booming out energy across the electromagnetic spectrum. Any star near the core would be bombarded by radio waves, optical light, X-rays, maybe even gamma rays. It’s hard to imagine life being able to arise on a planet orbiting a star near the center of an active galaxy.