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

Chapter 8. Bright Lights, Big Galaxy

YOU WALK OUTSIDE ON AN EARLY WINTER’S EVE AND cast your gaze upward. The constellations in the north and east reflect the tale of heroic Perseus, sent by King Cepheus and Queen Cassiopeia to rescue the maiden Andromeda from the sea serpent Cetus. You may chuckle, thinking that two curved lines of stars hardly bring to mind the vision of a young girl chained to a rock waiting to be eaten by a monster, but as you look carefully your eye is caught by a glimpse of something fuzzy just off Andromeda’s side. It’s hard to see, but it’s definitely there: a slightly elongated patch of light.

It seems suspended there, small and unassuming, hanging for all eternity in the sky. But, like so much about the night sky, that’s an illusion. You are seeing the great Andromeda galaxy, a massive spiral galaxy similar to our own Milky Way. And it’s headed our way.

If you could speed the clock up a few trillion times, such that millions of years appeared to elapse in mere seconds, you would see the Andromeda galaxy swell and grow before your eyes. Every passing moment would see it getting larger, until a few minutes later it looks like the whole sky is about to fall on you. You see stars around you suddenly wrenched up and away, forming a long thin line stretched into space, like a tendril reaching out to the other galaxy. The rest of the sky is oddly empty of stars, the Sun too becoming a part of a stellar stream stretching countless light-years, reaching into the space between the galaxies where stars are rare.

Suddenly, the Andromeda galaxy has flown by, shrinking in the distance somewhat, having literally passed through our galaxy like a ghost through a wall. However, over the next few accelerated minutes—actually, millions upon millions of years—you see it slow, stop, and then head back your way. Flash! It fills the sky in another pass, and then once again has moved away. But this time it doesn’t get as far. Andromeda swells one last time. Over your head you see the bright core of the galaxy merge with the core of our own. Above you hangs a vast cloudy ball, the remnants of the once mighty pair of galaxies, merged to form a single, larger galaxy.

Within minutes, the sky settles down. Everything now looks calm. You sigh with relief, glad that you have survived this cosmic encounter. What you don’t know is that a beam of matter and energy is headed your way from the heart of the new galaxy, and when it touches down on the Earth, the chaos of the merger will seem as bucolic as a peaceful springtime meadow.


Have you ever heard that a galaxy is like a city? A city has a downtown section, suburbs farther out, pockets of congestion, regions where there’s not much to see, and, of course, the occasional rough neighborhood. Galaxies are like that too. They have their regions of high and low activity, places that are exciting, places that are a bit duller. We even say they have a population—but instead of people, a galaxy is populated by stars, gas, and dust.

And, like a city, of course, there are places you really don’t want to go.

Yeah, you can see where this is headed.

If you live far from civilization, in a place with dark skies, then you have certainly stepped outside on a clear, moonless night. At first, when you do so, you may only see a few stars in the sky as your eyes slowly adjust to the gloam. But over time, as your pupils dilate and your eye automatically coats your retina with a light-sensitive protein that increases sensitivity, fainter stars will slowly become noticeable. The sky will become spangled with stars, thousands of them gently twinkling down upon you.

Along with the stars, you may see a faint glowing band across the sky. It almost looks like smoke, or a jet contrail. That swath of mist is called the Milky Way, named because it looks like a river of spilled milk across the sky. It has been known for thousands of years, since humans first noticed the night sky. In the early 1600s, Galileo turned his newly fashioned telescope to the Milky Way and was shocked to see that it was not a cloud, but was in fact made up of thousands of stars, all too faint to be seen individually.

This was the first clue that we live in a galaxy.93 A clue to its shape may come with a moment’s inspection of the brightest stars in the sky, revealing that they seem to stick near this milky band. Away from the band, stars are fainter and fewer in number.

In the eighteenth century, the great astronomers William and Caroline Herschel took this idea further: using a telescope, they counted up stars in different directions in the sky to try to determine the shape of the galaxy.

The idea goes something like this: imagine you are standing in a field. As the sky darkens, you are surrounded by fireflies. If you’re in the middle of the field you’ll see the same number of fireflies in every direction you look. But if you’re near the edge of the field, you’ll see fewer fireflies toward the edge than if you look across the field to the far side.94 The farther you can see, the more fireflies you see.

So it goes with stars. The Herschels reasoned that if they saw more stars in one direction than another, then that means the galaxy must be longer in that direction. They found the galaxy to be a flattened cigar shape with the Sun very near the center.


The shape of the galaxy as determined by William and Caroline Herschel in 1785. The Sun is near the center of a “grindstone”-shaped galaxy.


This method was repeated in 1906 by another great astronomer, Jacobus Kapteyn, using photographs instead of eyeball observations. He started his star-counting exercise, and in the end determined that the galaxy is roughly cigar-shaped, about 45,000 light-years across, with the Sun very near the center.

This idea was brilliant, but unfortunately it doesn’t work well in practice. In both attempts, the numbers, and position for the Sun, were way off. Why?

They didn’t know about dust. If cities have pollution, galaxies have dust.

It’s not the kind of dust you find clumping underneath your sofa or dimming your TV screen. Galactic dust is actually composed of complex carbon molecules. Stars create this dust in their stellar winds, while simultaneously blowing it into space.95 In sufficient quantities dust is opaque and blocks starlight.

This put the kibosh on the star-count method. Imagine now that instead of a field full of bugs, you are in a large room filled with smoke. The smoke is so thick you can only see a few feet in each direction (think billiards parlor). Since your vision is so limited, you have no idea what the shape or dimensions of the room actually are. You could be in a square room with walls just on the other side of the smoke, or you might be in a football stadium. Since your sight line is limited there’s no way to tell.


The Milky Way Galaxy is a grand design spiral, epically beautiful. This artist’s interpretation is based on actual observations, and accurately depicts the shape of our galaxy. The Sun is roughly halfway out from the center. Although the disk spans 100,000 light-years, it is only about 1,000 light-years thick, so viewed edge-on our galaxy would look very flat, like a pancake with a bulge in the middle.


Years after Kapteyn’s work, it was found that the galaxy was much larger than he had thought. This was determined using infrared and radio observations; both wavelengths of light can penetrate the dust that obscures visible light. Careful mapping of gas clouds, stars, and dust has revealed the true nature of the Milky Way Galaxy, and it’s a grand place indeed.

When you first visit a new city it’s a good idea to take a tour. So let’s take a stroll through the galaxy, starting near the Sun’s current position, and moving along to see the sights. Remember, sometimes even well-lit neighborhoods can hide some dangerous characters, so don’t be fooled by the beauty and apparent serenity of your surroundings.


The most prominent feature of the Milky Way is its flattened disk of stars, gas, and dust, all of which orbit the center of the galaxy itself (similar to the way the planets orbit the Sun). The disk is 100,000 light-years across and roughly 1,000 light-years thick,96 and is held together by its own gravity. It is composed of majestic, sweeping spiral arms, like a pinwheel. Spiral galaxies are fairly common in the Universe. Some are small and relatively obscure, and some are grand and huge, with well-defined arms. The Milky Way is one of the latter. In fact, very few spiral galaxies have been found to be bigger than the Milky Way.

The spiral arms are interesting. Because stars revolve around the center of the galaxy faster nearer the center (again, like the planets in the solar system), you might expect the spiral arms to eventually wind up like twine around a spindle. But they don’t. They are not permanent, fixed features, like branches in a tree. Instead, astronomers think they are more like celestial traffic jams. On a city highway, a traffic jam isn’t a fixed feature either; cars move in and out of the jam, but the jam persists. Similarly, as stars orbit the center of the galaxy they move in and out of the spiral arms, but because of a quirk in the way gravity behaves in a disk, the feature itself stays.

Gas clouds orbit the center of the galaxy much as stars do. When a gas cloud enters a spiral arm, it hits that gravitational traffic jam and slows down. If another cloud enters the arm right behind it, the two will collide. This interstellar fender bender compresses them both, and when clouds compress, they form stars. The stars born in this way have a range of masses, from very low to very high. The highest-mass stars are bright, and light up the arms. However, these are the shortest-lived (see chapter 3 on supernovae), and don’t live long enough to exit the spiral arms. Since the bright stars stay in the arms, the arms appear bright. Moreover, there are very few massive, bright stars compared to low-mass, dim stars, so the overall number of stars in the spiral arms is not much different from the number of stars between arms. It’s just that between the arms there are few or no bright stars. This makes the arms more prominent than they otherwise would be.

So they’re well-lit, pretty, and bustling with activity and traffic. And, like a busy section of the city, they have their dangers too.

For one thing, the fact that they are crowded is a major danger in itself. Not from collisions, though: the odds of any two stars colliding in a galaxy are incredibly low. In fact, assuming that stars are evenly distributed throughout the disk of the galaxy (a fair assumption), the odds of the Sun getting close enough to another star to even have their mutual gravitation affect each other is essentially zero! The average distance between stars in the disk of the Milky Way is huge: several trillion miles, while stars themselves are only roughly one million miles across. Imagine two flies in an empty box five miles on a side—what are the odds of those two flies even getting within a few yards of each other, let alone close enough to physically collide? That math works out to be the same for stars. On a human scale, the Milky Way is an incredibly empty place.

So stellar collisions will be extremely rare in the disk. As we’ve seen, though, you don’t need to be all that close to a star for it to affect you. A supernova within a few light-years would fit anyone’s definition of “bad.” A gamma-ray burst (GRB) can be thousands of light-years away and still put the hurt on us as well.97

Stars are small compared to the vast distances between them in the galaxy. But some objects are bigger—a lot bigger. This ups the odds of an encounter significantly. Such a cosmic rear-end collision would darken our days on Earth . . . literally.


When Kapteyn was counting stars, trying to figure out the shape of the galaxy, he had no idea that dust would mess up his statistics.

He certainly had no idea it could kill us all.

Stars make up about 90 percent of the normal mass of the Milky Way.98 The remaining mass is made up of gas and dust strewn between them. That may not sound like much, but it adds up to a whopping 20 billion times the Sun’s mass! That is a lot of litter, floating in the darkness of space.

Called the interstellar medium, or ISM for short, the majority of this material is in fact dark. It’s cold—hundreds of degrees below zero Fahrenheit—and mostly consists of hydrogen interspersed with heavier elements like helium, carbon, and oxygen. Some of it is dust, mixed in with the gas when it got blown out by giant stars and supernovae.

A lot of this material is smeared around the galaxy, like a layer of grime on a car’s windshield. It’s ethereally thin, with just a few atoms knocking around per cubic inch—the equivalent of a high-grade laboratory vacuum. But space is big, and even that small amount of matter adds up. If you go outside on a moonless summer’s night in the northern hemisphere, you might see the band of the Milky Way high overhead. If you look carefully, along the constellation of Cygnus, the swan, you can see that the diffuse glow of the galaxy is split in two lengthwise by a dark swath called the Great Rift. That is the effect of dust in the Milky Way: it obscures the stars behind it, blocking their light from reaching us. Galactic smog, if you will.

Not all of the ISM is diffuse, though: some of it is clumped. After viewing the Great Rift in Cygnus, wait six months and go outside on a winter’s night. Turn your gaze to Orion, the hunter. Below the famous three stars forming his belt you’ll see three fainter, more tightly aligned stars making up his dangling dagger. The middle star of the knife is not a star at all; even through binoculars it takes on a fuzzy appearance. Through a moderate telescope you can see that it’s actually a gas cloud, and in deep images with large telescopes its true nature is revealed: the great Orion Nebula is one of the largest complexes of gas and dust in the galaxy, with a total mass estimated to be thousands of times that of the Sun.

It’s about 1,500 light-years away, yet visible to the unaided eye—it’s bright.99 That’s because it’s a stellar nursery, the birthplace of thousands of stars. Many of these stars are massive, hot, and bright. In fact, a solid dozen stars inside the nebula will one day explode as supernovae (and then the nebula will get very bright). All the stars living out their lives inside the nebula light it up, making it brilliant and gaudy, the way the lights on Broadway illuminate the clouds above New York.


The magnificent Orion Nebula is one of the most beautiful objects in the sky. It is the location of intense star birth, and is lit from within by a dozen high-mass (and short-lived) stars. Located 1,500 light-years away, it is easily visible to the naked eye.


Such star factories are scattered around the galaxy, but coincidentally the Orion Nebula, one of the largest, is pretty close on a galactic scale. If the Milky Way were a football field, the Orion Nebula would be only one yard away.

So just how close can nebulae get to us? Everything in the galaxy orbits the center, all at slightly different speeds and trajectories. It’s possible that the Sun could pass very close to such a cloud, and in fact it gets even more likely when the Sun enters one of the spiral arms; as mentioned above, gas clouds pile up there. When the Sun moves into an arm, it’s like driving along the highway and suddenly plunging into a fog bank.

What would happen to us if we slammed into such a cloud?

The effects of a collision are actually fairly complex, and depend on a lot of factors, such as how many stars are forming, how close the Sun gets to them, how long the Sun spends in the nebula, and the detailed structure of the nebula on small scales.

We can generalize a bit, though. For example, the core of the Orion Nebula is where most of the action is: several very massive newborn stars are busily spewing out light across the electromagnetic spectrum in vast quantities. One complex of massive stars at the very heart of the nebula cranks out as much energy in just X-rays as the total energy the Sun emits! Even so, it would take a very close passage to these stars to affect us on Earth; even from a light-year or two away the X-rays from them would affect us far less than an average solar flare.

The ultraviolet emission isn’t too big a deal either. The brightest young star in the heart of the Orion Nebula is named 0321c Orionis,100 and it has a mass 40 times the Sun’s and a surface temperature 7 times hotter. Ultraviolet light floods out of such a star; 0331c’s ultraviolet output is millions of times that of the Sun. However, from a light-year away that emission is diluted greatly, and we’d receive only a fraction more UV than we do from the Sun.

In addition, 0341c blows out a stellar wind, and it’s beefy: it spews out 100,000 times as much matter as the Sun does in its solar wind, and at twice the velocity. However, again, from a light-year away the wind would be attenuated enough that the Sun’s magnetic field would protect us from the onslaught.

The most dramatic effect would be the visible one: from a light-year away, the brightest stars in the Orion Nebula would be incredible to see—0351c blasts out energy at a rate over 200,000 times that of the Sun! From a light-year away it would shine almost as bright as the full Moon. Other stars in the nebula would also be incredibly bright, and scattered through the sky; a truly dark night would be virtually unknown. This might affect some nocturnal species (see chapter 3) but overall it wouldn’t be too big a deal.


Dark clouds of dust haunt the Milky Way. The density of particles in them is very low compared with our atmosphere, but the clouds are so huge that they are opaque. They absorb the light from stars behind them, leaving what looks like a great hole in the sky. If you look carefully at this one, called Barnard 68, you can see the stars getting dimmer as you gaze from the outside of the cloud toward the center.


That’s not to say that a nebula is a cozy place to be. Perhaps the most dangerous aspect of passing close to the center of the Orion Nebula is that it would take a long time. Stars like 0371c have the unfortunate tendency to explode, with all the dangers involved (again, see chapter 3). Supernovae are dangerous within about 25 light-years—closer than that and the explosion does serious damage to the Earth’s ozone layer, causing a potential mass extinction. A close pass through the heart of the nebula means the Sun will be in the danger zone for close to 100,000 years.101 Massive stars live short lives of only a few million years before they explode, so there is a significant chance that plowing through a nebula like Orion will bring us dangerously close to an exploding star. Just one more fun thing to think about.

And there are two more dangers in this close encounter, both of which are invisible. Or not invisible so much as dark.

So far, I’ve only talked about beautiful nebulae illuminated by their newborn stars. But not all nebulae are like that; some have not yet formed stars. These are dark, cold clouds that go by various names, such as molecular clouds, Bok globules, or simply dark nebulae.

Some of them are fairly dense as cosmic objects go, with as many as 100 million particles of dust per cubic inch. To be sure this is still not terribly dense; Earth’s atmosphere at sea level is a hundred billion times denser! But these clouds can be very large, light-years across, and that adds up. Like a thick fog, they can completely absorb any starlight that falls on them. Many of them look almost like holes in space, so completely do they block light.

Interestingly, the exact effect on the Earth is difficult to predict were the solar system to plunge into such a cloud.102 Certainly, the amount of sunlight reaching the Earth could drop significantly; even a few percent diminution of sunlight could start an ice age. There are definitely dark nebulae in the galaxy dense enough to block that much sunlight.103

And what of the dust that physically mixes into Earth’s atmosphere when we plunge into a dense nebula? A group of scientists investigated what would happen to the Earth if this occurred, and they found that dust can accumulate in the Earth’s atmosphere, enough to darken the skies and significantly lower the Earth’s temperature. It could even cause a runaway ice age. They also determined that moderate ice ages can be triggered by less dense clouds, which we encounter somewhat more often. They estimate that we encounter such a cloud about every 100 million to 1 billion years or so, which means it’s a dead-on certainty that this has occurred several times in the Earth’s history. It’s probably happened a few times since complex life evolved on Earth too, though no specific ice age on Earth has been positively identified as having been triggered by a collision with a dark cloud.

However, there is another danger from getting too close to a nebula, and this time the details of the cloud aren’t so important. All that matters is, well, its matter.

Some interstellar clouds are incredibly massive, hundreds of thousands or even millions of times the mass of the Sun. A nearby passage means we will be affected by the gravity of all that mass. The direct effects on the Earth are minimal, actually, since we are so close to the Sun that its gravity will dominate.

But not all objects in the solar system are safely nestled in the inner solar system. Surrounding the Sun, well beyond the orbit of Pluto, is the so-called Oort cloud (named after the Dutch astronomer who postulated its existence), a vast collection of giant chunks of ice and rock, some of which can be hundreds of miles across. Some of these icebergs have orbits that bring them into the inner solar system every few dozen millennia, and when one of them comes, we see it as a beautiful comet.

Oort cloud objects typically stay well away from the Sun, hundreds of billions of miles out. It takes some sort of perturbing influence, some kind of shove, to change their orbits enough to drop them into the inner solar system. Such an effect may come from a passing star a few light-years away, for example; at the distance from the Sun of a typical Oort cloud object it takes just the thinnest whisper of a nudge to send them down.

If the Sun strays too close to a giant nebula, that whisper can turn into a shout. Some estimates of the Oort cloud put its population of orbiting icebergs in the trillions. Go back to chapter 1 and read about the damage a comet or asteroid impact can do. Now multiply those effects by ten, or a hundred, as comets rain down from the heavens after a close passage with a massive nebula.

Yikes. It’s hard to imagine the devastation wreaked by such an event. The Earth’s biosphere might just start recovering a few centuries after an impact when another comet would slam into us. How many mass extinctions in the dim history of our planet were due to the Sun skirting too close to a giant gas cloud?

It’s ironic—the Sun was almost certainly born in such a gas cloud 4.6 billion years ago. It may have once been surrounded by massive stars littering the sky, their stellar winds creating vast shock waves across the gas, compressing it into sheets and filaments that glowed like neon signs crisscrossing the sky.

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.


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.


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?


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.


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 motion 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.


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.


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 giant 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.

Even other galaxies may not be safe from such an unfriendly neighbor: 3C321 is a pair of galaxies, one of which is active. The active one is shooting out a jet directly at its partner 20,000 light-years away. The beam is creating all kinds of havoc in the victim galaxy, including ramming the clouds of gas there, irradiating the stars, and generally ruining what was probably a pretty nice neighborhood before all the mayhem started.

Which brings us to an interesting juncture. Can the Milky Way become an active galaxy? Can the galaxy itself become a danger to us?

In fact, yes it can. And it probably has been one in the past.

At the moment, the Milky Way’s black hole is napping—it takes incredibly sensitive gamma- and X-ray detectors to see any emission from it at all. For an SMBH to be active, a lot of material must be falling into it. Evidently ours is either not eating or not eating very much. We do see some energy coming out, but it’s very diffuse and very faint. Astronomers aren’t sure what’s causing this emission, and that very uncertainty of the source indicates that the Milky Way is not a booming active galaxy (or else the source would be obvious). So we appear to be safe.

But appearances can be deceiving. Studies have shown that there is quite a reservoir of gas near the black hole. Stars in the vicinity emit particle winds like the solar wind, and this matter can accumulate near the black hole, feeding it. These same studies show that the stream of particles can become clumpy, and when a big clump falls into the black hole, it can suddenly flare, becoming active for short periods. It emits vast energies for a few years before settling down again. These flares are most likely not very dangerous to us; the last one may have been as recent as 350 years ago—its effects are imprinted in the gas surrounding the galactic center, which can be more easily seen. X-ray observations of these clouds indicate that the last flare emitted energy at a rate 100,000 times higher than when the black hole is quiet. This sounds frightening, but remember, this happened recently as astronomical effects go, and humanity didn’t even notice.

Remember too that we’re located 25,000 light-years from the galaxy’s center, which is a lot of real estate between us and it. So it appears we’re not in any danger from such flares.

However, there are other reservoirs of gas near Sgr A*. Vast dark clouds of gas with more than a million times the mass of the Sun lurk nearby. They are currently stably orbiting the galactic center . . . currently.


When galaxies collide, beauty (and terror) can result. This galaxy, called the Tadpole because of its shape, had a recent encounter with another galaxy. The gravitational dance of the collision drew out a long streamer of gas from the Tadpole. In many such collisions, gas can be dumped into the centers of the galaxies, causing them to become active.


If you look at images of active galaxies, you might notice a trend: a lot of them are, well, funny-looking. They are distorted from the usual spiral or elliptical shape. Astronomers think this may be due to recent encounters with other galaxies, traffic accidents on a truly galactic scale. When two galaxies collide, their gravitational interaction can cause gas and dust to stream into their centers, where any supermassive black hole will eagerly gobble it up. This, in turn, will switch on the black hole, turning the recently quiet galaxy into an active one.

The Milky Way is not immune to such things. It has eaten many smaller galaxies in the past; in fact, it’s likely that most or even all large galaxies have grown through cannibalizing their neighbors. These types of encounters would have been more common in the past, when the Universe was smaller and galaxies were closer together. In fact, objects like quasars are all very far away, which means we see them when they were younger, in the past.108 It was a galaxy-eat-galaxy Universe back then, and it’s possible—even likely—that all major galaxies, including our own, were once active in their youth.

Encounters in recent times are more rare, but not unknown. The Milky Way is currently ingesting at least two different small galaxies, but these events are far too small to activate our SMBH. There are currently no nearby galaxies big enough and close enough (at least for now; see below) to do the deed, so most likely we’re safe from our own local active galaxy.

Of course, it’s possible that two clouds on different orbits around the black hole could collide, canceling each other’s momentum, sending them down into the monster’s maw. If that happened, the black hole could switch on and stay active for millennia, flooding the galaxy with vast levels of X-rays and streams of subatomic particles like a firehose on a cosmic scale.

The good news there is that this emission would be beamed, like a gamma-ray burst. Most likely, the beams would head up and down, out of the Milky Way’s plane and away from us. If that’s the case, we’re safe enough.

Of course, some galaxies have black holes in which the axis is tilted with respect to the plane, so it’s possible their beams could actually plow through the stars in the plane. But those are rare, and even if the Milky Way’s SMBH were one of them, the odds of a beam’s hitting us are probably only 1 in 30 or so.

I’d prefer longer odds myself, but then the series of events needed for us to be looking down a gamma-ray beam from the supermassive black hole in the Milky Way’s heart are already pretty precarious. I think we’re fairly safe.

And before you get too biased against supermassive black holes and their destructive powers, consider this: they may be necessary for life to arise.

Since every galaxy has a big black hole in its center, there is some reason to think that black holes play a role in galaxy formation. In fact, some characteristics of galaxies—like the way stars orbit the galaxy’s center—seem to scale with the central black hole’s mass. You might think that’s natural given how big the central black hole is, but remember: even a billion-solar-mass black hole is only a tiny fraction of the mass of a galaxy! The Milky Way is at least 200 billion solar masses, so our own supermassive black hole harbors only 0.002 percent of the total mass.

Theories abound, but it looks like the supermassive black hole in each galaxy formed at the same time the galaxy did. As stars formed and the matter forming the galaxy streamed into the center, the black hole accreted mass, becoming active, and blew out huge winds of particles and energy. These winds must have profoundly affected the galaxy around it, possibly even curtailing the size of the galaxy itself as it was forming. They would have influenced star formation, and the chemical content of those stars as well.

Sure, black holes can kill us, and in a variety of interesting and gruesome ways. But, all in all, we may owe our very existence to them.

Remember: when you stare into the abyss, sometimes it stares back at you.


There’s one more stop on our galactic tour, and technically it’s not really a danger from our own galaxy. But it involves the Milky Way, and honestly, it’s just too cool not to spend a moment on.

As mentioned earlier, our galaxy is not alone. Like a city surrounded by towns, several smaller galaxies hang out in our Local Group. But there’s also another big galaxy in the Local Group: the Andromeda galaxy. It’s a bit more massive than the Milky Way, so it’s the Minneapolis to our St. Paul (or the Baltimore to our Washington, D.C., or the Dallas to our Fort Worth, or whatever other cartographical analogy you like). Between the two of us, we totally dominate the Local Group.109

Estimates vary, but the best guess is that Andromeda is about 2.5 million light-years from our own galaxy. Because the two galaxies are each about 100,000 light-years across, this makes them unique in terms of scale: the distance between them is not that much bigger than their size. Stars are incredibly far apart compared to their sizes, as are planets. But galaxies are big, and can be close together . . . and that means they can interact.

Astronomers have measured the relative velocities of the two galaxies, and it looks as if the pair are bound together by their mutual gravity. In fact, there’s an even stronger sign that the two galaxies are doing a do-si-do.

As far as we can tell, almost all big galaxies in the Universe appear to be rushing away from us. The details of this aren’t important here—they’ll be in the next chapter in spades—but this means that over time, every big galaxy in the Universe will move away from us . . . except for one. You guessed it: Andromeda. That nearest big spiral is unique in the heavens because it is actually headed toward us.110


The Antenna galaxies (so called because of the long, curved antennae of gas and stars protruding from them) collided millions of years ago, and are in the process of merging. Their gas clouds are colliding on epic scales, causing massive amounts of star formation. Any spectators in those galaxies would have a fantastic view . . . for a while.


In point of fact, it’s screaming toward us: its velocity toward the Milky Way is about 120 miles per second, which is pretty fast (keeping up with our city theme, during the time it takes you to read this sentence, the Andromeda galaxy would have covered the distance from New York City to Boston). The problem is, we don’t know exactly what its transverse velocity is, its motion sideways relative to us. Think of it this way: if you’re standing in the street and a car is headed at you, that’s bad. But if it’s also skidding to the side quickly enough, it’ll miss you.

We don’t know for sure how much Andromeda is moving to the side. At its current distance from us, even a transverse velocity of hundreds of miles per second translates to a very tiny shift as seen by a telescope. However, it’s safe enough to assume that the transverse velocity is roughly the same as its velocity directly toward us, and some theoretical models back that up. That’s not enough for it to totally miss us.

So, given enough time, Andromeda and the Milky Way are due for a train wreck. What will happen?

Two astronomers decided to find out. T. J. Cox and Abraham Loeb at the Harvard-Smithsonian Center for Astrophysics modeled the interaction between the two giants over several billion years. What they found out doesn’t bode all that well for us.

The two galaxies accelerate toward one another as they close in. Faster and faster they approach, until they finally physically collide about two billion years from now. The collision is almost ethereal—stars are so far apart that in essence the two galaxies will pass right through one another. The odds of any two stars getting close enough to physically collide are practically zero.

In most galaxy collisions we observe today, the victims are suffering a burst of star formation.111 This is because gas clouds, unlike stars, are very large, so in a typical galaxy collision the chance of a cloud collision—of manycollisions—is a virtual certainty. When the clouds collide, they collapse and form stars. Many of these stars are massive and hot, so they light up the gas around them. Galaxy collisions in the Universe today advertise their presence by lighting up like neon signs.

However, according to the model created by Cox and Loeb, by the time the Milky Way and Andromeda merge a few billion years from now, much of the gas currently existing in the two will have already been used up to make stars. Unlike other galaxy collisions, our own won’t be accompanied by a starburst. This makes the collision safer for us; no starburst means no giant clusters of massive stars irradiating their environment, and no wave of supernova explosions destroying everything around them.

That doesn’t mean there’s no drama, however. During the collision, the shapes of the galaxies get distorted. Currently the Milky Way and Andromeda are both “grand design” spirals, with majestic spiral arms. But imagine you are a star on the galaxy’s edge, on the side facing Andromeda. As the other galaxy nears, you start to feel a gravitational tug from it, and eventually that pull is equal to the force you feel from your home galaxy. A star on the far side of the Milky Way, however, feels a greatly reduced pull since it is so much farther away from Andromeda. This has the effect of stretching out the galaxies, pulling them apart like taffy, forming long tentacles called tidal streams.

Over millions of years the two galaxies pass each other, whipping around in a curving path (depending on the amount of transverse velocity). The two long tails of stars, gas, and dust pulled out from the galaxies curve along with them, forming glowing tentacles hundreds of thousands of light-years long. From some distant galaxy, the two would look like some weird pair of marine creatures fighting to the death (or perhaps mating).

While the two galaxies pass through each other, they don’t have enough velocity to escape each other’s grasp. After about another billion years they fall back toward one another, repeating the sequence, and then again in less than another billion years. Finally, about five billion years from now, the two galaxies will have merged. Their cores will coalesce, and the matter ejected into the long tails will settle into a stable orbit. Instead of two spirals, the resulting merger will yield a single giant galaxy that is elliptical in shape—Cox and Loeb have dubbed it Milkomeda (I suppose Andromeway sounded too much like the name of some sort of pharmaceutical). In fact, many of the giant elliptical galaxies seen in the sky may be the result of such massive mergers; they are the junk heaps of cosmic collisions.

But what of the Sun? What happens to us during all this?

Interestingly, this whole event transpires during the lifetime of the Sun. While the Sun may be a red giant by the time it all ends (see chapter 7), it’ll still be around. Maybe.

Cox and Loeb’s model can make some predictions about the Sun’s fate. They find that after the first passage of the two galaxies, the Sun has a large chance of staying within the Milky Way’s disk. However, there is a small chance (about 12 percent) that it will be ejected into one of the long tidal tails. There is no danger from this, and in fact (as we’ll see in a moment) this may be the safest place for us to be. And the view! From that vantage point, we’ll be looking down on the collision with very little dust to obscure the scene. We’ll have box seats to one of the most colossal events in the Universe.

The chance of the Sun’s getting tossed out of the Milky Way becomes greater with each passage of the two galaxies. By the time the cores merge, the odds of the Sun’s being farther than 100,000 light-years from the center of the merger remnant are about 50 percent (and we’re better than 3 to 2 to be at least 65,000 light-years from the center). We’re currently about 25,000 light-years from the Milky Way’s center, so that’s a significant change.

In fact, during the merger there is a small chance (less than 3 percent) that we’ll swap sides, becoming bound to the Andromeda galaxy! While these are long odds, it’s an amazing idea. Stars tend not to be fickle in collisions, and stick with the ones who brought them, but a few will change allegiance given the chance.

There is also another possibility: there is a small chance—less than 1 percent, but it’s there—that the Sun will actually drop toward the center of the system. If this were to happen, then the Sun could actually get within a few thousand light-years of the merged cores of the two galaxies, and this would be very, very bad.

Remember, all large galaxies have supermassive black holes in their centers. Andromeda is no exception: at its heart lurks a black hole much larger than ours, weighing in at 30 million times the mass of the Sun (ours is only about 4 million). When the cores coalesce, the two monster black holes will merge, creating a single black hole with 34 million solar masses.112 Even a 1 percent chance of getting dropped near such a monster is a little higher percentage than I’d like. Still, if we can manage to escape getting swallowed by the black hole, there’s yet another problem: gas.

While there is not enough gas left over during and after the merger to form new stars, it takes far less gas falling into an SMBH to create an active galaxy. While not explicitly calculated by Cox and Loeb, it is implied in their models that some mass will drop toward the center of the merger, where it can form an accretion disk and be consumed by the black hole there. As you may recall, many active galaxies blasting out copious amounts of radiation and matter seem to have odd shapes, implying they recently suffered collisions.

If this were the case, then once again our galaxy—well, Milkomeda—will become active. Beams of matter and energy will blast out of the supermassive black hole in the core, and, if the Sun is in the wrong place at the wrong time . . . well, you know what happens to us: chapter 5 discussed these beams from a black hole. Now imagine them being a thousand times more powerful, with us in their path. If the Sun drops toward the core of the new galaxy and the supermassive black hole there decided to throw a fit, we’re in for a very bad ride. However, if the Sun is ejected off to 100,000 light-years away from the core, then the odds of intersecting one of those beams is rather small . . . and the work of Cox and Loeb indicates we have a far better chance of heading out, not in.

Of course, we’re talking about a time maybe five billion years from now. All politics is local, they say, and if we’re still around we’ll probably be contending with a star on its way to becoming a red giant and white dwarf. When your own small town’s politics are so messed up, who has time to worry about the big-city slickers and what they’re doing so far away?