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

by Philip C. Plait

  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.

  NASA, H. FORD (JHU), G. ILLINGWORTH (UCSC/LO), M. CLAMPIN (STSCI), G. HARTIG (STSCI), THE ACS SCIENCE TEAM, AND ESA

  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.

  ANDROMEDA STRAIN

  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.

  BRAD WHITMORE (STSCI) AND NASA

  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 many collisions—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 radiati
on 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?

  CHAPTER 9

  The End of Everything

  BLACK. NOTHING. EMPTY.

  Everything is dark. No stars dot the inky sky, no galaxies can be seen.

  They are all long since dead, gone, disintegrated as their very constituents have decayed into nothingness.

  Nothing has occurred in the Universe for countless years. It is a cold, almost entirely empty void.

  For trillions of trillions of years, this emptiness endures. But then, suddenly, in one tiny corner of the Universe no different from any other, a phase change snaps into existence. Like crystals growing in a saturated solution, this realignment in the very structure of space and time expands. It spreads outward at nearly the speed of light, enveloping more and more space.