Death from the Skies! (35 page)

 

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.

 

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

 

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.