Death from the Skies! (32 page)

 

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.

 

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.

 

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
1c 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;
1c’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.