Death from the Skies! (33 page)

In addition,
1c 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—
1c 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.

 

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
1c 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.
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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.
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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.
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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.
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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.