Bad Astronomy: Misconceptions and Misuses Revealed, from ...

BAD ASTRONOMY GOES HOLLYWOOD 247
are named, a typical nebula (Latin for “cloud”) is really not much
more substantial than a vacuum. The atoms in the vast cloud are
pretty far apart, but even a few atoms per cubic centimeter adds
up when you’re talking about a nebula trillions of kilometers thick.
These atoms can indeed bump into each other, allowing sound to
travel through the cloud.
Most processes that create “sound” in these nebulae, though,
are pretty violent, such as when two clouds smash into each other
or when a wind from a nearby star traveling at several kilometers
per second slams into the nebula and compresses the gas. These
processes generally try to push the gas around much faster than
the nebula can react; the atoms of gas “communicate” with each
other at the local speed of sound. If some atom is sitting around
minding its own business and another one comes along moving
faster than sound, the first atom is surprised by it. It’s literally
shocked: it didn’t know what was coming. When this happens to a
lot of material it’s called a shock wave.
Shock waves are common in nebulae. They compress the gas
into beautiful sheets and filaments, which we can “ooohh” and
“ahhh” at from our nice comfortable planet safely located a few
hundred light-years away. I imagine property values near the Orion
Nebula are at a premium. The view is unparalleled, and if you
choose your site correctly the ghostly whispers of swept-up atoms
will remain unheard.
2. . . . of a dense asteroid field . . .
Ever heard the term “asteroid swarm”? Well, it’s more like an
“asteroid vacuum.” In our solar system the vast majority of asteroids
are located in a region between Mars and Jupiter. The total
amount of area defined by the circles of their two orbits is about
one-quintillion (10 18 ) square kilometers. That’s a lot of room!
Astronomer Dan Durda puts it this way: imagine a scale model
of the solar system where the Sun is a largish beach ball a meter
(1 yard) across. The Earth would be a marble 1 centimeter ( 1 ⁄2 inch)
in size located about 100 meters (roughly the length of a football
field) from the Sun. Mars would be a pea about 150 meters away
from the Sun, and Jupiter, the size of a softball, about 500 meters
out.