Dust in the (Stellar) Wind

Week of April 3, 2000

Dust has been on my mind lately. My nose, too.

The first warm weekend where I live means mowing the yard for the first time, and that always kicks up some dust. A recent trip to the southwest parts of the United States (which will be the focus of a future Snack, I promise) really brought it home; I saw an actual dust storm in progress, and my allergies have been bugging me ever since.

But what does this have to do with astronomy? It made me realize: dust affects every aspect of astronomy. I talk about it a lot in these very Bitesize pages (it's mentioned no fewer than three dozen times in various pages), but I never really say what it is. It's time I did.

So what is ``dust'' when astronomers use the word? It's not the kind of dust you see around here on Earth. In your house, for example, dust is comprised of the dead skin cells, hair, fibers from clothes and dirt. Out in the desert, where I saw the duststorm it's finely powdered rock. But in space, it's something else.

Space is filled with dust. This has been known for a long time; you cen see it for yourself on a dark, cloudless night. Look for the Milky Way, the faint patch of light that is actually the glow from all the stars in our Galaxy. From a very dark site, you can see that the glow isn't smooth; it is broken up by irregular dark patches. In the constellation Cygnus, high in the summer for the northern hemisphere, there is actually a long dark streak through the Milky Way, called the Dark Rift. You southern folks can see a dark patch appropriately called the Coal Sack. Photographs show these even better.
Perhaps the most famous case is the Horsehead nebula, located in the constellation of Orion. There, the black patch looks eerily like a giant chesspiece, silhouetted against the red veil of glowing gas behind it.

What you are seeing in all these examples is dark material blocking the light from stars and gas behind it. The composition of the material itself remained a mystery for a long time. Now we know what it is: to quote Drs. Paul May and David Field (astronomers who study dust), dust is comprised of ``mainly of silica (SiO2), magnesium and iron silicates (e.g. olivine, orthopyroxine, forsterite), amorphous carbon or water ice''.
What that means is that the dust is a complex collection of different types of materials, most of which is actually rather common here on Earth. The particles of dust are not little spheres, though, they form long chains and clumps. ``Long'' is a relative term, of course; most of these clumps are a millionth of a meter or so across! But there's a lot of them.

Dust is everywhere. It's in our solar system, in our Galaxy, between galaxies. There may be some in your hair right now! It's also a very good absorber of visible light, which is why it's so easy to see against a bright background. As a matter of fact, it played a key role in our misunderstanding of the size of the Universe! Because our Galaxy is so filled with this stuff, we can only see so far before our view is blocked. It's like looking through a smoke-filled room. For a long time, it was thought that the Galaxy was much smaller than it really is, and that we are at the center! But now we have a better grasp of its true size, and the fact that we are about halfway to the edge of our vast spiral metropolis.

No one is quite sure just from where the dust comes. The most popular theory is that it is created in the atmospheres of red giant stars, and blown into space by the stellar winds from these stars. What's interesting is that we know very well that our own solar system, and therefore the Earth, formed from a collapsed cloud of gas and dust. Most of this material had exotic origins; the gas was probably left over from their formation of the galaxy itself and the heavy elements (like iron and radioactives such as uranium and the like) were formed in supernova explosions. But the dust, which eventually became part of the Earth itself, came from stars much like the Sun. They were older, to be sure; red giants are a late stage in a star's life. But these stars blew gentle winds out into space, which collided and intermingled with bigger clouds of gas, and these in turn collapsed to form, well, us.

So the next time you vacuum out the dust bunnies or wipe down a tabletop, think of this: the dust you are collecting (and will no doubt unceremoniously throw out) was actually formed from another, far more strange kind of dust, and it traveled a long, long way to get here.