Dust? We know dust as the pesky contaminant of our living room furniture... the ubiquitous and relentless enemy of tired elbows and of JohnsonWax
corporation. Often for astronomers, dust is just as pesky. Most of your
living room intuitions about dust hold for dust in space. The biggest
differences are the dust in space is generally simpler chemically and typically
smaller by a factor of 10-100. But, if this space dust were allowed to settle
on your coffee table, it would likely look much the same as the dust you're
used to.
Dust in space occurs nearly everywhere there is any other kind of matter in any
concentration. The solar system, for instance, is filled with dust, and if
you're very astute, you can observe this as Zodical light in the evening after
sunset.
History: In the early part of the century, the natures of the Galaxy and
Universe were generally misunderstood, so perhaps it's not suprising that dust
was not understood either. However, given that centuries of civilized humans
looked up at the Milky Way, and by the late 1800's, began to photograph it, it
is perhaps suprising that nobody recognized that the easily visible dark lanes
in the Milky Way were due to absorption, not structure. By the early 1900's,
people even generally agreed on the correct direction of the Galactic center,
and understood that the ghostly Milky Way is actually resolved into countless
points of light from stars. It is quite remarkable that people really didn't
seem bothered by the notion that directly toward the center of the Galaxy,
there just weren't nearly as many stars as just off-center.
However, as photographic techinques improved as did our understanding of the
Galaxy, astronomers quickly began to understand that the dark bands in the
Milky Way were due to obscuration, not a dearth of stars, and it was not long
before they also understood the obscuration to be caused by dust (as opposed to
gas).
Once astronomers understood that dust existed, they began to recognize just how
important it was. The first, and most obvious effect of dust is its most
simple, obscuration. Dust dims our view toward the center of the galaxy by as
much as 20 magnitudes (a factor of 100 million). In fact, astronomers are
