Hot, bright stars fill the arms of spiral galaxies. But what about the dark gaps between the arms? Are they filled with dark dust, as some researchers have supposed? Astronomers at the University of Chicago and the University of Alabama say no: they’ve found evidence that the gaps between spiral arms are indeed as empty as they look.
The astronomers studied pairs of galaxies in which one lies behind the other, so that the galaxies partially overlap along our line of sight. By measuring how much of the background galaxy’s light made it through various regions of the foreground galaxy, they could determine where the foreground galaxy was choked with light-absorbing dust.
Most of the dust turned out to be in the bright arms; the space between the arms and the outer parts of galactic disks were much more transparent. We find that it’s not unusual for only 15 to 20 percent of the light from a background galaxy to make it through a spiral arm, says Alabama astronomer William Keel. But at a location corresponding to where we are in the Milky Way, you’d have 80 percent making it through. The solar system lies in a gap between two of the Milky Way’s spiral arms-- which is fortunate for astronomers. If we were inside a dusty arm, says Keel, we might not be able to see other galaxies at all.
Why does the dust end up in the arms? Astronomers believe that density waves--in effect, stellar traffic jams--concentrate the dust there. Clumps of stars and dust create high gravity regions that suck dust in and delay its motion through the galactic disk. This compressed gas creates a wave of star formation that adds the bright but ephemeral stars that make spiral arms so striking.