Dust

When you look up at the sky on a summer night from a location far from city lights, you can see the Milky Way overhead. What you notice are the stars, but that is not all that can be seen in the sky. What lies between them?

This did not have a good answer until the 20th century, when spectroscopic observations provided the best information about what lay between stars, starting with the first detection of absorption lines by Johannes Hartmann, which was reported in 1904. At the same time, astronomers were taking advantage of advances in photography to study objects besides luminous objects like the moon, planets, stars, and nebulae from the Messier catalog. Astronomers, the most notable of whom was Edward Barnard, began to notice "dark nebulae," regions of the Milky Way that were oddly devoid of stars. Astronomers now refer to these regions as containing molecular gas and dust.

In the above image, gas is illuminated by several nearby, bright stars. At the same time, dark regions are seen where the gas and dust is too thick for light to penetrate.

The realization that the Galaxy contained absorbing dust was crucial to understanding the structure of the Galaxy. We now know that most of the ten billion stars in our Galaxy are obscured from our line of sight by dust in the disk of the Milky Way.