The Interstellar Medium
On a clear night when we look up into the sky we can see the stars, white dots on an apparently black background which appear to be forever unchanging. This is of course not the case, stars are born and stars die just on an incredibly large timescale (a star with similar mass to the sun could live for 1010 years). So where do stars form and where does the material from a dead star go to? The answer is the interstellar medium, dust and gas that collects in the regions between stars which contributes a small but significant percentage to the mass of our galaxy, the Milky Way.
The effects of the Interstellar Medium on light which has been emitted from stars leads to some of the most spectacular and wondrous sights in the night sky. An example of this is the Orion nebula, shown below. Many such images have been obtained using the Hubble Space Telescope, which is currently in orbit 360 miles above the earth.
The existence of these Interstellar gas and dust clouds has been appreciated since ancient time, however modern spectroscopy techniques have given us a greater understanding of the nature of these regions, as well as helping to understand the evolution of stars. Techniques such as the study of 21cm "spin-flip" radiation have allowed us to map the mass distribution of the galaxy and have lead to acceptable models of galactic rotation, although the elusive Dark Matter is yet to be fully understood.