The Interstellar Medium

Looking at the sky on a clear and dark night, the first and most impressive view is the myriad of stars twinkling on the firmament and emitting light from inconceivable distances. The space between them seems to be empty. Apart from other, apparently moving objects, such as comets and planets, it was thought for a long time that the universe was infinite and static, and Newton considered the stars to represent an absolute frame of reference. Progress in astronomy is often driven by technology. The increasing use of telescopes in the seventeenth and eighteenth centuries revealed structures hitherto unseen. Messier, hunting for comets, discovered similarly looking patches on the sky that were not moving and decided therefore to make a compilation (published in several catalogues of star clusters and nebulae until 1781), to avoid further confusion. Among them Andromeda (Messier 31, or M 31 for short) and Orion (M 42) are the most famous ones.{su1} The physical nature of the nebulae became only clear with the advent of spectroscopy in the nineteenth century, its main proponents being Fraunhofer, who discovered absorption lines in the spectrum of the sun, and Bunsen and Kirchhoff who associated spectral lines in the laboratory with light emitted by specific atoms. By then it had become evident that the space between the stars, the so-called interstellar medium (ISM), is by no means empty.