2.5 Using stars to probe the interstellar medium
The effects of interstellar material on starlight can be used to probe the properties of the interstellar medium itself. A few examples are:
The presence of particular interstellar atoms or molecules may be determined by identifying the observed spectral lines or bands.
The temperature of the gas may be determined from the relative strengths of different lines or bands produced by different energy state changes of the same atom or molecule.
The Doppler shift of spectral lines from interstellar gas can be used to infer the radial velocities of interstellar clouds along the line of sight to a star.
The amount of dust along the line of sight may be inferred from the reddening if the true colour of a star can be determined independently and compared with the observed colour.
From these and other observations we have a general picture of the interstellar medium:
The chemical elements are present in relative abundances that are not very different from the Solar System abundances. Thus, the relative percentages of atomic nuclei of hydrogen, helium and 'heavy' elements (atomic number Z > 2), are approximately 92% : 7.8% : 0.2%, though the proportion of an element present as ions, atoms, or combined in molecules or dust, does vary in different regions of the interstellar medium.
Dust accounts for roughly 1% of the mass of most types of region. The particles are very small - about 10−7 m to 10−6 m in diameter - and consist of some fraction of each of the less volatile substances found in the ISM, such as carbon and silicates. In the cooler regions of the ISM, substances with greater volatility also condense to form icy coatings on the grains, so there are regional differences in the composition of the dust. The gas is always dominated by hydrogen and helium, which are abundant and very volatile.
There is a wide variety of conditions present in different regions of the interstellar medium, with temperatures ranging from a few kelvin in dense star-forming regions to 106 K in supernova remnants. Densities vary from ~103 atoms per m3 in rarefied regions of the ISM to more than 1010 atoms per m3 in dense clouds.
The various types of region are far from quiescent, being racked by internal motions, and by physical and chemical transformations, often rapid compared with many astronomical changes. Each type of region is also highly structured, and far from uniform.