The interstellar medium

Although to all intents and purposes a single or binary star may be regarded as evolving isolated in empty space, not only is it a member of a very large system of stars – a galaxy – but it is also immersed in a medium of gas and dust, the interstellar medium. This background material (mostly gas) amounts, in our Galaxy, to a few percent of the galactic mass, some 109 M ⊙, concentrated in a very thin disc, less than 103 light-years in thickness (we recall that 1 ly ≃ 9.5 × 1015 m), and ∼105 light-years in diameter, near the galactic midplane. Its average density is extremely small, about one particle per cubic centimetre, corresponding to a mass density of 10−21 k gm−3 (10−24 g cm−3); in an ordinary laboratory it would be considered a perfect ‘vacuum’. The predominant component of galactic gas – of which stars are formed – is hydrogen, amounting to about 70% of the mass, either in molecular form (H2), or as neutral (atomic) gas (HI) or else as ionized gas (HII), depending on the prevailing temperature and density. Most of the remaining mass is made up of helium. The interstellar material is not uniformly dispersed, but resides in clouds of gas and dust, also known as nebulae. We have already encountered special kinds of such nebulae: planetary nebulae, supernova remnants and nova shells.