Diffuse and dust nebulae
The space between the stars is not empty: it is filled with rarefied gas (≈ 99 %, mostly hydrogen and helium) and dust (≈ 1 %) – the interstellar medium. A nebula is a cloud of gas and dust. An emission nebula is gas ionised by the ultraviolet light of nearby hot stars and glowing by itself (the Orion Nebula M42, ≈ 1350 light-years away, reddish because of Hα; a stellar nursery). A reflection nebula is dust scattering starlight (bluish; around the Pleiades). A dark nebula is dust blocking the stars behind it (the Horsehead, the Coalsack). A planetary nebula is the ejected shell of a medium-mass star (the Ring Nebula) and has nothing to do with planets. A supernova remnant is for instance the Crab Nebula (M1, 1054). Dust absorbs and reddens starlight (interstellar extinction), so the Galactic centre cannot be seen optically but shows up in the infrared and radio. Dense, cold (≈ 10–20 K) molecular clouds contract to form new stars.
With an adult in winter find the Orion Nebula (M42) with the naked eye and binoculars: the faint patch in Orion’s “sword” below the “belt”. It is an emission nebula; the eye barely sees its red colour – why? Discuss.