Sagittarius (Sgr) B2 is the most massive star-forming region in the Galaxy and the canonical HMSFR with probably the richest source of molecules detected to date, not least in the number of COMs recorded. The consequences of a variable and higher-than-standard cosmic ionization rates in this region close to the Galactic centre are discussed. They are seen to have a complex effect on COMs chemistries, offering both an unusual test bed for chemical evolution theory, while not being conditions representative of more widely observed HMSF cores. The particular case of cyanides and isocyanides stands out, and modelling that uses enhanced but extinction-dependent CR ionization rate brings best agreement between model results and observations. Nonetheless, the modelled column densities of some species are much lower than observed, and the physical structure profile of the regions appears to be responsible.

Here, I describe the size and structure of our home galaxy, the Milky Way, and the place within it of our own solar system. The number of stars within the galaxy is somewhere in the range from 100 billion to 500 billion. It now seems likely that the vast majority of these stars have planets. Habitable planets may be commoner in the galaxy’s disc than in its central bulge or outer halo. I use the constellation of Orion as a way to think about how to connect two-dimensional and three-dimensional pictures of the galaxy. Constellations are products of visual astronomy and are patterns in just two dimensions that disappear if we take a 3D perspective. In contrast, the Orion arm of the Milky Way is a real 3D structure whose existence is independent of our vantage point. The stars of the Orion constellation are all within the Orion arm. However, this correspondence is unusual. For example, the stars we see in the Sagittarius constellation are not in the Sagittarius arm. The final section of this chapter deals with the fact that galaxies are not static – they evolve over time. This evolution has consequences for the probability of life originating.