  This chapter draws upon astronomical observations and modeling to constrain
the prevalence of communicative civilizations in the galaxy. We discuss the
dependence of the Drake equation parameters on the spectral type of the host
star and the time since the galaxy formed, which allow us to examine
trajectories for the emergence of communicative civilizations over the history
of the galaxy. We suggest that the maximum lifetime of communicative
civilizations depends on the spectral type of the host star, which implies that
F- and G-dwarf stars are the best places to search for signs of technological
intelligence today.
