Abstract :
Until recently, Hα has been seen as the tracer of ionized gas, picking out both star formation and the late stages of stellar evolution. This has been reaffirmed, spectacularly, by the recent WHAM and SHS surveys. But the advent of large-area digital detectors creates a new role for narrowband Hα as a direct, simultaneous, measure of intrinsic stellar colour and reddening when e.g. r _ −Hα is combined with a nearby broad band colour e.g. r _ −I _. This new capability has been clearly demonstrated by the nearly-complete IPHAS survey.
