A Discussion on solar studies with special reference to space observations - Observations of stellar chromospheres

Abstract The term ‘stellar chromospheres’ is often used, but has never been defined. I understand by it phenomena analogous to the solar chromosphere in that the temperature increases outward in some parts of the lower atmosphere. From the observational point of view, of course, almost all stars would show a pinkish flash spectrum if we could eclipse them, but that phenomenon is of no special interest here; we are interested in temperature reversal, as well as flares and magnetic activity such as is associated with the sunspot cycle. We exclude shells. Unfortunately, the number of stars in which the flash spectrum is observable is severely limited, and we must detect the presence of chromospheres in the integrated stellar spectrum. This limits us to observations of a few lines which satisfy two conditions: (1) they must have a reasonable optical depth in the chromosphere, and (2) they must show different behaviour in the chromosphere than in the photosphere. In the visual region only three lines satisfy these conditions frequently: the H and K lines of Ca II and the 1083 nm* line of metastable helium. The former appear as doubly reversed emission lines, and this emission must come uniquely from the chromosphere; the 1083 nm line does not have a great optical depth, but its excitation temperature is so high that it could only appear at temperatures higher than that of the photospheres of most late-type stars.