The Discovery of Symbiotic Stars

Most of the stars in the HD catalogue have simple spectra, consisting of a bright continuum and a number of dark absorption lines. These are the “normal” dwarf, giant and supergiant stars that fall in distinct bands in the HR diagram, and form the basis for our current understanding of stellar evolution. However, the spectra of many HD stars display bright emission lines in addition to (or perhaps in place of) the more usual absorption features. Fleming (1912) produced the first comprehensive list of these “stars with peculiar spectra” and grouped them into various categories, including (i) novae, (ii) Otype stars, (iii) stars with bright hydrogen lines, and (iv) long-period variables. A small class of red variables was especially interesting: although their spectral characteristics appeared identical to those of other red long-period variables, their range of variability seemed small for their spectral type. Cannon later isolated another group of red stars with bright lines of H I and He II; these included the irregular variables Z And and SY Mus.

One of the first systematic spectroscopic studies of a peculiar emission-line star was Merrill's (1919) investigation of the enigmatic long-period variable R Aqr. He found conspicuously bright [O III] nebular lines about one month before a predicted maximum. The rest of the spectrum appeared to be that of a normal Md variable, with strong TiO absorption bands and sharp, bright H I lines. Bright emission lines of He II, C III, and N III appeared in 1926 and remained strong until 1933 (Merrill 1936). During these years, the amplitude of the Mira-like brightness variations diminished, although the period of the oscillations remained constant.