Artist's impression of the symbiotic star R Aquarii.
Credit: Dana Berry
A binary star system, the combined spectrum of which includes a molecular absorption band component, typically involving the presence of TiO bands, and an emission line component, such as emission lines of ions of higher ionization such as He II4686 Å and [O III]5007 Å. The components of a symbiotic star are a red giant and a small hot star, such as a white dwarf, surrounded by nebulosity. Spectra of symbiotic stars suggest that there are three regions which emit radiation: the individual stars themselves and the nebulosity that surrounds them both. The nebulosity is thought to originate from the red giant, which is in the process of losing mass quite rapidly either through a stellar wind or through pulsation. The symbiotic phase represents a late stage in stellar evolution and a brief span in the life of the binary. Because of the short timescale involved, symbiotic stars are rare objects. Less than 200 are known. The majority of these are in our own galaxy, the Milky Way, although 15 are extragalactic: one in the Draco dwarf galaxy, 6 in the Small Magellanic Cloud, and 8 in the Large Magellanic Cloud.
