Artist's impression of the symbiotic star R Aquarii. Credit: Dana Berry.
A symbiotic star is 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.
Symbiotic stars are likely progenitors of bipolar planetary nebulae and they could make up some of the systems that later explode as Type Ia supernovae, spectacular explosions visible across cosmological distances.
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. Only a few hundred are known, one of the closest of which is CH Cygni. The majority of these are in our own galaxy, the Milky Way, although some are extragalactic, including one in the Draco Dwarf Galaxy, and a number in the Small Magellanic Cloud and Large Magellanic Cloud, which are satellite galaxies of our own.