Symbiotic Stars

Symbiotic stars are binary systems where a red giant and a hot compact star orbit each other in a peculiar relationship. The name “symbiotic” comes from their composite spectrum showing features from both a cool star and a hot ionized nebula simultaneously - like two organisms living together. These systems produce some of the most complex and fascinating behavior in stellar astronomy.

The typical configuration pairs a cool red giant with a white dwarf or occasionally a main sequence star. The red giant sheds mass through a stellar wind, losing material from its distended outer layers. The compact companion orbits within or near this wind, accreting some of the ejected material. As gas spirals onto the hot companion, it heats up and emits ultraviolet radiation. This UV light ionizes the surrounding nebula created by the red giant’s wind, making it glow like a planetary nebula.

The resulting spectrum is bizarre. Cool absorption lines from the red giant’s photosphere appear alongside emission lines from ionized gas - hydrogen, helium, oxygen, and nitrogen at temperatures of 10,000 to 100,000 Kelvin. It’s like seeing a cool M-type star and a planetary nebula occupying the same location. This distinctive combination makes symbiotic stars immediately recognizable spectroscopically even when they appear as single points of light.

Symbiotic stars come in two types based on the red giant’s nature. S-type symbiotics contain normal red giants with typical stellar spectra. D-type symbiotics contain Mira variables - pulsating red giants that vary dramatically in brightness over periods of months. The pulsations modulate the mass loss rate, creating variability in the entire system. The ionized nebula brightens and dims as the wind density changes.

Orbital periods range from years to decades or even centuries. CH Cygni has a period of about 15 years. Mira AB, where the famous variable star Mira has a white dwarf companion, orbits over roughly 400 years. The wide separations mean the stars rarely eclipse each other, though when they do, the events reveal details about both components and the surrounding nebulosity.

Some symbiotic stars undergo nova-like outbursts. Material accumulating on the white dwarf’s surface can ignite in a thermonuclear runaway, similar to classical novae. The system brightens by several magnitudes over weeks, then slowly fades over months or years. Unlike classical novae that erupt once and go dormant, symbiotic novae can undergo multiple outbursts as the red giant continues feeding material to the white dwarf.

BTW symbiotic stars may be Type Ia supernova progenitors. If the white dwarf accretes enough mass to approach the Chandrasekhar limit of 1.4 solar masses, it could explode completely. The single-degenerate scenario for Type Ia supernovae requires exactly this setup - a white dwarf accreting from a companion star. However, finding enough symbiotic systems that could produce the observed supernova rate remains challenging.

Symbiotic stars create beautiful nebulae. The ionized gas from the red giant’s wind forms structures visible in narrow-band imaging. Some show bipolar shapes suggesting jets or focused outflows. Others display irregular nebulae shaped by the orbital motion and variable wind. R Aquarii shows spectacular jets and knots visible in Hubble images, created by periodic outbursts over centuries.

AG Draconis is one of the most-studied symbiotic stars. It undergoes frequent outbursts lasting weeks to months, brightening by 1 to 2 magnitudes. The system contains a hot white dwarf and a K-type red giant in a 550-day orbit. During quiescence, the white dwarf dominates the ultraviolet emission. During outbursts, optical brightness increases dramatically as the accretion rate surges or thermonuclear burning intensifies on the white dwarf’s surface.

The Blinking Planetary, NGC 6826, may actually be a symbiotic star rather than a true planetary nebula. The central star shows characteristics of both - a hot ionizing source surrounded by nebulosity, but with spectroscopic hints of a binary nature. Many objects classified as planetary nebulae might actually be symbiotic systems where the nebula forms from red giant winds rather than a single stellar death.

Observing symbiotic stars requires patience. Most are faint, around magnitude 10 to 13 during quiescence. Outbursts make them accessible to smaller telescopes. Spectroscopy reveals their true nature far better than photometry alone. The combination of cool and hot features, strong emission lines, and sometimes dramatic variability makes them fascinating targets for amateur spectroscopists tracking their behavior over months and years.