Refined distance measurements rescue a threatened theory

Artist's impression of the binary system SS Cygni, consisting of a white dwarf (right) stripping gas off a red dwarf star. The infalling gas forms an accretion disk, which according to theory flares periodically.

Bill Saxton, NRAO/AUI/NSF

Many violent astrophysical phenomena, from quasars to some types of supernovae, are probably driven by accretion: gas falling onto a compact object. Frequently, the energy involved then blasts light and matter back into space, which is why we can see them.

Cataclysmic variables, a type of recurring explosion involving a low-mass star and a white dwarf, probably fall into that category. However, an observation of the closest cataclysmic variable system—known as SS Cygni—casts doubt on that interpretation. The outbursts from SS Cygni were simply too bright to be accretion-driven by any known mechanism, leading some astronomers to doubt whether the process was responsible for this system and, by extension, other cataclysmic variables. Some even wondered whether it was involved in other phenomena, like bright galactic nuclei.

A new set of data could possibly come to accretion's rescue. J.C.A. Miller-Jones and colleagues measured the distance to SS Cygni very precisely using two networks of radio telescopes and an assumption-free geometry-based method. They determined the binary system is about 372 light-years from the Solar System, placing it about 28 percent closer than the previous distance measurement. If that's a more reliable estimate, the bright outbursts from SS Cygni could be explained using accretion, saving that theoretical model from a premature death.