SS433A close binary star in Aquila that is ejecting a pair of oppositely directed jets at very extreme speed. It lies at a distance of 5 kiloparsecs in the old supernova remnant W50. The star system appears faint optically but is characterized by strong hydrogen emission lines, which led to its inclusion in the Stephenson–Sanduleak (SS) catalog of such stars (1977); its optical variability has led to an alternative variable-star designation, V1343 Aquilae. It is an X-ray source, discovered by Ariel V in 1976. It is also a radio source and a gamma-ray source. The discovery that it has weak hydrogen emission lines that move considerably in wavelength led to intense investigation and analysis in 1978–81; the following picture has emerged.
SS433 comprises a fairly normal star that is transferring mass at a high rate to a compact companion; as in other X-ray binaries, the gas forms an accretion disk around the compact star. This hot accretion disk produces most of the system's light so that the nature of the normal star is not evident. It is probably a star of a few times the Sun's mass losing matter to a neutron star or black hole (see mass transfer). These two stars have an orbital period of 13 days, and the stars and the accretion disk form a partially eclipsing binary system.
Two jets of gas stream off the faces of the accretion disk, at a constant speed of 80 000 km s–1. The disk precesses in a period of 164 days, so that the jets trace out a cone on each side with an opening angle of 40°. The component of velocity along the line of sight thus varies periodically, and the changing Doppler effect causes the emission lines from the jets to move up and down the spectrum as the wavelength varies with the 164-day cycle. Radio observations show the precessing jets out to a distance of 0.05 parsecs; the Einstein Observatory recorded their X-ray emission in a broader band at distances up to 30 pc to each side, where they strike the surrounding supernova remnant W50. EXOSAT and Ginga observations showed that the innermost regions of each jet to be eclipsed by the normal star and to have a temperature of several million degrees.
SS433 may represent a transitory phase in the evolution of many X-ray binaries, and its apparent uniqueness may indicate that fast jets are only a short-lived phenomenon. The final outcome may be a Thorne–Żytkow object. Two other instances of stars located within supernova remnants resemble SS433 superficially, and a similar phenomenon is apparently occurring on a more moderate scale in the jet-emitting symbiotic star R Aquarii.