"Flying" in our southwestern skies in these early autumn evenings is one of the more prominent of the celestial birds: Aquila, the eagle, the eye of which is marked by the bright star Altair. Located near the top of the eagle's western "wing" is an object which, two decades ago, was considered to be one of the most mysterious objects known anywhere in they sky. While many of its mysteries have been cleared up since then, it still remains as one of the more enigmatic objects we've encountered in our quest to understand the goings-on in the surrounding universe.

The object in question was discovered in the 1960s by American astronomers Bruce Stephenson and Nicholas Sanduleak, who listed it in their catalogue of stars which exhibit bright features within their spectra ñ an indication that some kind of unusual activity is going on. In 1978 two other astronomers, David Clark and Paul Murdin, noted that this object, dubbed SS 433, coincided with the position of a strong source of radio waves, and also appeared to be the source of x-rays that had been detected by orbiting x-ray astronomy satellites. These are all indications of a compact object like a neutron star that is believed to be the leftover remains of a star that has exploded as a supernova, and since SS 433 is located near the center of a large, dim cloud of gas that appears to have been expelled in such an explosion, the earliest thoughts were that SS 433 is a neutron star more or less like others that are known.

Once detailed observations began of SS 433, however, it was found to exhibit many features never encountered before. Small, periodic changes in its brightness indicated that it is a member of a double star system, and like many such systems the features in its spectrum changed position on a fairly regular basis, a phenomenon due to the doppler effect as the star alternately moves toward us and away from us. However, while the brightness changes indicated a period of about thirteen days, the changes in the spectrum were indicative of a period of 164 days. What was much more surprising, however, was that the features in SS 433's spectrum indicated both motions at the same time -- it was sometimes popularly dubbed as the star "that is both coming and going" -- and the motions indicated by these features suggested a speed almost one-fourth of the speed of light. Furthermore, there were sometimes drastic changes in both the star's spectral features and its brightness from one night to the next.

Astronomers had never previously encountered such a bizarre object, with one astronomer referring to SS 433 with the Churchillian phrase "a riddle wrapped in a mystery inside an enigma." With the aid of developing ideas concerning the goings-on within high-energy environments, together with numerous observations with optical telescopes, radio telescopes, and x-ray astronomy satellites, a somewhat coherent picture of what was happening at SS 433 began to emerge by the mid-1980s.

SS 433 does indeed appear to be a double star system, with one of the stars being an "ordinary" star somewhat larger than our sun, and the other apparently being a neutron star as was originally thought. The two stars orbit around each other with a period of thirteen days, and meanwhile material from the "ordinary" star is pulled onto the neutron star. Rather than being pulled directly onto the star, however, this material falls into a large "disk" surrounding its companion. As it does so, it heats up to enormous temperatures, until it overloads -- at which time material is expelled along two high-powered "jets" that extend along the disk's rotation axis perpendicular to the disk itself.

It is these two jets that produce the simultaneous "coming and going" features in the star's spectrum, and the material flowing along these jets is indeed traveling at about one-fourth the speed of light. The flow and ejection of material is quite irregular, and sometimes large "clumps" of material are expelled, producing the irregular features sometimes seen in the spectrum. Meanwhile, the disk itself -- which provides over half of the light we see from SS 433 -- is tilted about 78 degrees to our line of sight, and this "precesses" -- i.e., wobbles like a top -- with a period of 164 days.

While this kind of scenario seems to explain the various observed phenomena in SS 433 and is generally accepted by most astronomers today, some questions still linger. It is still not entirely clear that the surrounding gas cloud is related to SS 433, although it appears that the two objects are at about the same distance from us. Furthermore, the reason for the disk's precession is not entirely understood; some astronomers have suggested that there could be a third star in the SS 433 system which could be responsible for this, although so far there has been no direct evidence for such a star.

One additional mystery concerns the fact that SS 433 appears to be a unique object, at least as far as we know. We do know of other so-called "x-ray binaries," together with several other apparent neutron stars that appear to have resulted from supernova explosions, but we don't know of any such objects that exhibit the precessing disk and extremely high-speed jets that we see in SS 433. Perhaps there was some unusual configuration of objects in the pre-supernova system that produced the set-up we see now. Or perhaps all, or most, similar systems pass through a stage like that exhibited by SS 433, but that it's very short-lived; indeed, there is some evidence that the precession period of SS 433 is rapidly decreasing. In any event, objects like SS 433 demonstrate that the universe will continue to throw us occasional curve balls that will force us to stop and think.