Astronomers don't even have a name for it, just a catalog number -- SS-433. And until about four years ago, it didn't even seem worth bothering with. But when this 14th-magnitude star finally did catch astronomers' eyes, a stream of epithets quickly followed. It was called the "most bizarre" object in the galaxy or, more kindly, a "truly unique" discovery, and even, somewhat misleadingly, a star that "seems to be coming and going at the same time."
Such was the sense of shock astronomers felt as they gradually realized that what they had once ignored as a moderately faint commonplace star may actually represent a previously unsuspected type of stellar evolution.
Intensive worldwide research, especially over the last three years, has clarified the astronomers' concept of this challenging object considerably. Nevertheless, a great deal of perplexity remains. Astronomers now have a working model of what may be going on. But they have only vague notions as to what the underlying causes are or what kind of object actually lies at the core of SS-433. Yet of one thing they are certain. Given the state of astronomical knowledge five years ago, they would never have suspected such a thing would exist, let alone set out deliberately to find it.
In 1977, the star was included along with hundreds of others when Nicholas Sanduleak and C. Bruce Stephenson at Case Western Reserve University made a catalog of objects with strong emission lines in their spectra and which appear on a set of Case University photos of a section of sky centered on the constellation Aquila in the Milky Way. Different elements in a star each emit light of a characteristic color. These characteristic emissions stand out as bright lines, called spectral lines, when light from the star is spread out in rainbow colors by a prism to form a spectrum. Thus SS-433 got its name from the initials of the two astronomers and from the fact that it is the 433rd entry in their special listing.
Meanwhile, quite unrelated observations of radio noise and X-ray emissions coming from the vicinity of Aquila began to converge to focus attention on SS- 433.
A prominent feature of the region is a roughly spherical nebula, known as W50 , which shows up clearly in radio observations and looks like the remnant of a supernova explosion. Supernovas are a normal phase in the evolution of many stars in which the star explodes, throwing off its outer layer, which then becomes an expanding cloud of dust and gas. The famous Crab Nebula is an example of this.
W50 has a pointlike source of radio and X-ray emissions smack in its center. This source turned out to coincide with that 14th-magnitude object on the Case Western plate -- SS-433 -- as David Clark and Paul Murdin of Britain's Royal Greenwich Observatory showed in 1978.
In June of that year, they photographed the SS-433 spectrum. To have a strong source of radio noise and X-rays coincide with an object that also has strong emission lines in its visible spectrum was striking enough. But the spectrum itself looked pecular. There were lines in it that Clark and Murdin couldn't identify. This caught astronomers' attention. As others began to study the spectrum -- especially Bruce Margon (now at the University of Washington) and colleagues at the University of California at Los Angeles -- astronomers realized they had a first-class mystery on their hands.
The puzzling features were spectral lines that were strongly shifted from their normal positions. These indicated material moving both toward and away from us at speeds a thousand times those that at which any large mass was expected to move within our galaxy.
When material moves toward us, the light it emits is bluer than when the material is at rest (the so-called blueshift). Likewise, light from material moving away is redshifted. SS-433 has three sets of spectral lines, respectively, showing a blueshift, a redshift, and relatively little shift (implying little motion toward or away from us).
The lines Clark and Murdin couldn't identify turned out to be lines of such familiar elements as hydrogen or helium. But they were so much redder and bluer than would have been expected that the scientists failed to recognize them. Their blueshifts and redshifts implied masses of material moving at a substantial fraction of the speed of light (it turns out to be 0.26 of light speed). Nothing moving that fast had yet been seen within our galaxy.
Furthermore, the combination of blueshifting and redshifting suggested material moving toward and away from us simultaneously. This led to the colorful, but erroneous, description of a star "coming and going at the same time." No one actually believed that. Moreover, the redshift and blueshift systems varied with a periodicity of about 164 days. Lines that were blueshifted became redshifted, and vice versa.
Commenting on this in April 1979, Margon summed up astronomers' perplexity by saying: "To be honest, I don't have any convincing idea what this really is, and neither does anyone else. We have no astrophysical parallel in this or any other galaxy that comes even close to this phenomenon. It is something completely different."
Two and a half years later, astronomers do have a "convincing idea" of what is going on, although they don't know the underlying cause and don't have ironclad proof of their concept.
This concept was proposed by Israeli theorist Mordecai Milgrom and further developed by Margon and his colleague George Abell in the United States and Martin Reese and A. Fabian in Britain. Eventually, many others have been involved. It envisions SS-433 as a binary star system in which a highly condensed member draws material from its companion star. This material flows in to form a disk around the condensed member, emitting X-rays in the process. Material from the disk then is thrown out in two narrow, high-speed jets. This whole system rotates, like a lawn sprinkler, so that sometimes a particular jet comes toward us and sometimes retreats.
This picture is consistent with what is known about SS-433. But it reveals nothing of the nature of the condensed object, if there is one, or of the mechanisms by which the jets operate. Margon, Clark, Murdin, and some others would like to see in this yet another way a star evolves after a supernova explosion. Depending on their mass, such stars now are thought to end up as a white dwarf, which is a highly dense state of matter, a neutron star, which is an even more dense state, or as a black hole.
Perhaps SS-433 represents an exotic route such stellar evolution can take when one member of a binary pair becomes a neutron star or black hole. Perhaps it even is a common phase in stellar evolution. However, as tronomers have neither the theoretical knowledge nor other examples of the phenomenon to help them resolve such questions. As Margon recently commented, "Until someone does find a second SS-433 . . . [it is] an unverified assumption that SS-433 is a common transient stage in the evolution of a binary X-ray star."
Meanwhile, astronomers have another cause for puzzlement and perhaps concern. SS-433 may be an exceedingly transient phenomenon. George Collins and Gerald Newsom of Ohio State University have reported a rapid slowdown in the 164-day period in which redshifts change to blue and vice versa. This implies that the system may be running down. Indeed, it might collapse within a few decades.
Collins, himself, is skeptical of that possibility. The slowdown may be only a temporary phase. Yet it may well be that astronomers have only a limited time to study SS-433. Thus at this writing, the intriguing object continues to justify the latest epithet flung at it recently by the amateur astronomy magazine Sky & Telescope -- "Enigma of the Century."