As an amateur astronomer, I was always facinated by Cygnus. Straddling the dense star lanes of the Milky Way, its five cruciform stars seemed hauntingly to mark the grave of some great, celestial leviathan. I loved to explore its star clusters and nebulae with my homebuilt telescope, endlessly wondering about the other treasures that might lurk behind the Great Rift's murky vail.
This facination did not end once I became a professional astronomer. Following a brief hiatus in graduate school where my research focussed on the more lucrative Glactic center, Cygnus once again became a part of my musings. Over the years, my collegues and I had explored small portions of Cygnus with a variety of powerful radio and infrared telescopes. We had studied a handful of its more spectacular stellar nurseries in great detail, hoping to learn more about how certain rare, massive stars were born in the Milky Way. Beneath light years of dust clouds, their radio and infrared emissions betrayed their presence. But there were still a good many things about the BIG PICTURE that eluded us. Like studying the human race by working in a hospital maturnity ward, we didn't know how typical our 'infants' were, or why they chose to be found only in certain dark corners of Cygnus. Then in 1983, the Infrared Astronomy Satellite (IRAS) was launched. Within one year its sensitive detectors mapped the entire sky, transmitting to Earth among its celestial prizes, a complete picture of Cygnus. It was a view quite unlike the one I had so often seen by eye on dark nights. The infrared light from heated dust and stars pierced the Great Rift, forming unfamiliar constellations. I could see at last the starry bones of the Swan shining through its enshrouding dusty skin. The handful of young stars my collegues and I had discovered were now joined by thousands of others. What were they? Where were they?
To even begin to make sense of the new infrared data I knew that I would need to aquaint myself with everything that was known about this complex region of the sky. It was a daunting task. The information provided by legions of astronomers filled hundreds of articles in the professional journals and spanned decades of painstaking effort. I spent many long summer days in the lab's research library pouring over dusty old articles, some written before I was born. My desk top soon became filled with dozens of journals opened to crucil articles. Although it might seem like a tedious task, this phase was actually tremendously enjoyable. It was like an Easter egg hunt in which important pieces to the mystery of Cygnus were scattered among hundreds of articles. My task was to uncover these puzzel pieces and fit them together as best I could into a coherent picture that also accounted for the new data from IRAS.
We live on a minor side street in the Milky Way called the Orion Spur, whose distant end lies somewhere beyond Cygnus. It is a street littered with myriads of dark clouds, clustering like houses along the way. In many of these clouds, whole families of stars are now struggling to be born. They are invisible to the naked eye, but shine brightly in infrared or radio light. Radio maps of steadily improving clarity have carefully charted the locations of vast clouds of ionized gas, and even the remnants of a few supernovae. Other maps showed where some of the dense molecular clouds were located, or the positions of fantastically luminous stars more than 100,000 brighter than our sun.
From my private tour of Cygnus I could see how the nearby stars that define the naked eye constellation were mostly stars within 1000 light years of the sun. There was nothing new to be learned here that would be worthy of publishing. But, beyond the Swan's shallow stellar skin I soon caught the glimmering of a more exciting mystery. Behind the Great Rift, luminous supergiant stars dimly seen from Earth formed an even deeper picture of Cygnus out to 6000 light years. From their scattered pattern in space, the rough outline of the Orion Spur and its more distant cousin the Perseus spiral arm began to take shape. I could even begin to see how the carpet of stars in the Orion Spur comes to an end some 9000 light years beyond Cygnus. This was an exciting discovery to me, much like my first hike to the top of Half Dome in Yosemite Park. But other astronomers had discovered this before me, so that I could not take credit for it.
Now that I had a pretty good idea where the bright stars were located, I would need to learn where all of the dust and gas clouds were located in which the young stars discovered by IRAS might be lurking. Unlike the pinpoint stars whose distances can be found from their brightness and luminosity, the distances to amorphous gas clouds can only be found by very indirect methods.
In a spiral galaxy like the Milky Way, stars chase each other in orbits around the central hub in a pattern of motion fixed by Newton's gravity and Kepler's laws. Stars deep in the Galactic core orbit the hub in a few millenia. Stars near the Sun or beyond, however, can take more than a quarter of a billion years to complete a single orbit. The Sun orbits the Galactic center at 500,000 mph, but other more distant stars and gas clouds beyond the Sun's orbit move more slowly and are steadily left behind. From this pattern of motion relative to the Sun, the speeds of gas clouds can be mathematically related to their distance. For decades astronomers have used large radio telescopes to map the Milky Way's spiral pattern in this way. They have cataloged the speeds of numerous clouds using the famous 21-centimeter hydrogen line.
Beyond Cygnus, two great streams of gas and stars flow away from us, carried by the Milky Way's slow rotation. The faster of the two moves at over 80,000 mph relative to the Sun. It is a great river of matter extending from Perseus, through Cygnus, and ending somewhere behind Scutum to form the great arc of the Perseus Arm. A slower moving river travels no faster than 50,000 mph and seems to join smoothly with nearby clouds in our corner of the Orion Spur. The vast multitude of the stars we see towards Cygnus are the flotsam and jetsam carried by this river which is our home in the Milky Way. It was once thought that the Orion Spur was a separate spiral arm nestled between the Sagittarius and Perseus Arms. But now we think it may be simply a minor spur of gas and stars jutting out from the Perseus Arm somewhere between Perseus and Orion.
Many of the dense, dark clouds in the Orion Spur have been mapped and form a bewildering unbroken curtain to earthbound radio astronomers. Like ghostly shadows on a wall, they overlap one another on the sky in a tight patina. Even their speeds are so close together, the Milky Way's velocity pattern cannot be used to sort their distances out. How strange that astronomers can tell if a cloud is in the Perseus Arm 20,000 light years away, but not distinguish between two nearby clouds only a few thousand light years apart! I was not the first astronomer to brood darkly over this failing of Mother Nature in designing the Milky Way. I knew that, despite my best of intentions at understanding Cygnus, I would have to live with this distance uncertainty in the very clouds I had hoped would be vital in understanding the IRAS data.
With such a fuzzy picture of how far away the star forming clouds were, what could I possibly hope to learn from the IRAS data? If I couldn't tell exactly where a particular infrared source was located, perhaps instead I could try to think of another more general question to answer? I knew I could at least distinguish from its speed whether an object was in the Perseus Arm or the Orion Spur. Perhaps this might lead to something interesting? The next steps in the research came quickly and took less than a single day to complete.
Did I really care about the clutter of all those foreground stars revealed by IRAS?...No?... Then why not just make them go away? So I wrote a simple computer program to delete all of the infrared sources that looked like ordinary stars. Ordinary stars are always brighter at short infrared wavelengths than at long ones, with a brightness that follows a simple mathematical law. Out of the 1,700 sources I started with, only 496 survived this step. I fully expected that these survivors would include among their multitudes the youngest stars beyond Cygnus that were still embedded in their cloudy nurseries. A simple calculation also told me that this collection might even include stars more than 100 times brighter than the Sun all the way out to the Perseus Arm. But which ones? From the IRAS data alone I would not be able to distinguish between nearby faint young stars in the Orion Spur, or more distant luminous ones in the Perseus Arm. What I needed was information about the speeds of these objects.
None of the faint infrared stars had ever been studied before, so I would have to be the one to make the velocity measurements. Fortunatly, the deadline for submitting observing proposals to the 12-meter radio telescope at Kitt Peak was only a week away, giving me plenty of time to write and submit a request for observing time to study these objects. My strategy would be to point the telescope at each of these 496 infrared sources and measure the strength and velocity of a particular carbon monoxide spectral line at a frequency of 250 giga cycles. The velocities of these lines would immediatly tell me which of the young stars were in the fast moving Perseus Arm, or in the slower Orion Spur.
Unlike being an amateur astronomer with your own telescope, you can't just say you want to use a multi-million dollar telescope "just to see what's out there". You must carefully explain in 1000 words or less what you hope to gain from the observation, and why you are the best qualified to be doing the research. This is where an astronomer must become a convincing salesperson for a research idea. My collegues and I stated that we were already familiar with the Cygnus region having published several research papers on its far infrared features in the past, and that this was a continuation of a study of its infrared emission. We mentioned that our's would be a comprehensive survey in Cygnus of star forming activity to very low stellar luminosities, both in the Orion Spur and in the Perseus Arm. The reviewers liked our reasoning and granted us about 9 days worth of observing time between 1989 and 1990.
Unfortunatly, we lost about 5 days due to bad weather which was a major dissappointment to us. It ment we would not be able to complete our survey, and would have to go through the entire proposal process all over again to plead for additional time next year. With time available to study only 70 of the 496 infrared sources, we knew beforehand what the result would look like. To be systematic, we studied only the brightest sources, and we expected that most of these would be the nearby ones in the Orion Spur. At least this first round of investigation was not likely to turn up very many objects from the much more distant Perseus Arm. Still, we had to make certain this was indeed the case.
Most of the infrared sources appeared as a single bright spectral line whose velocity could be easily read from the spectrum. As each source was detected, it was added to the appropriate bin on a graph that showed over which velocity ranges the Perseus Arm and Orion Spur would be found. By the time we were through, only one or two of the sources made their way into the Perseus bin. Virtually all of the bright sources were indeed at the velocities expected for the Orion Spur. Although this was a bit disappointing, it also ment that our next observing session at the 12-meter should be more profitable. Perhaps among the fainter sources to be studied then, more of the illusive Perseus Arm members will show up next time.
We eventually decided to publish our initial findings on where at least the bright infrared sources were located in Cygnus. The results were also presented at the 1991 American Astronomical Society convention in Philadelphia. Although discoveries such as these are not earth shattering and suitable for a cover story in Newsweek or Time magazine, they are an important step that must be taken to fully understand our neighborhood in the Milky Way. Many such steps are needed to advance science, along with the spectacular great leaps that can come our way once we have learned to walk.
My facination for exploring Cygnus remains a steadfast companion since those days long ago as an amateur astronomer. In the end I have learned a great deal, and that learning has transformed how I now think about Cygnus. Strangely, the Cygnus that forms my new reality is more real than the dark emptinesses I see on a starry night. Cygnus has become a living set of lines and points plotted on a figure; the ineffable pleasure of searching for knew knowledge in old journals; and the delight of adding my own discovery to understanding what lies behind the Great Rift. Perhaps in some future age, a Captain Kirk will boldly visit some of these infant star systems that I was the first to wonder about from afar.