Copyright by Ken Croswell. All rights reserved.

Introduction: A NEW GALAXY

It was hardly the astronomical discovery of the century. Indeed, the new galaxy that turned up on February 8, 1990, was so dim that the first to signal its presence was a computer. While scanning photographic plates of the sky, a team of British astronomers--and their faithful computer--uncovered the Sextans dwarf, one of the faintest galaxies ever found, right on the Milky Way's doorstep, orbiting our Galaxy just as the Moon circles the Earth.

When he discovered the new galaxy, Mike Irwin, an astronomer at the University of Cambridge, was not thinking about nearby dwarf galaxies, and he certainly was not trying to find any. Instead, Irwin was searching photographic plates for exactly the opposite--luminous quasars at the edge of the observable universe that can radiate a hundred times more light than the entire Milky Way Galaxy. Three years earlier, his scientific team had discovered what was then the most distant known object in the universe, a quasar some 10 billion light-years from the Earth.

Each of the photographic plates Irwin was examining showed a quarter million of the Milky Way's stars, a similar number of galaxies, and possibly a new quasar at a record-breaking distance. Any such quasar lay far beyond the Galaxy's stars, but on the plate the stars and the quasars all looked the same, a myriad of tiny specks. As a first step toward distinguishing the stars from the quasars, Irwin used a computerized machine to scan the plates, count the images, and measure their positions. The plate that he gave to the computer this February morning was of a small constellation called Sextans the Sextant, which lies just south of Leo. To Irwin, the plate had seemed ordinary, one of the thousands that his team was scanning.

In the afternoon, though, the output from the computer revealed an abnormality. "On this particular plate," said Irwin, "the computer detected an excess of objects in one corner. Having worked on dwarf galaxies before, I was immediately alerted to the fact that it might well be a dwarf galaxy, previously unknown, because I knew there weren't any in that part of the sky." Irwin therefore suspected that the excess images were stars beyond the Milky Way that belonged to this dwarf galaxy. In the log that day, he wrote down the object's coordinates along with the comment, "New Local Group galaxy???" To verify the apparent discovery, the next day Irwin gave another plate of Sextans to the computer, and six hours later the computer reported that the second plate bore the same enhancement of stars as the first. The new galaxy was real.

Because the galaxy in Sextans appeared large, and because its stars were spread out from one another, Irwin believed that it was nearby, in orbit around the Milky Way. At the time, nine other Milky Way satellite galaxies were known. Two were large and bright, but the other seven were much fainter. The last of these seven dwarfs had been found in 1977. By examining the new galaxy's individual stars, Irwin determined that Sextans was indeed nearby, in orbit around the Milky Way. It was the Milky Way's tenth satellite galaxy and its eighth dwarf, 295,000 light-years away.

The galaxy in Sextans is home to just a few million stars, whose total output of light does not even match that of the single brightest star in the Milky Way. "Without the computer," said Irwin, "there's no way Sextans would have been found. It's virtually impossible to find by eye. In fact, if you look under the microscope at the region of the plate where this object is, it's still very difficult to see. I've asked lots of people to try the test: here's the plate; the galaxy is on here somewhere; can you tell me where it is? And they can't find it."

Exciting though the discovery was, Irwin said that today he has almost forgotten it. "I've gotten very blase about this galaxy now," he said, noting that Canadian astronomer Sidney van den Bergh had jokingly criticized him for it. "He said that I'd ruined the nice symmetry of the Galaxy and its seven dwarfs--as in Snow White and the Seven Dwarfs. Now we've got eight, and it's one too many."

The entire universe, of course, harbors far more than just eight galaxies. Imagine that you could sail through space at a trillion times the speed of light: galaxy after galaxy would pass you by, each a conglomeration of innumerable stars. Galaxies litter the cosmos on every side, strewn across space like grains of sand over an endless beach. In the vast compass of the universe, each galaxy may seem insignificant.

But one galaxy stands out from every other: our own. The Milky Way Galaxy is the most important galaxy in the universe. We live in it, and we owe our lives to it; we circle one of its stars, and are made of material forged in countless others.

In recent decades, astronomers have delved into the intricacies of the Milky Way and painted a vivid portrait of our Galaxy's structure, evolution, and origin. Indeed, with the plethora of recent discoveries, the Milky Way itself almost seems to be a new galaxy, as new as the recent find in Sextans. But until now, no book for the general reader has presented these accomplishments or told the full story of our Galaxy--following the path from ancient times, when some viewed it as a river in the sky, to the present, when astronomers use it as a laboratory for the study of the formation of galaxies and even of the entire universe.

The Alchemy of the Heavens aims to be that book. Although other books describe how astronomers early in the twentieth century deduced the basic features of our Galaxy, The Alchemy of the Heavens also provides a close look at the equally exciting developments that have occurred since 1950. Moreover, the astronomers who made these recent discoveries here provide revealing behind-the-scenes accounts of how the discoveries were made, how well or poorly other scientists received them, and how one finding provoked the next. In this way the story of our Galaxy is much more than just one of science; it is also a deeply human story, full of colorful and controversial characters who sometimes struggled as much with one another as they did with nature.

Deciphering our Galaxy has been one of humanity's greatest--and most difficult---achievements. At first glance, the Milky Way might seem impossible to fathom. Because we live inside it, it is the only galaxy astronomers cannot see from the outside; its billions of stars superimpose themselves on one another like the leaves of trees tangled together in a huge tropical forest. Furthermore, the Galaxy is full of gas and dust that prevent astronomers from seeing most of its stars. For this reason, the stars that do light the night sky represent a tiny fraction of the Milky Way, making ours the one galaxy in the universe that astronomers will never be able to see in its full glory. Yet these obstacles make the story of the Milky Way richer and more colorful, forcing scientists to draw on a wide array of fields, from stellar evolution and nuclear physics to galactic dynamics and cosmology, in order to achieve their goal. Even today many of the greatest questions in Milky Way research have yet to be answered.

For these reasons, The Alchemy of the Heavens tells the history of our exploration of the Galaxy as it actually happened: as one discovery led to another, as one idea triggered another, as one observation overturned another. Like a symphony with unexpected chord changes, the story sometimes veers off in unexpected directions. For example, it might seem that this book should open with the beginning of the universe, with the big bang and what the Milky Way reveals about it. But astronomers' knowledge of this ancient era has come only recently, so this subject appears toward the end of the book.

A brief word, then, about the journey this book embarks on. The Alchemy of the Heavens begins with an overview of the modern conception of the Milky Way. The narrative then shifts back in time, to when the Milky Way was the stuff of myths and legends, and moves forward as astronomers began to take notice of it. The first revolution came during the 1910s and 1920s, when astronomers deduced the size of the Milky Way and our place within it.

The journey then grows more complex, as astronomers learned that nearby stars of different ages had different characteristics and held clues to past epochs in the Milky Way's life. These developments, which originated in the 1940s, stimulated a number of findings during the 1950s and early 1960s. Among them was the realization that most elements on Earth had been forged by the Milky Way's stars; and with that came the first detailed model of the Galaxy's origin and evolution. More recently, during the 1980s and 1990s, astronomers have used improved instruments to scrutinize the nearby stars and deduce far-reaching conclusions concerning the evolution of the entire Milky Way Galaxy.

Meanwhile, in the last few decades, scientists have attempted to augment this view by peering into other parts of the Galaxy. Using infrared and radio telescopes, some astronomers have studied the Galactic center, the point about which all else in the Milky Way revolves and the likely site of a massive black hole. Other astronomers have voyaged in the opposite direction, to the outer fringes of the Galaxy, which harbor nearly a dozen galaxies that orbit ours and abound with mysterious material that no one can see.

Finally, the Milky Way is in many ways a mirror of the entire cosmos, for our Galaxy has much to say about the universe beyond. In particular, since the oldest stars in the Galaxy contain elements that were manufactured just minutes after the big bang, these stars help cosmologists probe the origin of the universe. But the same stars also pose a paradox, for according to some estimates they are older than the universe itself--a logical impossibility that has yet to be resolved. As astronomers on Earth contemplate these issues, there may--or may not--be astronomers elsewhere doing the same, and the final chapter of this book draws on what astronomers now know about the Milky Way to discuss the possibility that intelligent life exists elsewhere in the Galaxy.

From Ken Croswell's The Alchemy of the Heavens. Copyright by Ken Croswell. All rights reserved.