At about the same time, Roscoe Frank Sanford submitted his PhD thesis “On Some Relations of the Spiral Nebulae to the Milky Way,” based on research he'd performed at the Lick Observatory in Northern California. A technically skilled astronomer, Sanford had carefully photographed dozens of distant, luminous pinwheel-shaped clouds, the enigmatic spiral nebulae, objects we know today as galaxies. What he found most remarkable was what he saw when he viewed these spirals edge-on: a dark, circuitous path that divided them in two. In a great leap of recognition, Sanford made the step from the out there to right here. He knew another such dark path amid the stars much closer to home—that of the Milky Way's dark clouds. At the time, “Milky Way” referred to the visible structure, whereas today it refers equally to the whole galaxy. That dark path, Sanford argued, implied that the spiral nebulae were in fact distant universes, similar to the Milky Way. Even more, it explained why we don't see these nebulae along the plane of the Milky Way: our view, Sanford concluded, is blocked by some obstructing matter, “whatever it might be.”
The final missing piece of this original dark-matter puzzle would come half a dozen years later from one Robert Julius Trumpler. Trumpler is hardly a household name, but, like Hubble, he fundamentally changed astronomers' view of the cosmos. He showed that it's dusty—that when we look to the heavens, we're looking through the equivalent of a cosmic dust storm—and, in some places, it's more dense than others, but overall, dusty all the way down. Trumpler didn't look for cosmic dust but rather stumbled on it, as a kind of latter-day Columbus happening on a new world. Born in Zurich at the end of the nineteenth century, Trumpler as a young man turned from banking, the career his parents encouraged him to pursue, to one that appeared less lucrative but into which he could pour his curiosity and perfectionist's attention to detail: astronomy. His early research involved positional astronomy, mapping the locations and motions of the stars. His first paper used astropositioning to determine the exact latitude of his university town, Göttingen, Germany. It was this Zen for cosmic mapping that would lead Trumpler to new lands. In 1919, the Great War having come to an exhausted end, Trumpler began work at the Lick Observatory on the same telescope that Roscoe Sanford had used to spot the dark lanes in spiral nebulae. Soon Trumpler was mapping the distribution and measuring the size and distance to open-star clusters—spherical collections of several thousand stars gravitationally bound to one another and orbiting the Milky Way like fish in a school swimming the galactic currents.
Trumpler surveyed a hundred open-star clusters and found an odd result. The expectation was that clusters having the same number of stars, brightness, and light fingerprint would be about the same diameter across. But they weren't. Trumpler's measurements seemed to indicate that distant clusters were larger than nearby clusters of the same type. It was as if star clusters closer to home were dimesized and that those farther away were silver-dollar-sized. This didn't make any sense. It would be returning to a pre-Copernican view of the heavens, in which the Earth was somehow a point of reference for structure in the cosmos, with star clusters increasing in size as they got farther from the Earth. Analyzing his results, Trumpler found another pattern: the light from the open-star clusters was blocked in a characteristic way with distance. Every 3,260 light-years, a star cluster's light dimmed by about two-thirds in every direction of the sky, except along the Milky Way's dark bands, where the dimming was more pronounced.
Trumpler also noticed that the light from more distant objects was not only diminished but also reddened. This reddening is different from the more famous red shift of starlight used by Hubble to calculate the speed of receding distant galaxies. In red shift, the entire spectral fingerprint of a star or a galaxy is shifted toward the red because the observed object is moving away from us. In what Trumpler observed, it wasn't that the spectrum shifted but rather that the interstellar material was blocking wavelengths at the bluer end of the spectrum while longer, redder wavelengths could pass through it. This is the same physical process that produces photogenic red sunsets. Trumpler realized that the way the light was filtered—what astronomers now call interstellar extinction—provided a wonderful clue as to what was blocking it. In order to redden the observed light, the size of the objects blocking the light must be about the size of the shorter blue wavelengths of light. There must be a fine haze between the stars, tiny bits of dust about the same size as particles of cigarette smoke or diesel exhaust.
In 1610, Galileo had used a telescope to see that the Milky Way is composed of countless stars, opening the way to a new understanding of the heavens. It had taken more than three and a half centuries for stargazers to see what was between those stars. Trumpler had discovered cosmic dust.
A NEW LAND BETWEEN THE STARS
Finding cosmic dust didn't grab headlines in the way that the discovery of an expanding universe did. Unlike Hubble, Trumpler isn't the moniker of a major telescope, yet the discovery of stuff between the stars was just as paradigm breaking as Hubble's discovery. Whereas Hubble added space by growth, Trumpler added space by increasing the available cosmic real estate. Trumpler had discovered more than dust; he'd discovered a vast unknown land, the interstellar medium. Space isn't empty. Even with all the advances of nineteenth-century astronomy and a deepening understanding of the stars, astronomers were still transfixed by the notion of the stars as points of light in a crystal-clear firmament. Even as late as the mid-1950s, there was a deep, powerful belief that space was empty, a realm as clean as polished black granite between the shining stars.
This age-old notion of interstellar emptiness held sway because astronomers found cosmic dust odd. Though for several centuries there'd been a sense that something was up with the dark bits, that idea didn't fit with our understanding of the cosmos. There was no cosmology or astrophysics to explain where this dust was coming from or going to. For example, as early as 1847, the astronomer Friedrich Struve had noted and calculated the rate of interstellar extinction of starlight, though he offered no mechanism to explain it. For more than a century, there'd been a form of dust denial, much like tired householders unwilling to face the dust bunnies growing in a home's corners. It was easier to alternately deny or ignore the dark bits in favor of the illuminating stories the stars had to tell.
By the 1950s, however, cosmic dust was working its way out of obscurity and into astrophysicists' thoughts, particularly into those of Fred Hoyle. In 1957, the same year that Hoyle copublished his paper on how the elements are forged in stars, he published another more speculative piece, his science-fiction novel The Black Cloud. The book tells the story of a massive cloud of interstellar matter, the enigmatic stuff between the stars. The cloud moves between the Sun and the Earth, blocking sunlight and threatening life on our planet. But a small group of smart Earthlings based at Cambridge University (where Hoyle happened to work at the time) learn to communicate with the cloud's alien intelligence and ask in that polite British way, “If you don't mind too much, could you please move?” The Earth was saved.
In the novel's foreword, Hoyle wrote: “I hope that my scientific colleagues will enjoy this frolic. After all, there is very little here that could not conceivably happen.” It was classic Hoyle: the appearance of self-deprecating humor followed by a “just try and challenge me” defense of his right to imagine. The fact was that most astronomers still viewed the real cosmic dark clouds simply as big, mysterious things that blocked light. Hoyle, and a handful of other astronomers, had a hunch that these dark bits were much more integral to cosmic ecology—and, somehow, that they were intimately connected with us.
In 1960, working with Chandra Wickramasinghe, a young postgraduate student from Ceylon (now Sri Lanka), Hoyle turned from the origin of the elements to the origin of the dust between the stars. Physicists use math to understand and visualize phenomena and objects they often never see, from the fusion reactions deep inside stars to the wave-particle duality of electrons. Among physicists, there are those for whom this understanding stays on
the page—they can do the math, but the object always remains as if in a dream reality. A few, however, develop a visceral, intuitive sense of the mathematics: it begins to live and breathe. Arguably more than any other astrophysicist at the time, Fred Hoyle had such a sense of the lives of stars. He'd experienced his professional coming-of-age with his work in stellar nucleosynthesis stars and had come to know stars as dynamic creatures that were born, that went through developmental stages, and that eventually died. When it came to thinking about the origins of cosmic dust, Hoyle approached the question with what were now the first inklings of cosmic ecology thinking.
With Wickramasinghe, Hoyle asked two fundamental questions when thinking about cosmic dust: What materials in the cosmos are common enough to create enough dust to produce the level of observed light extinction; and where could this material come from? The answers lay in Hoyle's favorite stellar product: carbon. In a scientific article published in 1962, Hoyle and Wickramasinghe proposed that the vast reaches between the stars were filled primarily with flakes of graphite, a substance known on Earth primarily for its use as pencil lead.
Through a series of detailed astrophysical calculations, the two men showed that, in red giant stars—which are factories of carbon production—carbon could condense out as microscopic flakes of graphite in the star's cooler upper atmosphere. This was a radical, and yet in some respects obvious, idea. After all, where did all that carbon the star made go? Once formed, the star's outflowing radiation pressure—the force exerted by light waves—would expel the graphite into interstellar space at supersonic speeds of more than six hundred miles a second. The graphite would literally be fired out by light waves as tiny black dust ships carried on the stellar wind. Later, Hoyle and Wickramasinghe showed that the interstellar-dust spectroscopic fingerprint fit cleanly with that of pure graphite, as well as with the observed interstellar extinction.
Over the next decade, Wickramasinghe led the study of cosmic dust as Hoyle's colleague rather than as his student. Based on elemental abundances, the two researchers and others predicted that stardust would also contain large amounts of silicate dust—mixes of silicon and oxygen such as silicon dioxide, or quartz, the main ingredient in beach sand and glass—and that supernova would produce dust rich in iron and manganese.
Hoyle and Wickramasinghe had established a new way to think about dust—not as a hindrance to seeing but as an avenue to understanding cosmic processes. Stars weren't merely blocked by interstellar dust, they were making it. But that was only part of the dark-matter mystery. Where did the dust go? What was happening in those dark clouds? Truly understanding dust's cosmic role would require looking not through dust but at it, and this would require a whole new way of seeing.
SEEING WITH STARDUST EYES
When it comes to seeing the universe, our unaided eyes deceive us. We have eyes evolved for life on Earth. They are exquisitely tuned to spotting the movement of potential prey, reading text on a computer monitor, or enjoying the eruptive range of color in a perennial garden at its summer peak. Yet when it comes to seeing beyond our planetary home into our cosmic neighborhood, we've come to realize that with our eyes alone, we're largely blind. This might come as a shock to anyone who has ever looked at the night sky, and indeed it was a change in perspective that was vigorously resisted by many, if not most, twentieth-century astronomers. After all, we can see the stars—and that's what's out there, right?
This change in perspective can be linked to a single historical moment that took place in the year 1800. That was the year the astronomer Sir Frederick William Herschel decided, in a quirky bit of Age of Enlightenment experimentation, to measure the temperature of a rainbow. Anyone else wanting to measure such a thing might have been considered eccentric, but Herschel, who'd just turned sixty-two and thus was beyond caring what anyone else thought, was a consummate explorer of the heavens. He built telescopes with his sister Caroline and, later, with his son John. In 1781, he'd discovered Uranus, the first new planet found since antiquity. While Herschel is famous for his planetary discovery—one that, mispronounced, continues to elicit peals of laughter from schoolchildren two centuries later—he opened a much larger window on the universe, one that extends way beyond our Solar System.
Intrigued by his observation that the different-colored filters he used for observing the Sun appeared to let through different amounts of heat, Herschel decided to experiment and see if the colors of the Sun's spectrum did indeed have different temperatures. He set up a glass prism and used three glass thermometers, their bulbs blackened to better absorb heat, to measure the temperatures of the individual colors of the Sun's spectrum from violet to red. Lo and behold, there were differences: the temperature of each successive color increased from violet, the coolest; to red, the hottest. What happened next shows Herschel to be an exemplar of that marvel that characterizes our species: unfettered curiosity.
Noticing the pattern, Herschel became curious and placed a ther mometer just a smidgen past the solar spectrum's red end, where there wasn't any visible light. He waited several minutes and checked the thermometer. It had the highest temperature of all. Herschel had dis covered a new kind of light: infrared—literally, below red—radiation. More than that, he'd shown that there are forms of invisible light. And he showed that what he called “calorific rays” were waves just like visible light—they could be reflected and refracted. After millennia of human civilization, Herschel took the first small step into an invisible celestial realm, the expansive realm of light that we can't see with only our own eyes. He'd made the first step into the terra incognita of the electromagnetic spectrum, opening the window to remarkable new ways of seeing.
Today we know that what is colloquially called “light” represents just a little more than a sliver of the full spectrum of electromagnetic radiation—from the most powerful gamma rays to the longest radio waves. It's easy to take for granted two centuries of human exploration and discovery of the electromagnetic spectrum. But unlike the maps produced by Columbus, Magellan, and Cook, the electromagnetic spectrum is a map of an invisible world of waves. Nevertheless, it is omnipresent, and we've become completely dependent on it. On any given day, we can wake up to radio waves; defrost a bagel with microwaves; text a message to a friend on our smartphone using slightly longer microwaves; turn on the television with the blink of the handheld remote's infrared eye; slather on sunblock to protect us from the Sun's UV rays; and, if need be, get a lifesaving look into our bodies with x-rays.
All these waves and rays are various ways to describe the same phenomenon: light. Each of them travels as waves, all at the speed of light: 186,000 miles a second. What differentiates them are the twin characteristics of wavelength and energy. The shorter the length of the light wave, the more energetic it is. Billionth-of-a-meter-long x-rays will pierce skin and soft tissue, stopped only by denser bone, while meter-long radio waves pass through our bodies like large lolling waves under a boat on the ocean. Most importantly for astronomers, just as with visible light, every wavelength of light carries information. We now know that while humanity is tuned to see the visible, the universe is shining in all the colors, or wavelengths, of the electromagnetic spectrum. The Stardust Revolution has been built not just on bigger and better telescopes but, more importantly, on completely different ways of seeing. And when we look at the heavens with different eyes, we see a different universe.
THE COLD AND DIRTY COSMOS
Only a handful of humans have spent as much time looking at the cosmos with stardust eyes as Michael Werner, the cherubic-looking, good-natured, white-socks-and-sneakers, bike-in-his-office lead scientist for NASA's Spitzer Space Telescope. Werner has literally spent his adult life looking at the cosmos in the infrared. He was part of a generation of astronomers who came of age with infrared astronomy in the 1960s. In 1968—the year before others of his generation gathered a four-hour drive away, at Max Yasgur's farm in upstate New York, to sing the songs of the dawning of the Age of Aquarius—Werner
was finishing up his PhD at Cornell University, thinking not about the stars but about what infrared eyes revealed about the gas and dust between them.
“For most astronomers, dust was just a nuisance because it was causing extinction,” says Werner. “Until infrared [astronomy] started, there really wasn't that much information or interest in dust per se.” Werner's thesis adviser at Cornell was Martin Harwit, a pioneer in infrared astronomy who'd worked with Fred Hoyle as a postdoctoral student. Harwit helped Werner get a postdoc position at the new Institute of Theoretical Astronomy established by Fred Hoyle at Cambridge University, where a new view of dust was dawning. The key to the mystery lay in the infrared, which opened a window for a new type of sky gazer, the cosmic-dust scientist.
During World War II, German scientists developed the first application of infrared sensors to see in the dark; this was the origin of today's night-vision combat technology. Using lead sulfide—a crystalline material that functions as a semiconductor—the German technology was the first to actually see in the infrared. The lead-sulfide detectors were photoconductors, which sensed the waves as light rather than as heat and produced an image in the same way that visible light waves do when striking the pixels in a digital camera. During the Cold War, the Americans and Soviets eagerly adapted and improved on this new way of seeing to spy on each other from the first satellites. These satellites could now literally see in the dark by sensing differences in temperature and, thus, in an object's infrared emissions.
While governments focused on seeing one another's secrets, a handful of astronomers wondered what infrared surprises the cosmos might hold. They realized, however, that they had a problem: much of the infrared radiation from space is absorbed by the Earth's atmosphere, except for several narrow windows of vision that provide a very limited infrared view. The molecules carbon dioxide, water, and methane—now well-known as greenhouse gases—absorb and trap not just heat (infrared) radiated from the Earth but also infrared radiation coming from space.
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