Cornell's Harwit led the effort in the United States to get infrared telescopes first high into the atmosphere and then into space in whatever way possible. This usually meant piggybacking on military or related high-altitude aircraft test-and-spy missions. In one case, Werner and Harwit used an infrared telescope—built by another infrared astronomy pioneer, Frank Low of the University of Arizona—that was jammed into an open door of a Learjet® during NASA's high-altitude testing of the plane. Harwit also negotiated space on a US military rocket and by doing so sent the first infrared telescope into space. Its view of the cosmos lasted only five minutes, but these five minutes firmly planted the seed of possibility.
In 1977, Werner began working on NASA's Shuttle Infrared Telescope Facility, SIRTF, a mouthful of an acronym renamed the Spitzer Space Telescope after its launch in 2003 in honor of Lyman Spitzer, the US astronomer who, in 1947, was one of the first to propose the idea of space-based telescopes. From his work with Harwit, Werner was fully aware of the enormous technical challenges and potential heartache of trying to see in the infrared from beyond Earth. He also knew that for all of Spitzer's challenges, the mission's success ultimately depended on one thing, which he called its prime directive: Spitzer had to be cold. Really cold. As much as it is a telescope, Spitzer is a sophisticated space-based extreme refrigerator. The Hubble Space Telescope's lesser-known space-based observational sibling, the Spitzer Space Telescope is among the coldest pieces of machinery ever created by humans. In the infrared, heat, as measured by temperature, is the equivalent of brightness. Thus, for an infrared telescope, the colder it is, the darker its view. A really dark-sky look in the infrared requires a really cold telescope. In essence, an infrared telescope can sense anything that's hotter than it is, including cosmic dust.
Even the Hubble Telescope's Earth orbit is too hot for Spitzer. Reflected and reradiated heat from the Earth gently warms orbiting satellites, so the Spitzer team arranged for its telescope to orbit the Sun, trailing in the Earth's orbital wake. When I visited Werner at the California Institute of Technology, home to the Spitzer Space Science Center and the nerve center for the Spitzer Space Telescope, the telescope was about ninety million miles from the Earth, at about the same distance from the Sun, with an invisible communications umbilicus transmitting home its marvelous images of the cosmos. When launched, Spitzer carried a tank of liquid helium that chilled its infrared detectors below the ambient temperature of space. The refrigeration unit worked similarly to a household fridge. However, rather than recirculating the helium-vapor coolant with a pump, the tiny amount of heat the Spitzer's electronics produced slowly boiled off the helium, dropping the sensors' temperature down to about –457°F. In comparison, liquid nitrogen—the substance that creates fantastic science-class demonstrations of crystalline bananas that shatter with a tap—boils at –320°F. For Spitzer, the ice cubes in your refrigerator are like intensely glowing charcoal embers.
At –457°F, the Spitzer and Werner got a deeper, darker view of the cosmos than anyone had ever achieved. This is critical, since the average temperature of dense dust and gas clouds ranges from –441°F to –414°F, whereas the more diffuse gas of the interstellar medium gets up to a balmy 1,340°F. In fact, for most objects in the universe—from you to planets to newborn stars and vast clouds of cosmic gas and dust—anything colder than 5,840.6°F is brightest in the infrared.
Deep-chilling the telescope dropped the level of background infrared brightness from the visible-light equivalent of high noon in the cloudless Sahara Desert to the deep darkness of a moonless night. This gave the Spitzer's extra-large, pizza-sized mirror—a little more than thirty-three inches in diameter—an infrared view of the heavens a million times darker than anywhere from the surface of the Earth. The Spitzer sees at wavelengths of 3 to 180 microns; a human hair is about 50 microns in diameter. What the Spitzer gives, quips Werner, is a view of the old, cold, and dirty cosmos. Old, because Spitzer sees ancient light red-shifted into the infrared. But it's the cold and dirty view that has helped transform astronomers' sense of the cosmos. With this infrared view, Spitzer and other infrared telescopes have revealed dust in a dramatic new light. Far from getting in the way of seeing, cosmic dust is a missing link in our connection with the cosmos.
THE DUSTY MISSING LINK
Stars don't shed only light and heat. They shed dust. In fact, in the new story of the Stardust Revolution, starlight is in some ways the secondary story. The light is the aftermath of stars' alchemical creation of the elements, and stardust is the first step between a star and you. Stardust is an evolutionary missing link, like finding the bones of an intermediary species between humans and an earlier primate. Before astronomers understood stardust, they could see that we and the stars were made of the same elements, and some astronomers dreamed that somehow we were deeply joined by this common language of the elements. But there was no mechanism, no pathway for understanding our link with the stars. Dust is that missing link. In stardust, we see how stars transform themselves into the beginnings of all we see around us. Where dust once obscured what appeared to be important, it now illuminates a new pathway to understanding.
It's a view illustrated in a historical retrospective by pioneering interstellar-dust researcher Mayo Greenberg, who titled one of his articles “In Dust We Trust.” There is a truth in the dust billowing out from stars across interstellar space, sometimes raging as great cosmic dust storms until these infinitesimal grains of soot and sand pile up into massive sculptural dunes, which, with primordial gases and the eternals of time and gravity, will give birth to a new generation of stars. Dust is one of the ways that generations of stars communicate. Stars are born in dusty cocoons and, in dying, are transformed into dusty nebulae, the raw materials for a next generation of stars.
In his office, Michael Werner searches through the Spitzer's online archive of hundreds of images for one of his favorites. When he finds it, his computer monitor exudes a sculptural aura of green, red, and white. “I call this the Continents of Creation,” he says, making a playful comparison to a famous Hubble Space Telescope image of a smaller section of the area, dubbed the Pillars of Creation. Continents is an appropriate word, for dust and gas are clearly seen as the structure of the universe. It's a celestial portrait of monumental scope. Here is a new land. After crossing an ocean of space and time, there, on the horizon, is not a little dust, a haze obscuring our view of something more important, but a vast glittering realm of dust and gas—majestic, mysterious, magnificent.
What's remarkable about these now-iconic infrared images—what's so odd in some ways that we struggle to comprehend what we see with our stardust eyes—is that where we're used to seeing points of light amid darkness, we're instead seeing a roiling landscape. There's a borderland of spires and canyons encircling a great ventricle-like central chamber, tens of light-years across. It appears as geography, as solid, because it's a stop-action snapshot. It is a vast cosmic pot of heated dust and gas. The tumultuous landscape gives the sense of process. There is both something here and something happening here. What's happening is one of the most astounding events in the cosmos: stars are being born. Astronomers call this cosmic territory in the constellation Cassiopeia, W5. It's like a hospital room number, for this is the image of a vast stellar birthing unit and nursery.
In looking at and through dust, infrared astronomy has turned the idea of the fixed, eternal star into that of the lives and ecology of stars, finally revealing the long-hidden secret of their birthplaces in dust. It's in this cold, dark realm that starlight first twinkles. That stars were still being born was itself a revolutionary concept. In 1941, the Spitzer Space Telescope's namesake, American astronomer Lyman Spitzer, made the first suggestion that stars aren't just eternal lights but were in fact being born in the interstellar matter. The comment, made in a scientific paper submitted to the Astrophysical Journal, was rejected by the anonymous referee as far too radical and speculative an idea, so Spitzer removed it from his paper.
But just after World War II, the Dutch American astronomer Bart Bok and his Harvard colleague Edith Reilly took Spitzer's idea of star formation from the condensation of interstellar matter—finally published after the war—one step further. They said they knew the probable location of these cradles of stellar creation: small, dense, dark clouds. Bok and Reilly documented a particular type of isolated, small, round, densely dark nebulae, today dubbed Bok globules. By astronomical standards, they're relatively puny, ranging in size from about ten thousand to thirty thousand times the distance of the Earth from the Sun, with masses of ten to one hundred times that of the Sun's. But Bok and Reilly suggested that these dark clouds were gravitationally collapsing to form stars. Akin to caterpillar cocoons, these dark clouds were sites of transformation and rebirth; in this case, the birth of stars. It was a tantalizing prediction, we hadn't yet witnessed star birth because it had been an event hidden by a curtain of dust.
In the early 1960s, just as Western dads were entering terrestrial hospital birthing rooms, Robert Leighton, a physics professor at Caltech, was about to pull back the curtain on stellar birthplaces. Leighton believed he'd find something new by surveying the sky with the equivalent of new eyes: using new wavelengths. Today, this is orthodoxy; then, it was closer to heresy. As a result, Leighton's epochal all-sky infrared survey, called the Two-Micron Survey (because it looked at the two-micron wavelength), was a shoestring operation. While, across town in 1965, NASA engineers with deep budgets were preparing a rocket to go to the Moon and were using infrared to observe other planets in the Solar System, Leighton and his PhD student Gerry Neugebauer pieced together a telescope from war-surplus infrared detectors and a mirror that was partially built in Leighton's office. They tested their homemade infrared telescope in an alley behind Leighton's Caltech office, and when it passed this backyard test, they mounted it in a garage-like building on nearby Mount Wilson. The telescope was programmed to automatically scan the night sky, each night imaging a different strip of sky.
The results of the first all-sky infrared survey were electrifying. Leighton and Neugebauer's view of the heavens was like nothing anyone had ever seen. There, along with the timeless stars of the night sky, were hundreds of others that no one had seen before, not even with the most powerful optical telescopes. These newfound stars included numerous monstrous stars, larger than any then known, which we now know to be newborn giant stars still enshrouded in their natal cocoons of cosmic dust. Looking with stardust eyes, Leighton and Neugebauer had revealed not just the old stars but, amazingly, new ones.
Since then, infrared astronomy has been the key to telling dust's story in the birth and death of stars from the edge of time to today. Infrared telescopes have provided astronomers with a previously dust-blocked delivery-room view of star birth. In 1983, using the first space-based infrared telescope—the Infrared Astronomical Satellite (IRAS)—American and European astronomers peered into hundreds of dark Bok globules with the telescope's infrared eye and realized that Bart Bok was right. There, inside a quarter of the globules, was a baby star, and from this astronomers concluded that every Bok globule would eventually birth a star. Compared with Bok globules, larger dark clouds, such as Werner's Continents of Creation, aren't singular delivery rooms but the equivalent of a chaotic birthing floor in a major urban hospital. There are dozens of stars at various stages of birth, from those still wrapped in dust cocoons to meaty kids ready to go home, their energetic cries disturbing the entire ward. The stellar winds from the first stars to form blow away the surrounding dust and gas that accumulates at the edges of the winds, creating other areas of star formation.
The only stars not born in dust were the cosmos' very first stars. At the edge of time, just as there were no stars, there was no dust—not a single grain, anywhere. With the universe's first stars, not only did the cosmic lights come on, but with the fusion of metals, from carbon on up, came the cosmos' first dust. Spitzer has spotted galaxies with dusty light fingerprints dating back to only 870 million years after the big bang. The cosmos' first stars are thought to have been massive ones that burned bright and ended their short, hot lives as supernovas. Thus, the cosmos' first dust probably appeared as these stars burned, and it came from the supernova fallout as the exploding stars' ejecta gradually cooled and condensed. This dust was soon mixed with dust from carbon-spewing AGB (asymptotic giant branch) stars. This stardust was the cosmos' first solids, its first steps toward forming rocky planets.
As with the developmental stages of a child, this first dust changed the dynamic of all that followed. Astronomers believe that this primordial dust enabled the emergence of the first smaller, Sun-like stars—much smaller than the massive stars that die as supernovas. Smaller stars can form only when the collapsing cloud of gas and dust from which they take shape has a way to cool itself. For stars to become small, they first must cool, and dust, it turns out, is the best material the cosmos has to offer for cooling embryonic stars. As a stellar birth cloud gravitationally falls in on itself, the inward pressure heats the core, causing it to expand, as does any hot gas. Dust, however, absorbs the heat and reradiates it at infrared wavelengths that can escape the cloud, thus cooling the protostellar core and enabling it to further contract, allowing smaller stars to form. These stars don't require as much gravitational compaction to counteract heating-induced expansion. The first dust thus performed midwifery for a next generation of smaller stars, ones that in their dying days were copious dust producers.
From clean beginnings, the universe has become very dusty. Astronomers see cosmic dust everywhere. There's obviously dust between stars, but dust has also managed to work its way out of galaxies to mix with the copious primordial gas of the intergalactic medium. This said, it's important to keep dust in perspective. Even after thirteen billion years of stardust production, dust still makes up a very small percentage of the interstellar material. The stuff in the space between stars is about 99 percent gas—mostly the primordial mix of three-quarters hydrogen to about one-quarter helium, with only about 1 percent dust. While 1 percent dust doesn't sound like much, if the Earth's atmosphere had the same dust composition, you wouldn't be able to see your shoes.
The key to cosmic dust's importance is that it isn't evenly distributed. Just as dust drifts around a home, accumulating as more or less voluminous motes, cosmic dust also accumulates in quiescent corners of the cosmos. Interstellar space is remarkably windy; every star, to varying degrees depending on its type, produces stellar winds that are both motion and matter. These winds are a combination of light waves and the particles of dust and vaporized atoms that they propel. They drive cosmic dust and gas into cosmic-scale dust motes, from globules to vast dunes. This cosmic dust eventually begins to gravitationally collapse—to fall in on itself. When it does, the dark, cold nebulae—the places of star birth—appear.
Although stars are born from dust and gas, it is in their death throes that they return dust, old and new, to the cosmos in a great ritual of cosmic regeneration visible in their beautiful death shrouds, the spectacular planetary nebulae. About three hundred years ago, the light reached Earth from a massive star that had exploded as a supernova about eleven thousand light-years away, across the Milky Way. Today we call this dead star's brilliant, glimmering remnant cloud of dust and gas Cassiopeia A, the leading edges of which are racing outward at up to 3,750 miles per second—in other words, lapping the Earth in three seconds—with a shock-wave temperature of about fifty million degrees Fahrenheit, vaporizing everything it encounters. While Cassiopeia A is vaporizing dust at its leading edge, it has also made dust. Lots of it. According to data obtained from the Spitzer, at least ten thousand Earths' worth of new stardust produced by the exploding star cooled in the days and weeks after the explosion.
Supernovas, the deaths of giant stars, are relatively rare compared with Sun-like stars, and it's from these more common, modest stars that most stardust is produced. After Sun-like stars have burned all their hydrogen, they
reach the equivalent of a midlife crisis, during which their cores collapse, becoming dense and hot enough to start burning helium as their primary fuel; this is the AGB stage. In 1969, the first infrared observations of these AGB-type stars found that they are shrouded in thick dust—sooty, sandy shells of graphite and silicates. In a similar fashion, the red supergiant star Betelgeuse—pronounced “Beetle Juice” and a star that, unlike our Sun, will end with a supernova bang—has become a prodigious dust factory in its dying days. On a clear night in the Northern Hemisphere, this notably red star, one of the brightest in the sky, shines at Orion's right shoulder. For all that shine, what's equally remarkable is the amount of dust that Betelgeuse is coughing out. Seen in the infrared, Betelgeuse is surrounded by a shroud of dust and gas, a vast glowing, clumpy shroud that extends out thirty-eight billion miles from the star's surface, about four hundred times the distance from the Sun to the Earth. Since the end of the last ice age, about ten thousand years ago, it has pumped out about our Sun's mass in dust, mostly silicates and aluminates—enough rock to eventually build hundreds of Earth-sized planets.
Thanks to infrared vision, cosmic dust no longer blocks light; it shines. In the infrared, it's possible to see dust for what it really is: microscopic minerals. The term dust conveys a sense of messiness and detritus. Carbonaceous stardust is often fluffy; the molecules bond to form powdery, spongelike masses. In this case, it's better named starfluff. But more often, it appears that the molecules that bond to form stardust do so in a highly ordered pattern that creates not the random-type dust that irritates homeowners but rather crystals—the elementary beginnings of every seaside pebble, farmer's field rock, or mountain cliff face on Earth, or anywhere else in the cosmos. With infrared eyes, we see that stars don't just produce light and heat, they also make rocks. The word mineral comes from the late fourteenth-century Latin term to describe something obtained from mining, dug out of the Earth. In the Stardust Revolution, we've extended the notion of mining and minerals to the stars, the field of astromineralogy. Infrared telescopes created the first generation of cosmic rock hounds.
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