| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature The Achievements of Chandra
Faster than a speeding bullet, more powerful than a locomotive, the amazing Superman of comic book fame could leap buildings with a single bound, capture villains out to destroy truth, justice, and the American Way, and still romance Daily Planet reporter Lois Lane on his weekends off. Among the Man of Steel’s most incredible talents was X-ray vision that let him see right through walls and mountains. The blue eyes behind Clark Kent’s geeky glasses could melt lead in midair, turn water into steam, analyze an object’s chemical composition, and spot asteroids millions of miles away. Superman might have been imaginary, but his DC Comic creators got at least some of their science right. X-rays are so powerful that only the most extreme processes in the universe can summon the energy to generate them. So while the universe appears stately and serene by visible light, the same view through X-ray vision reveals a violent place of seething shock waves, exploding stars, multimillion-degree gases, particles accelerated to nearly the speed of light, and matter trapped in the wrenching grip of black holes. Telescopes able to see X-rays are the only way to really study these exotic astronomical phenomena. Now, after nearly 30 years of planning and cooperation between more than 10,000 scientists, engineers, and technicians, humanity finally has an X-ray telescope with truly super powers: the Chandra X-ray Observatory. In the two years since it was launched, Chandra has already constructed a clearer picture of our universe. It has captured matter swirling into the ultimate abyss of black holes, peered into the tattered remains of exploded stars, and illuminated and measured the chemical compositions of these objects with unprecedented clarity. And it has gazed outward billions of light-years to the very edge of space and time to witness the first galaxies assembling from the detritus of the Big Bang. Not actually “rays,” X-rays are a turbocharged form of light with a wavelength too short to stimulate the chemical reactions in our eyes that allow us to see. A typical X-ray wavelength runs about one ten-billionth of a meter across—about the same width as a virus. That means that each “particle,” or photon, of X-ray energy packs 100 to 1,000 times more punch than its visible light counterpart. No wonder Superman’s creators bestowed him with X-ray vision! This high-energy world is dangerously toxic to life as we know it. The souped-up photons of X-rays can rip apart genetic material within cells and alter their chemistry, causing damage that can lead to genetic mutations, cancer, and even death. Lucky for us, a layer of oxygen and nitrogen atoms 100 miles up in Earth’s atmosphere sop up most of the ubiquitous cosmic X-rays headed for planet Earth. But this protective shield also makes it impossible for astronomers to study cosmic X-ray sources from ground observatories. In the late 1940s, physicist Herbert Friedman and colleagues with the Naval Research Laboratory used captured German V2 rockets to loft detectors high enough to detect the first X-rays from space. Their work showed that the sun’s million-degree corona is a powerful X-ray emitter. In the 1960s, physicists Riccardo Giacconi and Bruno Rossi of the Massachusetts-based company American Science & Engineering launched rockets equipped with better detectors, and hit pay dirt when their instruments registered the first X-ray sources outside the solar system. The first such source, Scorpius X-1 (so-named because it was the first X-ray source found in the constellation Scorpius), seemed to have no detectable counterpart in visible light. And yet it poured out 100 million times more X-ray energy than the sun. “This was one of the most energetic things we had seen in the universe, and we had absolutely no idea what it could be,” recalls Giacconi. Yet rocket-borne experiments suffered from one unavoidable drawback: they could only collect data for about five minutes before plummeting back to Earth. By the end of the 1960s, rockets had advanced X-ray astronomy as far as they could. To take the next step, astronomers needed to launch detectors that would remain in space for months or even years at a time. Giacconi led the team that launched the first X-ray satellite on December 12, 1970. The satellite was named Uhuru, the Swahili word for “freedom,” because it was launched off the coast of Kenya on that nation’s 7th anniversary of independence from Britain. The refrigerator-sized instrument detected 339 new X-ray sources and showed that many vary in intensity. Further observations by Uhuru established one of these sources, Cygnus X-1, as the first convincing example of a black hole. Over the succeeding decades, astrono-mers launched a series of bigger and better X-ray satellites, including nasa’s Einstein in 1978, the German-American rosat in 1990, and the Japanese asca in 1993. But even the most sophisticated of these orbiting observatories were primitive compared to the best telescopes studying visible light. If astronomers studying visible light were cruising in the telescopic version of Ferraris, X-ray astronomers were still stuck in horse-drawn buggies. In the 1970s, Giacconi and some of his colleagues, including Harvey Tananbaum of the Harvard-Smithsonian Center for Astrophysics, and Martin Weisskopf of NASA’s Marshall Space Flight Center hatched a plan for the first truly sophisticated X-ray observatory. Dubbed axaf, for Advanced X-ray Astrophysics Facility, the instrument was to be the X-ray equivalent of the Hubble Space Telescope. Initially funded by Congress in 1988, the ambitious program was scaled back by hundreds of millions of dollars during the federal budget crunch of the early 1990s. To save money, the axaf team radically redesigned the satellite by reducing the original six pairs of mirrors to four pairs, and cutting the four detectors to two. Scientists and engineers grumbled that axaf had become “ax-half,” but these cuts weren’t drastic enough to satisfy Congress. The design team then scrapped plans to place axaf in a 250-mile-high circular orbit where it could be serviced by Space Shuttle astronauts, opting instead for a highly elliptical orbit that would carry the telescope one-third of the way to the moon at its farthest point from Earth. The decision shaved billions of dollars from the program and kept it alive, but it also meant axaf would be out of the Shuttle’s reach in case any tuneup work was needed. nasa could ill-afford another Hubble mirror fiasco; axaf engineers would have to get it right the first time. Yet even a scaled-down version of AXAF offered a quantum leap in performance over all previous X-ray observatories—thanks to its mirrors. Like those on its predecessors Einstein and rosat, axaf would carry four pairs of specially built, barrel-shaped mirrors nested inside one another like Russian matrioshka dolls. Energetic X-ray photons punch right through normal mirror surfaces just like bullets zipping through Kleenex tissues. But the curved surfaces of the cylindrical mirrors reflect X-rays at grazing angles, causing the different wavelengths of X-ray energy to converge and focus at a point to the rear of the telescope.Chandra’s eight mirror cylinders were cast in a special glass called Zerodur that barely expands or shrinks when exposed to the 250 °F temperature swings of outer space. All are two-and-a-half feet long, and the outermost of each set are nearly four feet in diameter. The mirrors’ surfaces were ground and polished to such an extraordinary smoothness that, if scaled up to the size of California, their highest points would be about one inch tall. A coating of iridium, a rare earth metal highly reflective in X-rays, served as the final touch. Tests proved the mirrors worked even better than ordered; on Earth, their resolution is comparable to reading the letters on a stop sign from twelve miles away. Chandra was also equipped with two versatile detectors capable of taking both highly detailed photographs and X-ray spectra images. Astronomers are fond of saying that if a picture is worth a thousand words, a spectrum is worth a thousand pictures. X-ray spectra enable astronomers to determine the chemical composition, temperature, and motion of astronomical objects. That information would soon provide nature’s recipe for stellar coronae, the disks of gas and dust whirling around black holes, and the remnants of supernovae, or exploded stars. At last, X-ray astronomers had a telescope with a resolution comparable to the instruments in the best land-based visible light observatories. Ten times sharper than any X-ray telescope ever flown, axaf could detect sources 20 times fainter than anything its most powerful predecessors could find. By the time the spacecraft was completed in early 1999, it tipped the scale at nearly 25 tons and was the size of a Mayflower moving van. The project had cost $1.5 billion to develop, meaning each American taxpayer chipped in about $7.50. In December 1998, NASA renamed AXAF the Chandra X-ray Observatory after Subrahmanyan Chandrasekhar (1910-1995), a University of Chicago theoretical astrophysicist. Chandra (pronounced CHAN-drah), as he was affectionately known to friends, and colleagues, won the 1983 Nobel Prize for Physics for advancing human understanding of stars’ life cycles. “Chandra probably thought longer and deeper about our universe than anyone since Einstein,” says Sir Martin Rees, Great Britain’s Astronomer Royal. Launched on July 23, 1999 from the Space Shuttle Columbia, Chandra proved its worth almost immediately by solving a longstanding stellar mystery. Scientists have known for decades that when the most massive stars explode as supernovae at the end of their lives, the shock wave blasts a star’s outer layers into space. Gravity then causes the star’s core to collapse, forming a superdense object called a neutron star. These dead stars pack the mass of one to three suns into a city-sized sphere, so that a thimbleful of their material would weigh more than Mount Everest. According to theory, almost every supernova remnant should harbor a neutron star inside. Yet astronomers had searched in vain for signs of the neutron star within Cassiopeia A, an expanding shell of hot gas that is also one of the closest supernova remnants to Earth. Then Chandra’s second test image resolved a bright X-ray source at the center of Cassiopeia A. Scientists are betting that the source is Cassiopeia’s missing neutron star, and are trying to confirm that. Chandra images and spectra have thrown a monkey wrench into some of astronomers’ most cherished theories. For example, astronomers always believed that the explosion of a dying star should propel lighter elements such as carbon, nitrogen, and oxygen to the margins of the remnant, leaving heavy metals such as iron in the interior. But strangely, Chandra’s images and spectra of Cassiopeia A show pockets of iron concentrated at the outer edge, a result that sent astrophysicists scrambling back to their chalkboards. Ongoing Chandra observations will almost certainly resolve a nagging mystery surrounding one of the most famous supernova remnants, the Crab Nebula. In 1054 A.D., Chinese astronomers recorded the appearance of a “guest star” in the constellation Taurus—understood today as the supernova that created the Crab Nebula. In the millennium since then, the nebula has expanded to a diameter of six light-years, or 35 trillion miles. Yet the entire expanse remains brightly lit. A single 10-mile-wide neutron star at its center illuminates the entire Crab Nebula with the power of 100,000 suns, a phenomenon comparable to lighting the Rose Bowl for a night game with a single birthday candle. Detective Chandra found the likely culprit in the Crab mystery. The telescope’s exquisitely detailed images reveal a ring of charged subatomic particles whirling at nearly light speed around the neutron star in the supernova’s center. Astronomers believe the ring somehow transfers the high energy of the neutron star outward, lighting up the nebula from within. “It’s like finding the transmission line between the power plant and the light bulb,” says Arizona State University astronomer Jeff Hester. Hester and his colleagues are now using Chandra to take a series of snapshots of the ring. Once strung together into a movie, the images should show exactly how the ring transfers its energy to the nebula at large. Besides helping astronomers better understand bright objects like the Crab Nebula, Chandra has shed considerable light on objects of profound darkness: black holes. Prior to Chandra, astronomers knew of relatively small black holes such as Cygnus X-1, which are 5 to 20 times more massive than the sun, and supermassive black holes in the cores of large galaxies, which contain several million to several billion times the mass of the sun. But Chandra has found at least one black hole that falls in between these two extremes, with a mass between 500 and 100,000 times that of the sun, suggesting that, like Neapolitan ice cream, black holes come in at least three flavors. Chandra’s superb imaging and spectral resolution has taken astronomers right into the center of the supermassive black hole in the spiral galaxy NGC 5548. Chandra fingerprinted highly charged atoms of magnesium, neon, oxygen, nitrogen, and carbon in the multimillion-degree gas that surrounds it, providing the first detailed chemical analysis of the gases near a black hole. The information will help astronomers calculate how fast black holes consume matter, as well as how and why heavy elements become more common in the center of galaxies. Chandra is also helping to develop a picture of the universe in its infancy. Previous X-ray observatories detected the diffuse glow of X-rays pervading the universe from all directions. Chandra’s supersharp resolution has shown that this glow comes from tens of millions of X-ray sources spread throughout the universe. Most of these sources are the centers of young galaxies powered by massive black holes. These galaxies are so far away that we see them as they were when the universe was just a few billion years old, or one-fifth its current age. It has taken the X-rays from these galaxies 10 billion years to get here. “The Chandra data show us that giant black holes were much more active in the past than at the present,” Giacconi says. Because many of these galaxies are so far away or enshrouded in dust, astronomers would have no hope of detecting the massive black holes in these galaxies with visible light images alone. Astronomers expect Chandra’s discoveries to continue to transform humanity’s view of the universe for the rest of its estimated 10- to 15-year lifetime. During that span, Chandra will measure the expansion rate of the universe, pin down the masses of stars that explode as supernovae, and, perhaps most importantly, help identify the nature of dark matter—the enigmatic material that comprises at least 90 percent of the universe but neither reflects nor gives off any detectable light. That would be a feat even Superman would find impressive. Robert Naeye is editor of Mercury, the magazine of the Astronomical Society of the Pacific. He is also the author of the book Signals from Space: The Chandra X-ray Observatory, published by Raintree Steck-Vaughn in 2000. |
Summer 2001
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