The Chandra X-Ray Observatory opens the book on the high-energy universe.
- By James S. Schultz
- Air & Space magazine, March 2002
NASA/SAO/G. Fabbiano et al.
SITTING IN HIS CAMBRIDGE OFFICE, a silver scooter leaning discreetly against one wall, Claud Canizares ponders catastrophe. The director of the Massachusetts Institute of Technology's Center for Space Research and associate director of the Chandra X-ray Center knows it's a jungle out there, but he isn't talking about the high-tech tumult of Cambridge. What's on his mind today are fierce conflagrations like supernova Cassiopeia A, solar-system-spanning outbursts that would flash-fry soft human flesh in milliseconds. These and other incandescent churnings found in the universe's hot spots fascinate Canizares and his colleagues.
"Cosmic catastrophe is a central part of what happens astronomically," the astrophysicist says. "The phenomena are just so compelling at these high energies and temperatures. It's a peculiar aspect of human nature: Explosions, collapses, and cataclysmic events are fascinating in an of themselves. X-ray astronomy is the study of these sudden changes—the 'cosmic pathology,' if you will."
The X-ray universe is all around us, but is invisible to the naked eye and to conventional ground-based optical telescopes; X-rays in space can be detected only with extremely sophisticated sensors. That's why astronomers eager to study these pathologies depend on Chandra control center, which receives a steady stream of data from the orbiting Chandra X-ray Observatory. At the center, a staff of 50 oversee observatory operation, monitoring telemetry in real time, for one to two hours, three times every 24 hours. Transmission is two-way; operational commands are sent to the telescope based on observation targets proposed by astrophysicists, and then the spacecraft downloads to the center in 30 minutes the data it has collected during the previous eight hours. The operators convert the data in to images that the scientists who requested the observations can study and disseminate to other researchers around the world.
With its launch aboard the space shuttle on July 23, 1999, and subsequent deployment, Chandra became the third in a series of NASA's Great Observatories, joining the still-operational Hubble Space Telescope and the Compton Gamma Ray Observatory, which operated from 1991 to 2000, as orbiting instruments designed to peer ever deeper into the cosmos. Thanks to Chandra's sensitivity and sophistication, X-ray astronomy appears to have entered an era of unprecedented discovery. In particular, Chandra, which is named for the late Indian-American Nobel laureate Subrahmanyan Chandrasekhar, one of the foremost astrophysicists of the 20th century, is helping scientists to understand how black holes devour matter and energy.
At a September 2001 press conference, astrophysicists announced an example of their recent key discoveries: the cause of a violent, rapid X-ray flare observed in the vicinity of the supermassive black hole suspected to reside at the center of the Milky Way. A team of scientists led by MIT's Frederick Baganoff detected the flare while observing a source of strong radio emissions known as Sagittarius A*. The source suddenly emitted X-rays at a prodigious rate, roughly 45 times the expected rate. After three hours, X-ray intensity declined to pre-flare levels. The team concluded that the rapid rise and fall were compelling evidence that the emission resulted from matter, probably gas from a captured star, falling into the black hole.
Black holes have long stymied researchers. In the aftermath of the publication of Albert Einstein's theory of general relativity, German astronomer Karl Schwarzschild developed the concept of black holes as concentrated regions of extreme gravity. Astrophysicists calculated that black holes would form when massive stars several times larger than the sun die (smaller stars would evolve into less compressed bodies, such as white dwarfs or neutron stars).
Thus, when a massive star exhausts its internal thermonuclear fuels, it becomes unstable, gravitationally collapsing inward and becoming compressed to a particularly small—only a few kilometers in diameter—volume of super-high density. Nothing, not even light, can escape the powerful gravitational field produced by the black hole. Once snagged by the even horizon surrounding the singularity, black-hole-captured energy and matter appear to vanish completely from the universe.
Matter stripped from a companion start by a black hole can form a flat, pancake-like structure, known as an accretion disk. As material spirals toward the center of the disk, and eventually in the even horizon, it is heated by the immense gravity of the black hole, causing it to radiate X-rays, which are produced when matter is heated to millions of degrees. It is a point of no return, beyond which no one can say definitely what occurs. "We still don't understand the basic physics of gravity," says Stephen Murray, director of the High-Energy Astrophysics Division of the Harvard-Smithsonian Center for Astrophysics in Cambridge. "Are there wormholes, time warps, or can you extract information from the other side of an even horizon? We don't yet have nearly enough information to conclusively address such speculations."
With Chandra, the existence of black holes could at least be indirectly proven. Since 2000, the telescope has conducted observations that provide additional detail about black holes. For example, on September 12, 2000, astronomers announced that the observatory had pinpointed near the center of galaxy M82 an apparent black hole that could represent a missing link between smaller stellar black holes and the supermassive variety found at the centers of galaxies. The M82 black hole has the mass of at least 500 suns concentrated into a region about the size of our moon. Such a black hole would require conditions for its creation, such as the collapse of a "hyperstar" or the merger of many smaller black holes.
Chandra's power comes from its ability to make analytical sense of X-rays. Unlike ordinary light, X-rays are absorbed by Earth's atmosphere and thus can be detected only by instruments riding outside it. They are radiated under intense magnetic conditions, from gravitational forces, or in explosive environments. Thus, X-ray astronomers tend to observe those parts of the universe where the most violent events occur: exploding stars known as novas and supernovas, material near the event horizons of black holes, and supermassive black holes in the centers of active galaxies, clusters of galaxies, and extremely distant but powerful quasars. "It's a universe that's very different than what was imagined," says Riccardo Giacconi, who first suggested a Chandra-size orbiting X-ray telescope in the early 1960s. Giacconi is today an astrophysicist at Johns Hopkins University and president of Associated Universities Inc., which manages the multi-facility National Radio Astronomy Observatory. "Objects may be faint and far away, but [with Chandra] it's not a blur or a fog anymore."
Chandra, the product of a collaboration between NASA, the Harvard/Smithsonian Astrophysical Observatory, MIT, Pennsylvania State University, the aerospace company TRW, and a number of other academic, industrial, and commercial partners, has already been used to make many advances, including confirmation that widespread X-rays detected across the entire sky emanate from the collective emissions of single sources like the energetic, black-hole-fueled cores of galaxies, as well as other active galactic regions, such as stellar nurseries in star clusters. It has also given scientists increasingly detailed views of the environments of black holes; enabled the identification of early stages of star formation; and provided the composition of the extremely hot gases expelled during supernova explosions and from the outer layers of stellar atmospheres.
Chandra's journey into space began in 1976, when Harvey Tananbaum, now director of the Chandra X-ray Center, Giacconi, and seven colleagues submitted a proposal to NASA to build a space observatory capable of collecting and analyzing X-ray emissions from distant sources. Given a budge of $2 billion (slimmed down from $6 billion), with annual operating costs of $50 to $60 million, the researchers created an 11,000-pound spacecraft some 46 feet long and, including its solar panels, 65 feet wide. Because Chandra is designed to receive and analyze astronomical X-rays, its interior differs from that of an optical telescope. If X-rays were to hit a mirror head on, they would pass straight through. So Chandra's are cylindrical, angled so that X-rays graze off, are captured, and then are funneled to the observatory's instruments for processing.
One key mystery that analysis of the resulting images may ultimately reveal is the mechanism underlying massive gamma-ray bursts that emanate six to ten billion light-years away from Earth. The orbiting Compton Gamma Ray Observatory studied the phenomena, but scientists are still puzzling over them and hope that examining the bursts in X-ray wavelengths will offer additional insight. Could these occurrences represent two neutron stars colliding and coalescing? Perhaps they're super-supernovas—"hypernovas," as some have called them, the results of the detonation in the early universe of unstable, ultramassive stars 500 to 1,000 times larger than our sun. Or maybe they're an entirely different class of objects to which a name may some day be attached.
No doubt, say scientists, there are other astronomical conundrums that should yield in time to Chandra's observations. "Astronomy involves a lot of different types of physics and chemistry, but you can't just go into the laboratory to validate your theory," says Leon Va Speybroeck, a contributor to the original 1976 study proposing Chandra and today a telescope scientist with the Harvard/Smithsonian Astrophysical Observatory. "Understanding evolves over time. Someone can't do a single experiment and suddenly settle all questions."
Success has bred huge amounts of data, which is stored in two archives in Cambridge and one just south of Boston, where tape backups are transferred and placed in a guarded vault. Each shift, Chandra generates slightly more than 112 megabytes of data: that's 123 gigabytes per year. Astrophysicists are only now beginning to mine that enormous collection, a task that likely will take years. Although Chandra's operational life is officially five years, most of those affiliated with the project believe its instruments could last 15.
Many questions and answers are likely to be forthcoming as researchers schedule approximately 800 observing sessions each year. Objects under scrutiny will range from stars to gas clouds to nebulae in the Milky Way and beyond, as well as neighboring and distant galaxies. Observers want to answer basic questions: How do celestial objects form, mature, and perish? What is their nature? How do they behave? What in their basic form and function reveals the inner workings of the universe?
In the end, Chandra's deep view of the universe is no arcane scientific exercise but an exploration that brings a practical understanding home to Earthly doorsteps. Since its formation, the planet has been bombarded by extraterrestrial material. Among the arrivals have been the very elements today found in the crust and mantle, including those we rely on every day. According to Jeffrey Linsky, an astrophysicist at the University of Colorado who has used the telescope, "Your computer wouldn't work without supernovas. There's where the silicon comes from," he says. "The iron in our blood is from supernova." Indeed, humankind owes its very existence to stellar eruptions that give off material that drifts through the void, enriching the stellar medium and seeding planets, perhaps even life itself.
A Luminous Mind
Celebrated scientists, Greek and Roman gods, noted explorers: Names of the illustrious and esteemed have long been given to spacecraft on missions of exploration. "NASA's challenge to [the Chandra team] was to find a name that would also speak to people's imaginations," says Chandra X-ray Center director Harvey Tananbaum. "So we sponsored an essay contest."
The 1998 contest to rename the Advanced X-ray Astrophysics Facility, or AXAF, drew 6,000 entries, with suggestions ranging from "Isaac Asimov" to "Marie Curie," the Polish-born French physicist who coined the word "radioactivity" in 1898. One name seemed a favorite of many entrants: that of astrophysicist Subrahmanyan Chandrasekhar. The six-member selection committee agreed, and the orbiting telescope's official moniker became the Chandra X-ray Observatory.
Born in Lahore, India, on October 19, 1910, Subrahmanyan Chandrasekhar would eventually be called Chandra by friends and colleagues, a Sanskrit word meaning "moon" or "luminous." Studying first in India and then in England, Chandrasekhar was trained as a physicist before emigrating to the United States in 1937, where he taught at the University of Chicago for the remainder of his life. There he proved a popular teacher—shepherding 50 students to Ph.D.'s during his career—and prolific author, writing 10 books on a variety of topics, including the relationship between art and science.
Among his most notable works was the discovery of what is now known as the Chandrasekhar limit, which defines the physical limits on the mass of a white dwarf star. Earth's sun appears destined to become a white dwarf once its nuclear energy is entirely spent billions of years from now. By contrast, stars more massive either explode into novas or supernovas, leaving behind neutron stars, or collapse to form black holes.
In 1983, Chandrasekhar won the Nobel Prize for his studies of the physical processes important to the structure and evolution of stars. For 19 years, Chandrasekhar also served as editor of the Astrophysical Journal. He continued to write and teach at the University of Chicago until his death on August 21, 1995.