The Sciences

X-Ray Vision

A completely different view of ravenous black holes, exploding stars, colliding galaxies, and other wonders of the universe a human eye can't see

By Robert KunzigFeb 5, 2005 6:00 AM


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There is a lot to see in the night sky, but there is even more not to see. Our eyes have evolved to detect radiation within a narrow range of wavelengths that we call visible light. In essence, we peer out through a slit—and our view is what we used to think was the universe. Now we know better. The real drama happens in places where matter reaches temperatures of millions of degrees and shines mostly in X-rays our eyes cannot detect. The X-ray sky crackles with previously unimagined action: exploding stars, gas swirling into monster black holes, and pile-driver smashups of whole clusters of galaxies. All of this commotion is finally snapping into focus because of an extraordinary satellite, the Chandra X-ray Observatory, launched by NASA in 1999. What you see on these pages is the universe seen through the eyes of Chandra.

This perspective wouldn’t exist if not for a gung ho young physicist named Riccardo Giacconi, who in 1962 talked the Air Force into letting him launch a Geiger counter into space. NASA, then a brash young agency, had refused to do so. But Giacconi and his team at American Science and Engineering in Cambridge, Massachusetts, already had a contract with the Air Force to monitor atmospheric nuclear tests, and he knew the Air Force was hoping to get in on President Kennedy’s lunar program. He argued that his Geiger counter might detect X-rays from the moon and thus help determine its composition. “It was a good excuse,” he says now.

On June 18, 1962, Giacconi’s Geiger counter lifted off on an Aerobee rocket from the White Sands testing range in New Mexico. During the 350 seconds it spent above Earth’s X-ray-blocking atmosphere, it registered no emission from the moon but picked up an intense, unknown source in the constellation Scorpius. This was the first such source discovered, hence named Scorpius X-1. “Sco X-1 was a boomer,” Giacconi recalls. “We had no trouble detecting it.”

That result was a happy surprise. Researchers at the Naval Research Laboratory had previously detected X-rays from the sun’s hot outer atmosphere, but those rays were only a millionth as intense as the sun’s light. Detecting X-rays from another star light-years away seemed like a long shot. It turned out Sco X-1 was no ordinary star. It was thousands of times as luminous as the sun, and almost all that radiation was X-rays. “The great thing nature did for us is it invented a brand-new class of stars that nobody expected,” Giacconi says. When the Air Force realized he was looking at distant stars rather than at the moon, they ended his program. By then, however, X-ray astronomy had caught on.

The first scans of the X-ray sky were very coarse. Giacconi’s Geiger counter—essentially a box of electrified gas—was fine for recording the passage of X-rays but could not create a picture of the source. Even before Sco X-1, however, he had sketched out a design for a true X-ray-imaging telescope. In 1963 he and his colleague Herbert Gursky proposed to NASA a five-year plan that would culminate with a large, orbiting X-ray telescope. Thirty-six years and innumerable bureaucratic snafus later, NASA launched Chandra.

In that time, numerous simpler X-ray detectors went up. Giacconi, having worked on one of these experiments, dropped out of the project in 1981, before it had really begun. He passed the leadership of his team at the Harvard-Smithsonian Center for Astrophysics to Harvey Tananbaum, who kept the battles going nearly another two decades. The silver lining is that Chandra is a much better telescope than it would have been if it had been launched with 1960s technology.

Chandra’s fundamental design still follows the principles Giacconi sketched in the 1960s. He recognized that you cannot focus X-rays with a lens or reflect them straight off a mirror because they are so energetic they will burrow right in; that’s why such rays are so good for illuminating the insides of the human body. Giacconi’s solution was to direct the X-rays along the sides of a conical mirror, which would cause them to skip along, like pebbles off the surface of a pond. To help pull in faint objects, Chandra contains a series of nested mirrors that each funnel X-rays to a sharp focus at its narrow end, where a camera sits. And to achieve the desired image clarity—equivalent to reading a stop sign 12 miles away—the mirrors are polished to near perfection, with no bumps larger than six atoms high. In a nice twist of redemption, the job was done successfully by Hughes Danbury Optical Systems, the company that had previously botched the mirror for the Hubble Space Telescope.

What Giacconi could not have foreseen in his original plan, drawn up before the digital revolution, was Chandra’s workhorse camera, the ACIS. It detects X-rays using the same kind of silicon chip, called a charge-coupled device, that is in every digital camera. An array of 10 chips in ACIS gives it the unique ability to measure both the position and the energy of the incoming rays with high accuracy. Since each element radiates X-rays at a characteristic set of energies, or wavelengths, ACIS can reveal the composition as well as the appearance of objects. “You can make a picture just of silicon. That’s the power of Chandra—not just that the mirrors are so good,” says astrophysicist Una Hwang of NASA’s Goddard Space Flight Center.

CENTAURUS A This disturbed object is one of the brightest galaxies in the X-ray sky. A composite view (above) exposes the many types of activity behind its tremendous brilliance. X-rays (top inset) highlight the hottest regions of Centaurus A, including two enormous arcs of energized gas. A huge explosion around 10 million years ago seems to have hurled this material outward and heated it to millions of degrees. Visible light (second inset) shows a vast, elliptical grouping of stars bisected by a dark lane of dust, which astronomers interpret as the remains of a spiral galaxy that collided with a larger elliptical galaxy. Radio waves emitted by cool hydrogen (third inset) trace a pool of gas left behind by the explosion and merger. A broader mix of radio wavelengths (bottom inset) reveals two giant jets of hot gas streaming from the center. A monster black hole, as massive as a billion suns, is the likely source of all the commotion.

Courtesy of NASA/UMASS/D.Wang et al.


Chandra’s view of our galaxy’s center exposes what is hidden in visible-light photos. The X-ray perspective is dotted with torrid white dwarfs and neutron stars and bathed in multimillion-degree gas. The white blob at the center contains a massive black hole surrounded by infalling material, which, oddly, is not much brighter than some of the stars around it. “The big puzzle is why a 3 million solar mass black hole is so faint,” says astronomer Fred Baganoff of MIT. Perhaps a nearby supernova swept a lot of gas out of the black hole’s reach.

THE MOON Riccardo Giacconi, who predicted a space-based device could detect lunar X-rays, was right. Where sunshine illuminates the moon’s surface (right), atoms there emit X-rays. Chandra can pick up the faint signal (far right). X-ray noise from Earth’s uppermost atmosphere causes the speckles on the moon’s night side.

So what do these wonderful pictures show? Above all, they expose a universe dominated not by starlight and the fusion energy that creates it but by an energy source no one expected before the discovery of Sco X-1: gravity in its rawest and most extreme forms.

When a massive star, much larger than the sun, exhausts its nuclear fuel, it collapses under its own gravity and then rebounds explosively: That’s a supernova. For thousands of years the expanding gases are so hot they emit X-rays. Young supernova remnants such as Cassiopeia A are among the most beautiful objects in the X-ray sky. But at the heart of the fireworks lurks an unbeautiful monster, the star’s collapsed core.

If the original star was more than 10 times as massive as the sun, what’s left is an incredibly dense neutron star. If it was more than 25 times as massive, the remnant is an even smaller and more bizarre black hole. In the 1960s and 1970s, Giacconi’s team provided some of the first strong evidence that black holes were not just the fever dreams of theorists. The objects give themselves away by what they do to their neighbors. Thanks to its intense gravity, a neutron star or a black hole in orbit with an ordinary star snatches gas from its companion. That gas, swirling and plunging into the gravitational pit, becomes so hot it emits X-rays—a lot of X-rays. A proton falling into a neutron star releases 50 times as much energy as a proton fusing with another inside the sun. That’s why Giacconi’s detectors could detect Sco X-1 (now recognized as a neutron-star binary) even though it lies some 1,000 light-years from Earth.

On the grand scale, even Sco X-1 is a pip-squeak. Astronomers now think that the center of our Milky Way is home to a black hole nearly 3 million times as massive as the sun. With Chandra, they are watching its sputterings in unprecedented detail. Other galaxies harbor even more powerful black holes, billions of times as heavy as the sun. When stars or vast clouds of gas fall into such massive objects, the resulting X-ray blaze can be seen across the universe.

Chandra also observes many other ways that gravity sculpts the cosmos. The potent mutual attraction that holds together huge clusters of galaxies traps hot gas that feverishly emits X-rays. The contours of these emissions trace the collisions and mergers that created these clusters and that continue to tear apart and reshape the individual galaxies within. Galaxies can run headlong into each other, sparking the furious birth of hot, short-lived stars; some of these go on to create the next generation of neutron stars and black holes. Chandra has watched groups of new stars pulling themselves together and has monitored one hugely unstable star, Eta Carinae, which seems on the verge of pulling itself apart.

What Chandra sees, near and far, is nature at its most energetic. “Gone is the classical conception of the universe as a serene and majestic ensemble,” Giacconi told the audience in Stockholm in December 2002, when he received a Nobel Prize for his work. “The universe we know today is pervaded by the echoes of enormous explosions and rent by abrupt changes.” No other X-ray telescope, existing or planned, provides a sharper look at that action than Chandra. “These are the best pictures we’re going to see for a long time,” Una Hwang says.


Young stars in the cluster RCW 38, about 6,000 light-years away, are a lot hotter than our sun. Dust blots out the stars’ visible light, but Chandra can pick up some of their cloud-penetrating X-rays (left). The stars are embedded in a cloud of hot X-ray–emitting gas and electrons (blue). The electrons may have wafted off the young stars and then been energized by the intense magnetic field around an unseen neutron star or a black hole somewhere inside the cluster. An infrared image of the RCW 38 region (right), created by the Two Micron All Sky Survey, shows the envelope of gas and dust that is still forming new stars.


In this interacting pair of galaxies, located near the handle of the Big Dipper, the small one has triggered bursts of star birth in its larger neighbor. A visible-light image (far right) captures the resulting spiral glow that gives the Whirlpool its name. Chandra’s X-ray observations (near right) show that some of the new stars may have gone on to become a novel kind of black hole. Black holes that arise from dying stars typically seem to have about 10 times the mass of the sun and heat their surrounding gas to tens of millions of degrees. But the black holes in the Whirlpool have temperatures of less than 4 million degrees Celsius, indicating that the clouds of hot gas swirling around them are bigger and more spread out. They may be a new class of midsize black holes, weighing 100 solar masses or so, which could have formed either by the collision of smaller black holes or by the death of supermassive stars. In this false-color Chandra image, red indicates the lowest-energy X-rays and green denotes the medium-energy rays. Blue marks the highest energy rays, which are more than a thousand times as powerful as visible light.


Two kinds of mass have been missing from astronomers’ view of the universe: matter that emits only X-rays and matter that emits nothing at all. Chandra has shown that giant clusters of galaxies are filled with extremely diffuse gas—less than an atom per cubic foot—that is extremely hot, around 10 million degrees Celsius. In the small galaxy group HCG 62 (left), the gas is concentrated in the core (red). The bright cloud at the center of Abell 2125 (middle) envelops hundreds of galaxies. One day it will most likely merge with the cloud at its lower right; thus do clusters grow. In the Fornax cluster (right) the core cloud is swept back like a comet’s tail toward the top of the image, indicating it is moving through even more diffuse gas on a collision course with the galaxy at lower left. Astronomers think these motions are guided by filaments of dark matter, exotic particles whose presence is known only by their gravitational pull.


The most luminous star in our galaxy is dimmed by obscuring matter and by its great distance—7,000 light-years away. Eta Carinae, roughly 120 times as massive as the sun, is now practicing for an explosion that will tear it apart, probably creating a black hole. What you see in visible light (left) are two giant lobes of gas and stardust cast off by the unstable star, hidden within. The orange horseshoe in Chandra’s X-ray image (right) surrounds those visible lobes; it may be the remnant of an outburst that happened a millennium ago. The blue cloud and the white central dot consist of 60-million-degree gas shooting outward, perpendicular to the lobes, at millions of miles per hour. The white spot is around 100 times the size of Pluto’s orbit. The star itself lies buried deep inside it.


This tortured cloud is the remnant of a supernova explosion that was brilliantly visible in 1054. The predecessor star was a bit closer to us than Eta Carinae but far smaller. Instead of a black hole, it left behind a rotating neutron star, or pulsar, that spins 30 times a second and shoots beams of particles from its poles. The pulsar is the white central dot in this series of images, which Chandra took over several months. A wind of particles seems to be spreading out from the neutron star’s equator like wispy smoke rings, but they are traveling at half the speed of light.

Cassiopeia A

Just before it explodes as a supernova, a massive star is like an onion, with layers of different chemical compositions atop one another. In this color-coded Chandra image of the supernova remnant Cassiopeia A, the bluish-violet areas are the remains of the star’s innermost core, consisting largely of iron, the last element the star created before it exploded. The red is silicon from the next layer out. Green marks the spherical shock wave that precedes the great mass of material still rushing outward three centuries after the star detonated. Most of this material cannot be seen with optical telescopes. “They don’t get the big picture that X-rays show,” says NASA’s Una Hwang, who made this image. “Optical observers are looking just at the tip of the iceberg.”

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