X-Ray Vision

How the Chandra X-ray Observatory changed our understanding of the universe. 

NASA’s Chandra X-Ray Observatory had its “grand opening” 25 years ago. On August 12, 1999—less than a month after it was ferried into orbit by space shuttle Columbia—Chandra’s protective sunshade door flipped open and the $2 billion observatory captured its first images. Scientists saw in stunning detail the debris field of a massive star, Cassiopeia A, which had exploded more than three centuries earlier. 

Astronomers refer to the initial image captured by a telescope as “first light.” But Chandra doesn’t see the universe through visible light, as optical telescopes have done for centuries. Instead, Chandra’s instruments can “see” high-energy electromagnetic radiation known as X-rays. The same type of radiation doctors have used for over a century to peer inside the human body now enables astronomers to probe deep into outer space. 

The cosmos is crackling with energy, filled with objects that produce a vast range of radiation at wavelengths that are either too long or too short to be perceptible to the human eye. Taken together, this electromagnetic spectrum tells the story of the universe. Through radio telescopes, astronomers can study radiation left behind by the Big Bang. Ultraviolet light reveals the presence of very hot stars, while infrared telescopes, such as the James Webb Space Telescope, reveal cool distant objects such as planets. 

“Chandra’s most critical accomplishment has been showing the importance of multi-wavelength astronomy: By combining James Webb Space Telescope data with Chandra data, we are now able to understand our universe in unprecedented ways,” says Ellen Stofan—a former chief scientist at NASA who is now the Under Secretary for Science and Research at the Smithsonian. 

X-rays enable scientists to observe some of the most violent and energetic phenomena in the universe—the astronomical equivalent of tornado chasing. For a quarter of a century, Chandra has made it possible to watch as stars and gas are pulled into black holes, to explore collisions between clusters of galaxies, and to observe the shockwaves produced by exploding stars. 

The Chandra telescope—named after Nobel Prize-winning theoretical physicist Subrahmanyan Chandrasekhar—isn’t the first nor the only orbiting X-ray observatory, but it’s the most powerful. Chandra is equipped with four pairs of mirrors that are the cleanest, most precisely aligned ever built. What’s more, the mirrors were ground and polished to a smoothness of just a few atoms. 

Chandra’s precise mirrors give the observatory an angular resolution of just 0.3 arcsecond. (Angular resolution is the smallest angle at which a telescope can discern that two objects are separate from one another.) This ability makes Chandra 1,000 times more powerful than the first orbiting X-ray telescope, which flew aboard Skylab in the early 1970s. “Chandra is unique, because its angular resolution gives us very sharp images,” says Giuseppina Fabbiano, an astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. 

With great resolution power comes great discoveries. Observations of Cassiopeia A, for example, continued long after Chandra’s first light, making it one of the most intensely studied supernova remnants in astronomy. 

More specifically, Chandra has made it possible for scientists to map the elements that constitute Cassiopeia A and its surroundings. That’s because X-rays are produced by electrons, which travel around the nucleus of an atom. Elements such as oxygen and sulfur each have distinctive X-ray signatures determined by how many electrons each of them possesses. Using this method, researchers discovered prodigious amounts of key cosmic ingredients in the remnants of Cassiopeia A, including one million Earth masses worth of oxygen. Other observations of Cassiopeia A have detected carbon, nitrogen, phosphorus, and hydrogen. These elements, along with oxygen, are the foundation for DNA. Put another way, the elements that make up our bodies were literally made in the stars. 

Beyond Cassiopeia A, the image that is most often associated with Chandra is the Bullet Cluster, which was formed by a high-speed collision of two galactic clusters—making it the most energetic event known in the universe since the Big Bang. 

What makes the Bullet Cluster truly extraordinary is that it provides tangible evidence for the existence of dark matter. Scientists began speculating about dark matter when they realized there isn’t enough visible matter to account for the gravitational force that holds spinning galaxies together. The discovery implied two possibilities: Either our existing laws of gravity needed to be modified or some type of unseen matter accounted for 85 percent of the mass in our universe. 

Dark matter does not reflect, absorb, or emit light, thus posing the riddle: How do you see that which cannot be seen? The answer is that while dark matter can hide, it can’t hide its effects. Dark matter can be traced indirectly by measuring how it warps space through gravitational lensing, when the light from a distant source is distorted and magnified. 

The super-heated gas in the Bullet Cluster emits tremendous X-ray energy, providing Chandra with a superb view of the scene of the galactic collision. Astronomers also gathered optical images of the same cluster from the Hubble Space Telescope and two observatories in Chile: the Very Large Telescope and the Magellan Telescope. Next, the researchers mapped the location of the mass in the cluster by pinpointing where the background light from other galaxies was distorted by the effects of gravitational lensing. 

When astronomers overlaid the images of the super-heated gas with the locations of the gravitational distortions, they saw something very odd. After the galaxies collided, the movement of the hot gas in the Bullet Cluster had been slowed by a drag force, similar to air resistance. But the area of the Bullet Cluster that contained the most mass had moved to a point in space that was ahead of the gas. While the hot gas had been slowed by the drag force, the dark matter had kept moving, oblivious to any external force that wasn’t gravity. Astronomers had been searching for a smoking gun for the existence of dark matter: The Bullet Cluster provided it. 

Black holes, like dark matter, are elusive objects that can be detected only by their effects on the surrounding environment. Their tremendous gravitational force draws in gas, dust, and sometimes even entire stars—creating a disk of matter swirling into the maw of the black hole. Friction between particles in the disk heats them to millions of degrees, producing X-rays that serve as beacons to astronomers. Over the years, Chandra has found hundreds of black holes, including some hidden to other telescopes by walls of dust. 

Black holes get larger as they consume matter, sometimes growing into supermassive black holes that typically dwell at the centers of galaxies. Among supermassive black holes, one type is of special interest to astronomers: active galactic nuclei (AGN). 

AGNs are supermassive black holes that emit extraordinary amounts of high-energy radiation as they consume gas and dust. In fact, they produce so much optical light, they outshine all the stars in their host galaxy combined. AGNs intrigue astronomers because they might be the key to understanding how galaxies form. We still don’t know, for example, how the dust and clouds created in the aftermath of the Big Bang turned into stunning spiral clusters of stars (like our own Milky Way). 

Fabbiano and her research team have spent some 15 years studying data from Chandra to better understand the nature and role of AGNs. She and other astronomers now believe that AGNs do more than greedily consume nearby stars in their host galaxies—rather, galaxies and their AGNs co-evolve through a complex interplay of feeding and feedback. AGNs, for instance, spit out jets of particles moving near the speed of light that heat up the clouds in their host galaxy. Astronomers look at such phenomena and ask questions. Do AGNs enable the creation of stars? Or do they inhibit star formation? Perhaps the answer is both, but at different eras during a galaxy’s evolution. 

“Chandra has changed our understanding of AGNs from focusing all the action on the supermassive black holes” to a view that they are “both important players,” says Fabbiano. “The standard AGN model must now be modified to include the many ways the AGN-galaxy interaction plays out.” 

In sum, Chandra has racked up many accomplishments for a telescope that was initially expected to last only five years. Budget cuts might finally bring the observatory’s mission to an end. Nonetheless, Fabbiano sees this as an opportunity for an upgrade to an even bigger and better X-ray observatory, which was proposed in a 2020 report published by the National Academies of Sciences, Engineering, and Medicine. 

While Chandra’s future is in doubt, its legacy is assured. “When I was in elementary school, I used to check books out of the library that would talk about these theoretical things called black holes that might exist,” says Stofan. “The idea that we went from something being entirely theoretical when I was a child to now, where we’re producing images of them, is mind-blowing. Chandra is similar in that we are now able to see these highly energetic processes, such as the explosion of stars and the collision of galaxies, through an entirely new lens of X-ray vision. Ultimately, Chandra’s discoveries are rewriting textbooks in libraries across the world for other elementary students to pick up and read.” 

Mark Strauss is the managing editor at Air & Space Quarterly.

This article is from the Fall 2024 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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