X-rays are a form of high-energy, electromagnetic radiation released by extremely hot objects. In the early 20th century, astronomers realized that space should be flooded with X-rays from the sun. Moreover, the sun’s X-ray spectrum would reveal a lot about the processes at work within the star. However, X-ray astronomy was not possible until the advent of rockets and satellites. Despite their energy, X-rays are easily absorbed, which is why they are so good at imaging the body. Water vapor in Earth’s atmosphere effectively blocks X-rays from reaching the surface—a good thing for life,
because high-energy X-rays can cause damage and mutations when they impact on soft, living cells.

The first glimpse of the sun’s X-rays came in the late 1940s, during a US Naval Research Laboratory (NRL) program to study Earth’s upper atmosphere. A team led by US rocket scientist Herbert Friedman fired German V-2 rockets into space equipped with X-ray detectors—essentially modified Geiger counters. These experiments provided the first incontrovertible evidence of X-rays from the sun. By 1960, researchers were using Aerobee sounding rockets to detect X-rays, and the first X-ray photos of the sun were taken from an Aerobee Hi. Two years later, the first cosmic X-ray source was detected.

“Nothing is going to happen unless you work with your life’s blood.” Riccardo Giacconi

Extrasolar X-rays

Riccardo Giacconi, an Italian astrophysicist then working for American Science and Engineering (AS&E), had successfully petitioned NASA to fund his team’s X-ray experiment. The team’s first rocket misfired in 1960, but by 1961 it had a new, improved experiment ready for launch. This instrument was one hundred times more sensitive than any flown to date. Using a large field of view, the team hoped to observe other X-ray sources in the sky. Success followed a year later: the rocket aimed its camera first at the moon and then away from it. What the camera saw came as a complete surprise to the team. The instrument detected the X-ray “background”—a diffuse signal coming from all directions—and a strong peak of radiation in the direction of the galactic center.

Stars like the sun emit about a million times more photons at visible light frequencies than they do as X-rays. The source of the X-ray signals, by contrast, radiated a thousand times more X-rays than light. Although a small, barely visible point in the sky, the source was pumping out one thousand times more X-rays than the sun. Furthermore, certain physical processes were taking place within the source and these had never been seen in the laboratory. After weeks of analysis, the team concluded that this must be a new class of stellar object.

Search for the source

There was no candidate in the solar system to account for the intense radiation. The most likely source was named Scorpius X-1 (Sco X-1 for short) after the constellation within which it was located. Herb Friedman at the NRL confirmed the result using a detector with a larger area and better resolution than the AS&E instrument. Sco X-1 is now known to be a double star system and is the brightest, most persistent X-ray source in the skies.

Further launches revealed a sky dotted with X-ray sources, both galactic and extra-galactic. In a short space of time, the team had detected a disparate set of celestial oddities emitting X-rays. These included supernova remnants, binary stars, and black holes. Today, more than 100,000 X-ray sources are known.

The Chandra X-ray Observatory was launched by NASA in 1999. It was initially planned to operate for five years, but was still in use in 2016.

Toward Chandra

By the mid-1960s, instruments were becoming ever more sensitive. Detectors were able to record X-rays one thousand times weaker than Sco X-1 just five years after Giacconi’s discovery. Initially proposed by Giacconi in 1963, Uhuru, the first satellite dedicated solely to X-ray astronomy, was launched in 1970. It spent three years mapping X-rays. This all-sky survey located 300 sources, including a bizarre object in the center of the Andromeda galaxy, and it earmarked Cyg X-1 as a potential black hole. Uhuru also found that the gaps in galaxy clusters are strong sources of X-rays. These apparently empty regions are in fact filled by a low-density gas at millions of degrees Kelvin. Although thinly spread, this “intercluster medium” contains more mass than that of all of the cluster’s galaxies combined.

In 1977, NASA launched its High Energy Astronomy Observatory (HEAO) program. HEAO-2, renamed the Einstein Observatory, was equipped with highly sensitive detectors and revolutionized X-ray astronomy. With its fused quartz mirrors, the telescope was a million times more sensitive than that of Giacconi’s 1961 discovery rocket. Einstein observed X-rays emanating from stars and galaxies, and even from planetary aurorae on Jupiter.

Eager to probe the X-ray background further, Giacconi once again proposed an advanced telescope. In 1999, this became the Chandra X-Ray Observatory, the third of the orbiting Great Observatories. Chandra is the most powerful X-ray telescope ever built, tens of billions of times more sensitive than the early detectors. Its phenomenal performance outstripped all expectations and its mission lifetime was tripled from five to 15 years. As of 2016, however, its mission is ongoing. Chandra’s outstanding technical firsts include detecting sound waves coming from a supermassive black hole. The X-ray data, when combined with optical observations from the Hubble Space Telescope and infrared data from the Spitzer Space Telescope, have provided stunning images of the cosmos.

Active regions of the sun are revealed by combining observations from many telescopes. Highenergy X-rays are shown in blue; low-energy X-rays green.

Realm of the X-rays

X-ray astronomy observes the highest-energy objects in space: colliding galaxies, black holes, neutron stars, and supernovae. The energy source behind this activity is gravity. As matter falls toward a massive concentration of material, particles collide and accumulate. They give up their energy by emitting photons, which at these speeds have X-ray wavelengths (0.01–10 nanometers, or billionths of a meter)—equivalent to temperatures of tens of million of degrees. The same mechanism, is at work in a wide range of dramatic phenomena: active stars more massive than the sun, for example, produce strong solar winds and significant amounts of X-rays. “X-ray binary star” systems, in which mass transfers from one star to its partner, also produce intense radiation.

“The universe is popping all over the place.” Riccardo Giacconi

Seeing black holes

When stars explode at the end of their lives, the blast waves from the supernova compress the interstellar medium, causing the gas to release Xrays. Left within what remains of the supernova, the massive star continues life as a neutron star or a black hole. Turbulence generated by material being torn apart as it is sucked into a black hole will also produce X-rays. The radiation being pumped out causes the outer layers of the supernova remnant to fluoresce in a range of colors.

Certain galaxies have centers that outshine all the billions of stars in the galaxy itself, with emissions that are bright at all wavelengths. The center of such an “active galactic nucleus” is assumed to contain a supermassive black hole. Material falling toward the centers of galaxy clusters—the largest structures in the universe—also shines in X-rays, and is not visible in other light frequencies. Chandra has now taken two “deep field” images of the Xray background—23- and 11-day exposures of the northern and southern hemispheres of the sky. X-ray instruments of the future may help scientists see how black holes are distributed.

Observations in the X-ray spectrum reveal hidden structures. The larger blobs in this patch of sky from an ESA X-ray survey are galaxy clusters; smaller dots are black holes.


Born in Genoa, Italy, in 1931, Riccardo Giacconi lived in Milan with his mother, a mathematics and physics high school teacher. She instilled a love of geometry in the young Riccardo. Giacconi’s first degree was from the University of Milan. With a Fulbright Scholarship, he moved to Indiana University in the US, and then to Princeton, to study astrophysics.

In 1959, Giacconi joined American Science and Engineering, a small firm in Cambridge, Massachusetts. AS&E built rocket-borne monitoring equipment for measuring electrons and artificial gamma-ray bursts from nuclear weapons. Giacconi was tasked with developing instruments for X-ray astronomy. He was at the heart of most of the breakthroughs in X-ray astronomy, and in 2002, he was awarded a share of the Nobel Prize in Physics for his contributions to astrophysics. In 2016, he was still working in his mid-80s, as principal investigator for the Chandra Deep Field-South project.

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