How Things Work: Chandra X-Ray
The Chandra X-Ray Telescope, explained.
For more than eight hours last fall, the Chandra X-Ray Observatory stared at a nondescript galaxy 240 million light-years away. In that time, one of the detectors intercepted exactly four X-ray photons. It sounds like a meager harvest, but those four packets of energy helped astronomers realize that the galaxy contained a type of exploding star that had never been observed before.
Chandra, which is named for Nobel Prize-winning astrophysicist Subrahmanyan Chandrasekhar, who first calculated the ultimate fate of stars like our sun, is the largest and most sensitive X-ray telescope ever built. The spacecraft can produce full-color images of X-ray-emitting objects while measuring the intensity at each X-ray wavelength.
Stars, galaxies, and other astronomical objects all produce light, with a mix of wavelengths that depends on the object’s composition and temperature. Cool interstellar gas clouds, for example, emit primarily longer, infrared wavelengths. Medium-hot stars like our sun peak at visible wavelengths, while the hottest stars shine brightest in the ultraviolet. X-rays come from the hottest objects of all, such as clouds of gas between galaxies or the bands of gas spiraling into black holes.
Earth’s atmosphere absorbs X-rays, so X-ray astronomers must place their telescopes in space. The Chandra telescope was launched by the space shuttle Columbia in 1999 and is today operated by the Chandra X-Ray Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Chandra completes one orbit around Earth roughly every two and a half days. Its highly elliptical path takes it up to 83,000 miles away. Most of the time, this path keeps Chandra clear of the Van Allen belts, rings of radioactive particles encircling Earth, so the telescope has to shelter its instruments from the radiation for only a small portion of each orbit.
An optical telescope uses a large, curved-glass primary mirror to gather light, but X-rays would penetrate such a mirror’s reflective coating; an X-ray telescope’s mirrors must be facing almost perpendicular to the path of incoming light so that the photons graze the surface like stones skipping across a pond.
Chandra has four pairs of mirrors. X-rays hit the top mirrors in each pair, then skip down to the secondary mirrors. “You need two bounces to have X-rays come to a focus,” says Martin Weisskopf, Chandra project scientist at NASA’s Marshall Space Flight Center in Huntsville, Alabama, which manages the program. Each mirror is most efficient at reflecting a particular range of X-ray wavelengths.
After bouncing off the mirrors, the X-rays travel down a 26-foot tube toward the telescope’s scientific instruments, located at the other end.
Devices called gratings can be moved into the light path between the mirrors and the instruments. The gratings contain thousands of narrow openings that segregate the X-rays by wavelength. The intensity of radiation at each wavelength reveals the abundance of different elements, along with the object’s density, temperature, and motion toward or away from the telescope.
Beyond the gratings are the scientific instruments. The primary one, called the ACIS, for Advanced CCD Imaging Spectrometer, uses a charge-coupled device detector, similar to those found in digital cameras, to record the position of each X-ray that strikes it, along with the X-ray’s energy level. In many cases, this information can be used to determine which chemical elements are present.
Most targets for Chandra are selected months in advance. But some time is reserved to study targets that appear suddenly, like the exploding stars known as supernovae. Such was the case with Supernova SN2006gy, which was discovered September 18, 2006, by an automated search program at the University of Texas’ McDonald Observatory.
As astronomers began studying the star, they realized that it was an oddball. Compared to other supernovae, it took longer to reach peak brightness, it faded more slowly, and at maximum, it was several times more powerful.
Supernovae fall into two broad categories. One type is the destruction of a star at least 8 to 10 times as massive as the sun. Its core collapses to form a neutron star or black hole and its outer layers fall in, then explode. The other type is the complete destruction of the dead core of a star, known as a white dwarf. If the white dwarf steals enough gas from the surface of a nearby companion star, a nuclear explosion can occur, blasting the white dwarf to smithereens. Supernova SN2006gy seemed to fit in the latter category—until Chandra took a look at it.
A team led by David Pooley of the University of California at Berkeley used the telescope to peer into the star’s galaxy 56 days after SN2006gy’s discovery. The four X-ray photons it counted were “a clear, no-question-about-it detection,” says Weisskopf. “Depending on the assumptions you make about the nature of the object that exploded and its history, you expect to see different amounts of X-ray emission. With the white-dwarf theory, we should have seen not four photons but 40,000.” (Four photons weren’t enough to enable the scientists to determine which elements were generating the radiation.)
To explain the blast, University of Texas astronomer J. Craig Wheeler resurrected a model from the 1960s that says the original star must have been at least 100 times as massive as the sun. The core of such a star is so dense and hot that some of its energy is converted to matter—pairs of electrons and their anti-matter equivalents, positrons. With less radiation pushing outward, the star’s oxygen core began to collapse, triggering a thermonuclear explosion that ripped the star to bits.
Astronomers are still studying SN2006gy to confirm the mechanism. When they do, they will have to credit Chandra and its four little X-ray particles.