How Things Work: Chandra X-Ray
The Chandra X-Ray Telescope, explained.
- By Damond Benningfield
- Air & Space magazine, January 2008
(Page 2 of 2)
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.