ROSAT/Galactic Center
Credit: The ROSAT Mission/Max-Planck-Institut für extraterrestrische Physik

Diagnosing the Heart of the Galaxy

"Tomography" is a technique in which a 3-dimensional picture is built up slice-by-slice from 2-dimensional images. In medicine, positron emission tomography (PET) or magnetic resonance imaging (MRI) is used to generate 3-d "X-rays" of the human body to provide vital information about many forms of pathology. Tomography is also used by X-ray astronomers to generate detailed images of matter in the universe. A tomographic image of the center of the Milky Way Galaxy, obtained by the ROSAT X-ray observatory, is shown above. The images represent X-rays of low (0.1-0.5 keV, upper left) to high energies (1.5-2.5 keV, lower right). X-rays are absorbed by gas and dust lying between us and the X-ray source; because high energy X-rays have greater "penetrating power" than low energy X-rays, high energy X-rays can pass through more of the interstellar medium than low energy X-rays. Since the energy of the X-ray emission is a measure of the amount of interstellar absorption, objects that emit only high energy X-ray emission are typically more highly absorbed than objects which emit low-energy X-rays. Thus, by measuring the x-ray energy being emitted, astronomers can develop a picture of the mass distribution between us and the X-ray emitting source.

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Page Author: Dr. Michael F. Corcoran
Last modified April 17, 2001