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This image shows an example "deep sky" observation with ROSAT revealing the
faintest objects ever detected by an X-ray satellite. The region shown, only
3/4 of a degree across, contains at least 50 faint X-ray objects, most of which
are very distant quasi-stellar objects (QSOs or quasars) with enormous
intrinsic brightness in both the visible band and in the X-ray band. The
optical and X-ray emission from quasars is believed to be powered by matter
falling into a black hole at the centre of the host galaxy. Because quasars are
the most numerous emitting objects visible at ROSAT's sensitivity,
ROSAT observations are one of the best ways of finding quasars, and ROSAT
studies are being used to find out how quasars evolve and to probe the
large-scale structure of the universe.