LEDAS 
Leicester Database and Archive Service
 
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Quasar field

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.

 
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