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# Long description

The two panels of this image show black-and-white photographs of small, typical regions of the night sky, taken through a telescope and shown in negative colours. Both images show numerous stars. In the left-hand image, small circles have been drawn around two stars. In the right-hand image, a larger circle has been drawn around a prominent galaxy. The number of photons falling within these circles can be counted to give the flux of light from each star or galaxy, as described in the main text.