To assign a magnitude to astronomical objects that are not point sources, such as galaxies and nebulas, astronomers treat them as if all the light from them to the telescope came from a point. So the Orion nebula, whose integrated magnitude is 6, doesn't look as bright as a magnitude 6 star.
If light from all the stars (excluding the sun) were combined in a single star, it would have an apparent magnitude of -6.7, much less than the full moon.
The range of brightness of everything seen in the sky before the 1990's can be summed up in two gigantic leaps of apparent magnitude: the sun is about 25 magnitudes (1010) brighter than the brightest star, Sirius, and Sirius is 25 magnitudes brighter than the faintest star that can be photographed by the 200-inch telescope on Mt. Palomar.
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Last revised: 8 March 2008.