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extended source

  1. A source whose angular size exceeds the resolution of the instrument being used to observe it. Extended sources, such as many galaxies and nebulae, can be resolved by telescopes, whereas most stars cannot and are therefore classes as point sources.

  2. In radio astronomy, formerly a source whose angular extent could be measured, as distinguished from a point source. Now, one which has a large angular extent and is strongest at longer wavelengths, as distinguished from a compact source.

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