This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2. The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System.
Image Credit: ESO/Digitized Sky Survey 2
Explanation from: http://www.eso.org/public/images/eso1241e/