An emission nebula.
Source: http://www.spacetelescope.org/images/html/heic0619a.html (TIFF image 36 MB). http://en.wiktionary.org/wiki/File:EmissionNebula_NGC6357.jpg
Author: NASA, ESA and Jesœs Ma­z Apellÿniz (Instituto de astrof­sica de Andaluc­a, Spain). Acknowledgement: