Deep-sky object
Also called: DSO, deep sky.
Anything in the night sky beyond the Solar System that isn't a single star: star clusters, nebulae and galaxies.
Deep-sky objects are the faint, fuzzy treasures of the sky: open and globular clusters, nebulae and galaxies. Most are far too faint to see without optical help, and many look like soft grey glows rather than the colourful pictures in books.
Observers usually count double stars among them too, and catalogs such as the messier catalog and the NGC are made almost entirely of them.
At the eyepiece
Deep-sky objects reward a dark sky, a dark-adapted eye and patience more than magnification: see dark adaptation and averted vision.
In AstroPilot
Explore lists every deep-sky object in AstroPilot's catalog, ranked for your sky, your instrument and tonight's Moon.