Deep sky

Deep sky or Deep Sky Object (DSO, for short), which differs from deep space, is a term used by amateur astronomers to describe mostly faint astronomical objects outside the solar system, such as star clusters, nebulae or galaxies. These objects are hundreds to billions of light-years distant from Earth.

Nearly all clusters and nebulae are contained within galaxies, and there are a number of galaxies visible to the naked eye. They are, in order of closeness, the Milky Way, the Large Magellanic Cloud (approximately 160,000 light-years away), the Small Magellanic Cloud (about 200,000 light-years away), and the Andromeda Galaxy, (about 2.5 million light-years away).

Types of deep sky objects:

*Star clusters
**Open clusters
**Globular clusters
*Nebulae
**Bright nebulae
***Emission nebulae
***Reflection nebulae
**Dark nebulae
**Planetary nebulae
*Galaxies
*Quasars

These are classified by the Messier catalogue of 110 objects and the much more comprehensive New General Catalogue which contains nearly 8,000 objects. Many sets of these and other objects from more specialised catalogues such as the Uppsala General Catalogue are used by amateurs as a test of their observing skills and their equipment. The Messier marathons occur at a specific time each year and involve observers trying to spot all 110 objects in one night. A much more demanding test known as Herschell's 400 is designed to tax larger telescopes.

ee also

* [http://www.deepskyobserving.com/How-to-observe-deep-sky-objects.htm How to observe deep sky objects]
* [http://www.space-and-telescope.com/MessierObjects.aspx Clickable table of Messier objects]