Deep-sky object

Any astronomical object that is not an individual star

"Deep sky" redirects here. For the racehorse, see Deep Sky (horse). For the 2023 documentary, see Deep Sky. For the band, see Deepsky.

Several nebulae in the constellation Orion commonly called deep-sky objects

A deep-sky object (DSO) is any astronomical object that is not an individual star or Solar System object (such as Sun, Moon, planet, comet, etc.).[1][2] The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies. This distinction is practical and technical, implying a variety of instruments and techniques appropriate to observation, and does not distinguish the nature of the object itself.

## Origins and classification

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Classifying non-stellar astronomical objects began soon after the invention of the telescope.[3] One of the earliest comprehensive lists was Charles Messier's 1774 Messier catalog, which included 103 "nebulae" and other faint fuzzy objects he considered a nuisance since they could be mistaken for comets, the objects he was actually searching for.[3] As telescopes improved these faint nebulae would be broken into more descriptive scientific classifications such as interstellar clouds, star clusters, and galaxies.

"Deep-sky object", as an astronomical classification for these objects, has its origins in the modern field of amateur astronomy. The origin of the term is unknown but it was popularized by Sky & Telescope magazine's "Deep-Sky Wonders" column, which premiered in the magazine's first edition in 1941.[4] Houston's columns, and later book compilations of those columns, helped popularize the term,[5] each month giving the reader a guided tour of a small part of the sky highlighting well-known and lesser-known objects for binoculars and small telescopes.

## Observations and activities

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Map of the constellation Cygnus marking the location of bright and more challenging deep-sky objects

There are many amateur astronomical techniques and activities associated with deep-sky objects. Some of these objects are bright enough to find and see in binoculars and small telescopes. But the faintest objects need the light-gathering power of telescopes with large objectives,[6] and since they are invisible to the naked eye, can be hard to find. This has led to increased popularity of GoTo telescopes that can find DSOs automatically, and large reflecting telescopes, such as Dobsonian style telescopes, with wide fields of view well suited to such observing.[7] Observing faint objects needs dark skies, so these relatively portable types of telescopes also lend themselves to the majority of amateurs who need to travel outside light polluted urban locations.[8] To cut down light pollution and enhance contrast, observers employ nebular filters, which are designed to admit certain wavelengths of light and block others.

There are organized activities associated with DSOs such as the Messier marathon, which occurs at a specific time each year and involves observers trying to spot all 110 Messier objects in one night. Since the Messier catalog objects were discovered with relatively small 18th-century telescopes, it is a popular list with observers, being well within the grasp of most modern amateur telescopes. The Herschel 400 Catalogue is also a popular list with observers and is considered more challenging; it was designed for larger telescopes and experienced amateur astronomers.[9]

## List of deep-sky object types

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There are many astronomical object types that come under the description of deep-sky objects. Since the definition is objects that are not Solar System objects or individual stars, examples include:[10]

* Black holes[11] and active galactic nuclei[12]
* Nebulae
* Bright nebulae
* Emission nebulae
* Reflection nebulae
* H II regions
* Diffuse ionized gas
* Planetary nebulae
* Supernova remnants
* Dark nebulae[13]
* Galaxies[14]
* Star clusters
* Open clusters
* Globular clusters[15]

## See also

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* Amateur telescope making
* Celestial cartography
* Shallow sky
* Star catalogue
* Star hopping

## References

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1. ^ Fred Schaaf (1998). 40 Nights to Knowing the Sky: A Night-by-Night Sky-Watching Primer. Henry Holt and Company. p. 113\. ISBN 978-0-8050-4668-7.
2. ^ Ian Ridpath (2001). The Illustrated Encyclopedia of the Universe. Watson-Guptill Publications. p. 273\. ISBN 978-0-8230-2512-1.
3. ^ a b Edward W. Kolb; Rocky Kolb (1999). Blind Watchers of the Sky: The People and Ideas that Shaped Our View of the Universe. Oxford University Press. p. 174\. ISBN 978-0-19-286203-7.
4. ^ Charles Anthony Federer (1942). Sky and Telescope. Sky Publishing Corporation.
5. ^ Fred Schaaf (1998). 40 Nights to Knowing the Sky: A Night-by-Night Sky-Watching Primer. Henry Holt and Company. p. 13\. ISBN 978-0-8050-4668-7.
6. ^ Grant Privett; Paul Parsons (2001). The Deep-Sky Observer's Year: A Guide to Observing Deep-Sky Objects Throughout the Year. Springer Science & Business Media. p. 11\. ISBN 978-1-85233-273-0.
7. ^ Rajiv Gupta (September 2004). Observer's Handbook 2005. Royal Astronomical Soc of Canada. p. 73\. ISBN 978-0-9689141-8-2.
8. ^ The Observer's Handbook, Royal Astronomical Society of Canada, p. 63
9. ^ Cavin, Jerry D. (2011). The Amateur Astronomer's Guide to the Deep-Sky Catalogs. Springer New York. p. 149\. ISBN 9781461406563.
10. ^ W.H. Finlay (2003). Concise Catalog of Deep-sky Objects: Astrophysical Information for 500 Galaxies, Clusters and Nebulae. Springer Science & Business Media. p. 10\. ISBN 978-1-85233-691-2.
11. ^ Black hole at the Encyclopædia Britannica
12. ^ Active glactic nucleus at the Encyclopædia Britannica
13. ^ Nebula at the Encyclopædia Britannica
14. ^ Galaxy at the Encyclopædia Britannica
15. ^ Star cluster at the Encyclopædia Britannica

* Neil Bone, Wil Tirion, Deep Sky Observer's Guide. Firefly Books, 2005. ISBN 1-55407-024-4.
* Jess K. Gilmour, The practical astronomer's deep-sky companion. Springer, 2003. ISBN 1-85233-474-6.
* Jack Newton, Philip Teece. The Guide to Amateur Astronomy. Cambridge University Press, 1995. ISBN 0-521-44492-6.
* W. H. Finlay, Concise Catalog of Deep-sky Objects: Astrophysical Information for 500 Galaxies, Clusters and Nebulae. London: Springer, 2003. ISBN 1-85233-691-9. Includes the Messier objects, Herschel 400 & more.
* Roger Nelson Clark, Visual astronomy of the deep sky. CUP Archive, 1990. ISBN 0-521-36155-9.

## Further reading

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* Burnham's Celestial Handbook by Robert Burnham Jr. (Volume One, Volume Two, Volume Three at Google Books)
* Deep Sky Observer's Guide by Neil Bone, Wil Tirion. Firefly Books, 2005. ISBN 1-55407-024-4.
* The practical astronomer's deep-sky companion by Jess K. Gilmour. Springer, 2003. ISBN 1-85233-474-6.
* Concise Catalog of Deep-sky Objects: Astrophysical Information for 500 Galaxies, Clusters and Nebulae by W. H. Finlay. London: Springer, 2003. ISBN 1-85233-691-9. Includes the Messier objects, Herschel 400 & more
* Visual astronomy of the deep sky by Roger Nelson Clark. CUP Archive, 1990. ISBN 0-521-36155-9.

## External links

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* Deep Sky Observers Companion: Online deep sky object description database and observing planner
* clell.de, Deep Sky collections and catalogs similar to Messier's Archived 2021-05-09 at the Wayback Machine
* Sketch gallery of deep sky objects by Wes Stone

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* Astronomy
* Stars
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