Editing Nebula

{{short description|Body of interstellar clouds}}
{{other uses}} 
[[File:Trifid Nebula by Deddy Dayag.jpg|thumb|right|upright=1.5|True color image of the [[Trifid Nebula]], showing complex gas and plasma structure]]

A '''nebula''' ({{langnf|la||cloud, fog}};<ref>{{OEtymD|nebula}}</ref> {{plural abbr|'''nebulae'''}} or '''nebulas''')<ref>''American Heritage Dictionary of the English Language, Fifth Edition''. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula</ref><ref>''Collins English Dictionary – Complete and Unabridged, 12th Edition 2014''. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula</ref><ref>''Random House Kernerman Webster's College Dictionary''. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula</ref><ref>''The American Heritage Dictionary of Student Science, Second Edition''. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula</ref> is a distinct luminescent part of [[interstellar medium]], which can consist of ionized, neutral, or molecular [[hydrogen]] and also [[cosmic dust]]. Nebulae are often star-forming regions, such as in the [[Pillars of Creation]] in the [[Eagle Nebula]]. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form [[star]]s. The remaining material is then thought to form [[planet]]s and other [[planetary system]] objects. 

Most nebulae are of vast size; some are hundreds of [[light-year]]s in diameter. A nebula that is visible to the human eye from [[Earth]] would appear larger, but no brighter, from close by.<ref>{{cite news|url=http://www.universetoday.com/99989/in-reality-nebulae-offer-no-place-for-spaceships-to-hide/|title=In Reality, Nebulae Offer No Place for Spaceships to Hide|author=Howell, Elizabeth |date=2013-02-22|work=Universe Today}}</ref> The [[Orion Nebula]], the brightest nebula in the sky and occupying an area twice the angular diameter of the full [[Moon]], can be viewed with the naked eye but was missed by early astronomers.<ref>{{cite book|url=https://books.google.com/books?id=gCI9AAAAIAAJ&pg=PA98|title=Visual astronomy of the deep sky|author=Clark, Roger N. |year=1990|page=98|publisher=Cambridge University Press|isbn=9780521361552}}</ref> Although denser than the space surrounding them, most nebulae are far less dense than any [[vacuum]] created on Earth (10{{sup|5}} to 10{{sup|7}} molecules per cubic centimeter) – a nebular cloud the size of the [[Earth]] would have a total mass of only a few [[kilogram]]s. Earth's air has a density of approximately 10{{sup|19}} molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 10{{sup|4}} molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters. Some nebulae are variably illuminated by [[T Tauri star|T Tauri]] variable stars.

Originally, the term "nebula" was used to describe any diffused [[astronomical object]], including [[galaxy|galaxies]] beyond the [[Milky Way]]. The [[Andromeda Galaxy]], for instance, was once referred to as the ''Andromeda Nebula'' (and [[spiral galaxies]] in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by [[Vesto Slipher]], [[Edwin Hubble]], and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight. He also helped categorize nebulae based on the type of light spectra they produced.<ref>{{Cite web |date=March 19, 2020 |title=What is a nebula? |url=https://spacecenter.org/what-is-a-nebula/ |access-date=June 27, 2021 |website=Space Center Houston}}</ref>

==Observational history==

[[File:Hs-2009-25-e-full.jpg|thumb|right|Portion of the [[Carina Nebula]]]]

Around 150 AD, [[Ptolemy]] recorded, in books VII–VIII of his ''[[Almagest]]'', five stars that appeared nebulous. He also noted a region of nebulosity between the [[constellation]]s [[Ursa Major]] and [[Leo (constellation)|Leo]] that was not associated with any [[star]].<ref>{{citation
  | first=P. | last=Kunitzsch | date=1987
  | title=A Medieval Reference to the Andromeda Nebula
  | journal=[[ESO Messenger]]
  | volume=49 | pages=42–43
  | url=http://www.eso.org/sci/publications/messenger/archive/no.49-sep87/messenger-no49-42-43.pdf
  | bibcode=1987Msngr..49...42K | access-date=2009-10-31 }}</ref> The first true nebula, as distinct from a [[star cluster]], was mentioned by the [[Astronomy in the medieval Islamic world|Muslim Persian astronomer]] [[Abd al-Rahman al-Sufi]] in his ''[[Book of Fixed Stars]]'' (964).<ref name=Jones>{{Cite book|title=Messier's nebulae and star clusters|author=Jones, Kenneth Glyn |publisher=Cambridge University Press|date=1991|isbn=0-521-37079-5|page=1}}</ref> He noted "a little cloud" where the [[Andromeda Galaxy]] is located.<ref name="rasqj25">{{cite journal
  | last=Harrison | first=T. G. |date=March 1984
  | title=The Orion Nebula&nbsp;– where in History is it
  | journal=[[Quarterly Journal of the Royal Astronomical Society]]
  | volume=25 | issue=1 | pages=70–73
  | bibcode=1984QJRAS..25...65H }}</ref> He also cataloged the [[Omicron Velorum Cluster|Omicron Velorum]] star cluster as a "nebulous star" and other nebulous objects, such as [[Brocchi's Cluster]].<ref name=Jones/> The [[supernovas]] that created the [[Crab Nebula]], [[SN 1054]], was observed by Arabic and [[Chinese astronomy|Chinese astronomers]] in 1054.<ref name="Lundmark">{{cite journal | last1 = Lundmark | first1 = K | year = 1921 | title = Suspected New Stars Recorded in the Old Chronicles and Among Recent Meridian Observations | journal = Publications of the Astronomical Society of the Pacific | volume = 33 | issue = 195 | page = 225 | bibcode = 1921PASP...33..225L | doi = 10.1086/123101 | doi-access = free }}</ref><ref name="Mayall">{{cite journal | last1 = Mayall | first1 = N.U. | year = 1939 | title = The Crab Nebula, a Probable Supernova | url = http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1939ASPL....3..145M&link_type=ARTICLE&db_key=AST| journal = Astronomical Society of the Pacific Leaflets | volume = 3 | issue = 119 | page = 145 | bibcode = 1939ASPL....3..145M }}</ref>

In 1610, [[Nicolas-Claude Fabri de Peiresc]] discovered the [[Orion Nebula]] using a telescope. This nebula was also observed by [[Johann Baptist Cysat]] in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by [[Christiaan Huygens]], who also believed he was the first person to discover this nebulosity.<ref name="rasqj25" />

In 1715, [[Edmond Halley]] published a list of six nebulae.<ref> {{cite journal
  | last=Halley | first=E. | date=1714–1716
  | volume=XXXIX | journal= [[Philosophical Transactions]]
  | pages=390–92 | title=An account of several nebulae or lucid spots like clouds, lately discovered among the fixed stars by help of the telescope}} </ref> This number steadily increased during the century, with [[Jean-Philippe de Cheseaux]] compiling a list of 20 (including eight not previously known) in 1746. From 1751 to 1753, [[Nicolas-Louis de Lacaille]] cataloged 42 nebulae from the [[Cape of Good Hope]], most of which were previously unknown. [[Charles Messier]] then compiled a catalog of 103 "nebulae" (now called [[Messier object]]s, which included what are now known to be galaxies) by 1781; his interest was detecting [[comet]]s, and these were objects that might be mistaken for them.<ref name="hoskin2005">{{cite journal| last=Hoskin | first=Michael | date=2005| title=Unfinished Business: William Herschel's Sweeps for Nebulae | journal=[[British Journal for the History of Science]] | volume=43 | issue=3 | pages=305–320 | bibcode=2005HisSc..43..305H | doi=10.1177/007327530504300303 | s2cid=161558679 }}</ref>

The number of nebulae was then greatly increased by the efforts of [[William Herschel]] and his sister, [[Caroline Herschel]]. Their ''Catalogue of One Thousand New Nebulae and Clusters of Stars''<ref>{{cite book|title=Philosophical Transactions|url=https://archive.org/details/philosophicaltr04unkngoog|year=1786|publisher=T.N.|page=[https://archive.org/details/philosophicaltr04unkngoog/page/n479 457]}}</ref> was published in 1786. A second catalog of a thousand was published in 1789, and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity rather than a more distant cluster.<ref name="hoskin2005" />

Beginning in 1864, [[William Huggins]] examined the spectra of about 70 nebulae. He found that roughly a third of them had the [[emission spectrum]] of a [[gas]]. The rest showed a continuous spectrum and were thus thought to consist of a mass of stars.<ref>{{cite book
  | author=Watts, William Marshall
  | author2=Huggins, Sir William
  | author3=Lady Huggins | title=An introduction to the study of spectrum analysis
  | publisher=Longmans, Green, and Co.
  | date=1904
  | pages=[https://archive.org/details/anintroductiont00hugggoog/page/n106 84]–85 | access-date=2009-10-31
  | url=https://archive.org/details/anintroductiont00hugggoog
}}</ref><ref name="struve37">{{cite journal
  | last=Struve | first=Otto | date=1937
  | title=Recent Progress in the Study of Reflection Nebulae | journal=Popular Astronomy | volume=45
  | pages=9–22 | bibcode=1937PA.....45....9S }}</ref> A third category was added in 1912 when [[Vesto Slipher]] showed that the spectrum of the nebula that surrounded the star [[Merope (star)|Merope]] matched the spectra of the [[Pleiades]] [[open cluster]]. Thus, the nebula radiates by reflected star light.<ref>{{cite journal
  | last=Slipher | first=V. M. | date=1912
  | title=On the spectrum of the nebula in the Pleiades
  | journal=Lowell Observatory Bulletin | volume=1
  | pages=26–27 | bibcode=1912LowOB...2...26S }}</ref>

In 1923, following the [[Great Debate (astronomy)|Great Debate]], it became clear that many "nebulae" were in fact galaxies far from the [[Milky Way]].

Slipher and [[Edwin Hubble]] continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light.<ref name="struve37" /> In 1922, Hubble announced that nearly all nebulae are associated with stars and that their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter (including all [[O-type main sequence star]]s), while nebulae with continuous spectra appear with cooler stars.<ref>{{cite journal
  | last=Hubble | first=E. P. |date=December 1922
  | title=The source of luminosity in galactic nebulae.
  | journal=Astrophysical Journal | volume=56
  | pages=400–438 
  | doi=10.1086/142713 | bibcode=1922ApJ....56..400H
| doi-access=free}}</ref> Both Hubble and [[Henry Norris Russell]] concluded that the nebulae surrounding the hotter stars are transformed in some manner.<ref name="struve37" />

==Formation==
[[File:Nursery of New Stars - GPN-2000-000972.jpg|thumb|[[NGC 604]], a nebula in the [[Triangulum Galaxy]]]]
There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the [[interstellar medium]] while others are produced by stars. Examples of the former case are [[giant molecular clouds]], the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its [[stellar evolution]].

[[Star-forming region]]s are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their [[ultraviolet radiation]] [[ion]]izes the surrounding gas, making it visible at optical [[wavelength]]s. The region of ionized hydrogen surrounding the massive stars is known as an [[H II region]] while the shells of neutral hydrogen surrounding the H II region are known as [[photodissociation region]]. Examples of star-forming regions are the [[Orion Nebula]], the [[Rosette Nebula]] and the [[Omega Nebula]]. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years. 

Other nebulae form as the result of [[supernova]] explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces.  One of the best examples of this is the [[Crab Nebula]], in [[Taurus (constellation)|Taurus]]. The supernova event was recorded in the year 1054 and is labeled [[SN 1054]]. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a [[neutron star]].

Still other nebulae form as [[planetary nebulae]]. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into [[red giant]]s and slowly lose their outer layers during pulsations in their atmospheres.  When a star has lost enough material, its temperature increases and the [[ultraviolet radiation]] it emits can [[ion]]ize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a [[white dwarf]].
{{clear}}

==Types==

<gallery mode="packed" class="center">
File:Hubble Sees a Stellar "Sneezing Fit" (11467249715).jpg|[[Herbig–Haro object|Herbig–Haro]] [[HH 161]] and [[HH 164]].<ref>{{cite news|title=A stellar sneezing fit|url=http://www.spacetelescope.org/images/potw1350a/|access-date=16 December 2013|newspaper=ESA/Hubble Picture of the Week}}</ref>
File:Omega Nebula.jpg|The [[Omega Nebula]], an example of an [[emission nebula]]
File:Horsehead-Hubble.jpg|The [[Horsehead Nebula]], an example of a [[dark nebula]].
File:NGC6543.jpg|The [[Cat's Eye Nebula]], an example of a [[planetary nebula]].
File:PIA04533.jpg|The [[Red Rectangle Nebula]], an example of a [[protoplanetary nebula]].
File:SNR 0509.jpg|The delicate shell of [[SNR 0509-67.5|SNR B0509-67.5]]
File:Tycho xrayonly.jpg|[[SN 1572|Tycho Supernova remnant in X-ray light]]
File:Southern Ring Nebula by Webb Telescope (2022).jpg|[[NGC 3132|Southern Ring Nebula]], Planetary Nebula
File:Webb captures detailed beauty of Ring Nebula (NIRCam and MIRI images) (weic2320a).jpg|[[Ring Nebula]] in the northern constellation of [[Lyra]]
</gallery>

===Classical types===
Objects named nebulae belong to four major groups.  Before their nature was understood, [[galaxy|galaxies]] ("spiral nebulae") and [[star cluster]]s too distant to be resolved as stars were also classified as nebulae, but no longer are.
* [[H II region]]s, large diffuse nebulae containing ionized hydrogen
* [[Planetary nebula]]e
* [[Supernova remnant]]s (e.g., Crab Nebula)
* [[Dark nebula]]e
Not all cloud-like structures are nebulae; [[Herbig–Haro object]]s are an example.

===Flux Nebulae===
{{Main|Integrated Flux Nebula}}
{{excerpt|Integrated Flux Nebula}}

===Diffuse nebulae===

[[Image: Carina Nebula by ESO.jpg|thumb|The Carina Nebula is an example of a diffuse nebula]]

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.<ref name="Messier">{{cite web|url=http://messier.seds.org/diffuse.html|publisher=SEDS|title=The Messier Catalog: Diffuse Nebulae|access-date=2007-06-12|archive-url=https://web.archive.org/web/19961225125109/http://messier.seds.org/diffuse.html|archive-date=1996-12-25}}</ref> Diffuse nebulae can be divided into [[emission nebula]]e, [[reflection nebula]]e and [[dark nebula]]e.

Visible light nebulae may be divided into emission nebulae, which emit [[spectral line]] radiation from excited or [[ion]]ized gas (mostly ionized [[hydrogen]]);<ref name="shu1982">{{cite book
  | author = F. H. Shu
  | title = The Physical Universe
  | publisher = University Science Books
  | date = 1982
  | location = Mill Valley, California
  | isbn = 0-935702-05-9
  | url-access = registration
  | url = https://archive.org/details/physicaluniverse00shuf
  }}</ref> they are often called [[H II region]]s, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.<ref name="shu1982" /> Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called [[dark nebula]]e.<ref name="shu1982" />

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of [[infrared]] emission, chiefly from [[cosmic dust|dust]] within the nebulae.<ref name="shu1982" />

===Planetary nebulae===
{{Main|Planetary nebula}}
[[File:A hazy nebula.jpg|thumb|The [[NGC 1501|Oyster Nebula]] is a [[planetary nebula]] located in the constellation of [[Camelopardalis]]]]
Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved [[asymptotic giant branch]] stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a [[white dwarf]].<ref name="shu1982" /> Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in [[star formation]] regions.<ref name="shu1982" /> They are [[H II region]]s, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.<ref name="shu1982" />

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12&nbsp;billion years after its formation.<ref name="s">{{cite book
  | author = Chaisson, E. 
  | author2 = McMillan, S. 
  | title = Astronomy: a beginner's guide to the universe
  | edition= 2nd
  | publisher = Prentice-Hall
  | date = 1995
  | location = Upper Saddle River, New Jersey
  | isbn = 0-13-733916-X}}</ref>

====Protoplanetary nebulae====
{{Main|Protoplanetary nebula}}
{{excerpt|Protoplanetary nebula}}

===Supernova remnants===

[[Image:Crab Nebula.jpg|thumb|The [[Crab Nebula]], an example of a [[supernova remnant]]]]

A [[supernova]] occurs when a high-mass star reaches the end of its life.  When [[nuclear fusion]] in the core of the star stops, the star collapses.  The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.<ref name="shu1982"/> The expanding shell of gas forms a [[supernova remnant]], a special [[diffuse nebula]].<ref name="shu1982"/> Although much of the optical and [[X-ray]] emission from supernova remnants originates from ionized gas, a great amount of the [[radio wave|radio]] emission is a form of non-thermal emission called [[synchrotron emission]].<ref name="shu1982"/> This emission originates from high-velocity [[electron]]s oscillating within [[magnetic field]]s.

== Examples ==
{{div col|colwidth=18em}}
*[[Ant Nebula]]
*[[Barnard's Loop]]
*[[Boomerang Nebula]]
*[[Cat's Eye Nebula]]
*[[Crab Nebula]]
*[[Eagle Nebula]]
*[[Eskimo Nebula]]
*[[Carina Nebula]]
*[[Fox Fur Nebula]]
*[[Helix Nebula]]
*[[Horsehead Nebula]]
*[[Engraved Hourglass Nebula]]
*[[Lagoon Nebula]]
*[[Orion Nebula]]
*[[Pelican Nebula]]
*[[Red Square Nebula]]
*[[Ring Nebula]]
*[[Rosette Nebula]]
*[[Tarantula Nebula]]
*[[Waterfall Nebula]]
{{div col end}}

===Catalogs===
*[[Gum catalog]] (emission nebulae)
*[[RCW Catalogue]] (emission nebulae)
*[[Sharpless catalog]] (emission nebulae)
*[[Messier Catalogue]]
*[[Caldwell Catalogue]]
*[[Abell Catalog of Planetary Nebulae]]
*[[Barnard Catalogue]] (dark nebulae)
*[[Lynds' Catalogue of Bright Nebulae]]
*[[Lynds' Catalogue of Dark Nebulae]]

==See also==
* [[H I region]]
* [[H II region]]
*[[List of largest nebulae]]
* [[List of diffuse nebulae]]
* [[Lists of nebulae]]
* [[Molecular cloud]]
* [[Magellanic Clouds]]
* [[Messier object]]
* [[Nebular hypothesis]]
* [[Orion molecular cloud complex]]
* [[Timeline of knowledge about the interstellar and intergalactic medium]]

== References ==
{{Reflist}}

== External links ==
*[http://messier.seds.org/nebula.html Nebulae], SEDS Messier Pages
*[http://fusedweb.pppl.gov Fusedweb.pppl.gov]
*[https://bibnum.obspm.fr/solr-search?q=n%C3%A9buleuse Historical pictures of nebulae], digital library of Paris Observatory
 
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