Nebula

[[File:Veil_Nebula_(partial)_(noao-veile).jpg|thumb|right|165px|The Veil Nebula. No, it does not look like {{wpl|Max Stirner||,}} [[pareidolia|even a bit.]]]]
{{Astronomy}}
A '''nebula''' ("cloud" or "fog" in [[Latin]],<ref>[http://www.etymonline.com/index.php?term=nebula Nebula], Online Etymology Dictionary</ref> plural '''nebulae''')<ref>''American Heritage Dictionary of the English Language, Fifth Edition''. S.v. "nebula." Retrieved November 23, 2019, from https://www.thefreedictionary.com/nebula</ref> is a distinct part of the interstellar medium, having a larger concentration of gases (mostly [[hydrogen]] in either [[atom]]ic or [[Molecule|molecular]] form and [[helium]]) plus interstellar dust and heavier elements than helium.

Despite nebulae being both quite large, with sizes of up to hundreds of light years, and denser than the space that surrounds them, by [[Earth]] standards are extremely rarefied and equivalent to a vacuum; while air has a density at sea level of 10<sup>19</sup> molecules per cubic centimeter, a dense nebula would just have some thousands of molecules in such volume. This also means they don't work at all as in the second movie of ''[[Star Trek]]'', where the "Enterprise" and the "Reliant" played hide-and-seek in one, and in addition to that since their {{wpl|surface brightness}} do not change they (like other extended bodies as galaxies, for that matter too) from close by they would appear just larger but not brighter (i.e., if the {{wpl|Great Orion Nebula}} was ten times closer it would appear one hundred times brighter by virtue of the {{wpl|Inverse square law}} but its surface area would now be one hundred times larger (10<sup>2</sup>=100) too so things would not change, meaning it would offer to the naked eye quite a show from a place faraway of city lights and pollution (it can be spotted without optical aid, even from sites with {{wpl|light pollution}}), but from a city save for its brightest embedded stars would be much less impressive or even invisible).<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>

==Types==
Four major types of nebulae are classified, even as our knowledge has improved with time other types have been added on. Earlier on, [[Galaxy|galaxies]] and {{wpl|star cluster|star clusters}} that could not be resolved into stars were lumped together in this classification but that's no longer the case save in very old astronomy textbooks.

===Emission nebulae===
Also known as H II regions. In them, hydrogen is ionized<ref name=definition>{{cite book |url= http://www.oxfordreference.com/view/10.1093/acref/9780199609055.001.0001/acref-9780199609055-e-1759?rskey=tB5idX&result=11 |title=A Dictionary of Astronomy: H&nbsp;II region |edition=2nd rev. |publisher=Oxford University Press |date=2012 |author=Ian Ridpath |access-date= 24 December 2015 |isbn=9780199609055|doi=10.1093/acref/9780199609055.001.0001 }}</ref> by the ultraviolet emission of young, massive, [[star]]s ([[Star#Types of stars|spectral type O and very early B]]) that have formed within them, causing them to shine with a characteristic red color. Like such stars, they're short-lived in astronomical terms (just some millions of years), given that both the powerful {{wpl|stellar wind|stellar winds}} of these stars and their death as supernovae will end up driving most of the gas away.<ref name="popIII_HII">{{cite journal |author1=Alvarez, M.A. |author2=Bromm, V. |author3=Shapiro, P.R. |date = 2006 |title=The H II Region of the First Star |journal=Astrophysical Journal |volume=639 |issue=2 |pages=621–632 |doi=10.1086/499578 |bibcode=2006ApJ...639..621A|arxiv = astro-ph/0507684 |s2cid=12753436 }}</ref> They include some of the most photogenic objects of the sky as the already mentioned Great Orion Nebula.

===Reflection nebulae===
{{wpl|Reflection nebula|Reflection nebulae}} shine by reflecting the light of nearby star(s), that emit insufficient ultraviolet radiation to ionize the gas, and as the former are also sites of recent star formation. Since blue light is more easily scattered than red light, they appear bluish in photos, with some exception as the red supergiant star {{wpl|Antares||,}} that is surrounded by one of such color. Sometimes, as in the famous {{wpl|Trifid Nebula||,}} they may share space with emission nebulae.

===Dark nebulae===
{{wpl|Dark nebula|Dark nebulae}} are so dense and contain so much interstellar dust that they obstruct the light, at least visible light (infrared and radio waves can bypass them), of objects behind them thus appearing as dark clouds silhouetted against a bright background. For example, the band of the [[Milky Way]] as can be readily seen with the naked eye by observing {{wpl|Great_Rift_(astronomy)|from the Cygnus constellation southwards|.}} The southern hemisphere also offers a view of the famous {{wpl|Coalsack nebula||,}} close to the constellation of the Southern Cross.
Dark nebulae are also known as "molecular clouds", given that the hydrogen within them is in molecular form, with some of them being {{wpl|Molecular cloud#Giant molecular clouds|quite large}} in both mass and size, often forming large complexes, and most notably they are the places where stars are being born, once the density is high enough to start such process (eventually transforming into emission and/or reflection nebulae, depending of what kind of stars are being born into them).<ref>{{Cite journal|last1=Di Francesco|first1=James|last2=Hogerheijde|first2=Michiel R.|last3=Welch|first3=William J.|last4=Bergin|first4=Edwin A.|date=November 2002|title=Abundances of Molecular Species in Bernard 68|journal=The Astrophysical Journal|volume=124|issue=5|pages=2749–2755|arxiv=astro-ph/0208298|bibcode=2002AJ....124.2749D|doi=10.1086/344078|s2cid=119078546}}</ref><ref>[http://www.eso.org/public/outreach/press-rel/pr-1999/phot-29-99.html ESO - eso9934 - Secrets of a Dark Cloud]</ref> They can also contain other molecules, detectable thanks to radio astronomy.

The smallest dark nebulae are named {{wpl|Bok globule|Bok globules}} and are cocoons of cold gas and dust surrounding stars in formation.

===Planetary nebulae===
{{wpl|Planetary nebula|Planetary nebulae}} are the graves of low- and intermediate-mass (0.5&nbsp;- ~8 solar masses) stars, consisting of shells of gas emitted by the dying suns in their latest {{wpl|Asymptotic giant branch|evolutionary stage}}, surrounding the former core of the star, now collapsed into a [[Star#White dwarf|white dwarf]], whose intense ultraviolet radiation ionizes the surrounding gases (i.e., they're very small H II regions<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 were given their name by the first astronomical observers who were initially unable to distinguish them from [[planet]]s, and who tended to confuse the former with the latter, which were of more interest to them.

It's thought that our [[Sun]] will produce a planetary nebula when it dies around 7.6 billion years from now.<ref name="Schröder2002">{{cite journal
  |author1=Schröder, K.-P. |author2=Connon Smith, Robert | title= Distant Future of the Earth and Sun revisited
  | journal=Monthly Notices of the Royal Astronomical Society
  | date=2008
  | volume=386
  |issue=1 | pages=155–163
  |arxiv = 0801.4031 |bibcode = 2008MNRAS.386..155S |doi = 10.1111/j.1365-2966.2008.13022.x |s2cid=10073988 }}</ref>

The lifetime of these objects is very short (just some dozens of thousands of years), as both the gases end up being so far away that the white dwarf cannot ionize them, and as it cools it emits less ultraviolet radiation becoming unable to excite them efficiently. A previous, still shorted-lived, phase is the {{wpl|Protoplanetary nebula}} that consists of a small reflection nebulae caused when the central star is not hot enough to ionize the expelled gases (i.e., its core is not still naked and mass loss has not revealed it).<ref name="sahaietal2005">{{cite journal
  | author= R. Sahai
  | author2= C. Sánchez Contreras
  | author3= M. Morris
  | title=A Starfish Preplanetary Nebula: IRAS 19024+0044
  | journal=Astrophysical Journal
  | date=2005
  | volume=620
  | issue= 2
  | pages=948–960
  | bibcode=2005ApJ...620..948S | doi = 10.1086/426469
| s2cid= 1790433
 | url=https://trs.jpl.nasa.gov/bitstream/2014/40924/1/04-3294.pdf
  }}</ref> Also, in theory they should appear as spherical shells surrounding the white dwarf core; however and as usual nature is ''far'' more complicated than that, and companion star(s) and planets, magnetic fields, among other processes not fully understood can produce a lot of different morphologies,<ref>{{cite journal
 | bibcode = 2005ApJ...635L..49K
 | last1 = Kwok
 | first1 = Sun
 | last2 = Su
 | first2 = Kate Y. L.
 | date = December 2005
 | title = Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881
 | journal = The Astrophysical Journal
 | volume = 635
 | issue = 1
 | pages = L49–52
 | doi = 10.1086/499332}}</ref> not just because of projection effects as we see them from different angles.<ref>{{cite journal | first = Z | last = Chen |author2=A. Frank|author3=E. G. Blackman|author4=J. Nordhaus|author5=J. Carroll-Nellenback| s2cid = 119073723 | title = Mass Transfer and Disc Formation in AGB Binary Systems| journal = Monthly Notices of the Royal Astronomical Society | volume = 468 | issue = 4 | pages = 4465 | date = 2017 | doi = 10.1093/mnras/stx680 |arxiv = 1702.06160 |bibcode = 2017MNRAS.468.4465C }}</ref>

===Supernova remnant===
{{wpl|Supernova remnant|Supernova remnants}} are the high-mass (>8 solar masses) equivalent of planetary nebulae, formed when a massive star reaches its end of life and explodes as a supernova, even if other types of supernova as {{wpl|Type Ia supernova|Type Ia supernovae}} can leave behind one too, with fast expanding (thousands of kilometers per second, if not more) ejecta surrounding a [[Star|Neutron star]] or a [[black hole]], even if in some cases the explosion may be asymmetric and {{wpl|Pulsar kick|kick out elsewhere}} {{wpl|Zombie star|the stellar remnant|.}} Like planetary nebulae, they're short-lived objects in astronomical terms (thousands of years).

===Integrated flux nebula===
{{wpl|Integrated Flux Nebula|Integrated flux nebulae}} are a kind of reflection nebulae that shine reflecting the light of the entire Milky Way, being far away from the galactic plane. They're ''very'' faint objects, requiring up to hours of exposition to appear in photos.<ref>{{Cite web |date=2017-03-25 |title=MWCatalogue |url=http://www.galaxyimages.com/MWCatalogue.html |access-date=2023-05-05 |archive-url=https://web.archive.org/web/20170325074937/http://www.galaxyimages.com/MWCatalogue.html |archive-date=2017-03-25 }}</ref>

===A final caveat===
Most nebulae have a surface brightness too low to be observed, be with the naked eye or with telescope even with {{wpl|Astronomical_filter#Nebular_filters|filters}} designed for such purposes and will appear just in long-exposure pictures. Besides that, even in much brighter ones as the Great Orion Nebula or the {{wpl|Lagoon Nebula||,}} their surface brightness is not enough to appreciate the vivid colours so famous in photos of them. Unless you're using a ''very'' large (and expensive) telescope they'll (like galaxies too) appear "greyish" and colorless<ref>[https://astronomy.stackexchange.com/questions/28837/can-we-see-the-color-of-nebulae Can we see the color of nebulae? |access-date=2023-07-02]</ref>. Conversely, planetary nebulae except for some {{wpl|Abell 39|large, low surface brightness, ones}} tend to appear small, up to {{wpl|IC 4997|starlike}} in small telescopes, requiring high magnification and/or one of those filters mentioned above to unmask them.

If you plan to buy a telescope, hoping you'll see the same as in those colourful pictures, you have been warned. Especially if it's one of those crappy department store telescopes with magnifications in the hundreds and that come in boxes covered in those garish deep-space pictures released by [[NASA]].

==Gallery==
<gallery>
Pillars of creation 2014 HST WFC3-UVIS full-res.jpg|A section of the Eagle Nebula, known as the "Pillars of Creation"
Orion Nebula - Hubble 2006 mosaic 18000.jpg|The famous Great Orion Nebula.
Integrated Flux Nebula Surrounding Polaris - Kush Chandaria.jpg|A faint integrated flux nebula that surrounds Polaris, the North Star.
Image:Crab Nebula.jpg|The Crab Nebula, what remains of a massive star than went supernova back in 1054 in the Taurus constellation.
NGC7293 (2004).jpg|The Helix Nebula, also known as the "Eye of God" or the "Eye of Sauron",<ref>{{wpa|Helix Nebula}}</ref> a famous planetary nebula in the constellation Aquarius
R Coronae Australis region.jpg|A reflection nebula in the southern constellation of Corona Australis.
Dark Rift 2012.jpg|Dark nebulae silhouetted against the Milky Way.
</gallery>

==References==
{{reflist|2|80%}}

[[Category:Astronomy]]

Categories: Astronomy