Editing Nebula (section)

{{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>