Editing Interstellar cloud

{{short description|Accumulation of gas, plasma, and dust in space}}
[[File:Carved by Massive Stars.jpg|thumb|A small part of the [[emission nebula]] [[NGC 6357]]. It glows with the characteristic red of an [[H II region]].<ref>{{cite web |title=Carved by Massive Stars |url=http://www.eso.org/public/images/potw1334a/ |work=ESO Picture of the Week |publisher=[[European Southern Observatory]] |access-date=13 September 2013}}</ref>]]

An '''interstellar cloud''' is an accumulation of [[gas]], [[Plasma (physics)|plasma]], and [[cosmic dust]] in [[galaxies]]. Put differently, an interstellar cloud is a denser-than-average region of the [[interstellar medium]], the [[matter]] and radiation that exists in the [[Interstellar space|space]] between the [[star system]]s in a galaxy. Depending on the [[density]], [[size]], and [[temperature]] of a given cloud, its [[hydrogen]] can be neutral, making an [[H I region]]; ionized, or plasma making it an [[H II region]]; or molecular, which are referred to simply as [[molecular cloud]]s, or sometime dense clouds. Neutral and [[Ionization|ionized]] clouds are sometimes also called ''diffuse clouds''. An interstellar cloud is formed by the gas and dust particles from a [[red giant]] in its later life.

==Chemical compositions==
The chemical composition of interstellar clouds is determined by studying [[electromagnetic radiation]] that they emanate, and we receive – from [[radio wave]]s through [[visible light]], to [[gamma rays]] on the [[electromagnetic spectrum]] – that we receive from them. Large [[radio telescope]]s scan the intensity in the sky of particular [[frequency|frequencies]] of electromagnetic radiation, which are characteristic of certain [[molecule]]s' [[electromagnetic spectroscopy|spectra]]. Some interstellar clouds are cold and tend to give out electromagnetic radiation of large [[wavelength]]s. A map of the abundance of these molecules can be made, enabling an understanding of the varying composition of the clouds. In hot clouds, there are often ions of many [[chemical element|elements]], whose spectra can be seen in visible and [[ultraviolet light]].

Radio telescopes can also scan over the frequencies from one point in the map, recording the intensities of each type of molecule. Peaks of frequencies mean that an abundance of that molecule or atom is present in the cloud. The height of the peak is proportional to the relative percentage that it makes up.<ref>{{Cite web |author=Project Leader Dr. Lochner |date=November 2009 |title=Spectra and What Scientists Can Learn From Them |url=http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html |archive-url=https://web.archive.org/web/20141109040237/http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectra.html |archive-date=Nov 9, 2014 |access-date=12 February 2010 |publisher=Goddard Space Flight Center, NASA}}</ref>

==Unexpected chemicals detected in interstellar clouds==
[[File:Diving into the Lagoon Nebula.OGG|thumb|View inside the [[Lagoon Nebula]].]]
Until recently,{{When|reason="Recently" as of when?|date=February 2024}} the rates of reactions in interstellar clouds were expected to be very slow, with minimal products being produced due to the low temperature and density of the clouds. However, [[Organic compound|organic molecule]]s were observed in the spectra that [[scientist]]s would not have expected to find under these conditions, such as [[formaldehyde]], [[methanol]], and [[vinyl alcohol]]. The reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. The fact that they were found indicates that these [[chemical reaction]]s in interstellar clouds take place faster than suspected, likely in gas-phase reactions unfamiliar to organic chemistry as observed on earth.<ref>{{Cite web|url=http://www.nrao.edu/pr/2001/vinylalco/|title=Scientists Toast the Discovery of Vinyl Alcohol in Interstellar Space|date= October 2001|author=Charles Blue|publisher=[[National Radio Astronomy Observatory]]|access-date=9 February 2010}}</ref> These reactions are studied in the [[CRESU experiment]].

Interstellar clouds also provide a medium to study the presence and proportions of metals in space.  The presence and ratios of these elements may help develop theories on the means of their production, especially when their proportions are inconsistent with those expected to arise from stars as a result of [[Nucleosynthesis|fusion]] and thereby suggest alternate means, such as [[cosmic ray spallation]].<ref>{{cite journal | last1 = Knauth | first1 = D. | last2 = Federman | first2 = S. | last3 = Lambert | first3 = D. | year = 2000 | title = Newly Synthesized Lithium in the Interstellar Medium | journal = Nature | volume = 405 | issue = 6787| pages = 656–658 | doi=10.1038/35015028 | pmid=10864316|bibcode = 2000Natur.405..656K | s2cid = 4397202 }}</ref>

==High-velocity cloud==
{{Main|High-velocity cloud}}
[[File:IRAS 10082-5647.jpg|thumb|[[Reflection nebula]] IRAS 10082-5647 observed by the [[Hubble Space Telescope]].]]

These interstellar clouds possess a velocity higher than can be explained by the rotation of the [[Milky Way]].<ref>Navarro, J. F., Frenk, C. S., & White, S. D. M. 1995, [[Monthly Notices of the Royal Astronomical Society]], 275, 720</ref> By definition, these clouds must have a v<sub>lsr</sub> greater than 90&nbsp;km s<sup>−1</sup>, where v<sub>lsr</sub> is the local standard rest velocity. They are detected primarily in the [[hydrogen line|21 cm line]] of neutral [[hydrogen]],<ref>{{Cite web |title=Dark Matter- More Than Meets the Eye |url=http://genesismission.jpl.nasa.gov/educate/scimodule/Cosmogony/CosmogonyPDF/MilkyWaySurpriseST.pdf |archive-url=https://web.archive.org/web/20190611235414/http://genesismission.jpl.nasa.gov/educate/scimodule/Cosmogony/CosmogonyPDF/MilkyWaySurpriseST.pdf |archive-date=Jun 11, 2019 |access-date=12 February 2010 |publisher=[[NASA]]}}</ref> and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.

Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, or [[tidal force|tidally-displaced]] [[matter]] drawn away from other galaxies or members of the [[Local Group]]. An example of the latter is the [[Magellanic Stream]]. To narrow down the origin of these clouds, a better understanding of their distances and [[metallicity]] is needed.

High-velocity clouds are identified with an HVC prefix, as with [[HVC 127-41-330]].

==See also==
* [[List of molecules in interstellar space]]
* [[Nebula]]
* [[Interplanetary medium]] – [[interplanetary dust]]
* [[Interstellar medium]] – [[interstellar dust]]
* [[Intergalactic medium]] – [[Intergalactic dust]]
* [[Local Interstellar Cloud]]
* [[G-Cloud]]

==References==
{{reflist}}

==External links==
* [http://www.cosmos.swin.edu.au/entries/highvelocitycloud/highvelocitycloud.html High Velocity Cloud] — The [[Swinburne Astronomy Online]] (SAO) encyclopedia.
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