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Interstellar medium
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==History of knowledge of interstellar space== [[File:Herbig-Haro object HH 110.jpeg|thumb|[[Herbig–Haro object]] [[Herbig-Haro 110|HH 110]] ejects gas through interstellar space.<ref>{{cite news|title=A geyser of hot gas flowing from a star|url=http://www.spacetelescope.org/news/heic1210/|access-date=3 July 2012|newspaper=ESA/Hubble Press Release}}</ref> ]] The word 'interstellar' (between the stars) was coined by [[Francis Bacon]] in the context of the ancient theory of a literal [[sphere of fixed stars]].<ref>{{Cite book |last=Lord Verulam, Viscount St Albans |first=Francis |title=Sylva Sylvarum, or A natural History in ten Centuries |publisher=W. Lee |year=1627 |location=London |pages=§ 354–455}}</ref> Later in the 17th century, when the idea that stars were scattered through infinite space became popular, it was debated whether that space was a true vacuum<ref>{{Cite book |last=Boyle |first=Robert |url=https://quod.lib.umich.edu/e/eebo/A28966.0001.001/1:1?rgn=div1;view=toc |title=The Excellency of Theology Compar'd with Natural Philosophy |publisher=Printed by T.N. for Henry Herringman |year=1674 |location=London |pages=178 |chapter=}}</ref> or filled with a hypothetical fluid, sometimes called ''aether'', as in [[René Descartes]]' [[Vortex theory of gravity|vortex theory]] of planetary motions. While vortex theory did not survive the success of [[Newtonian physics]], an invisible [[luminiferous aether]] was re-introduced in the early 19th century as the medium to carry light waves; e.g., in 1862 [[Robert Hogarth Patterson|a journalist]] wrote: "this efflux occasions a thrill, or vibratory motion, in the [[Aether (classical element)|ether]] which fills the interstellar spaces."<ref>{{Cite book |last=Patterson |first=Robert Hogarth |title=Essays in History and Art |year=1862 |pages=10 |chapter=Colour in nature and art, Reprinted from Blackwood's Magazine}}</ref> In 1864, [[William Huggins]] used spectroscopy to determine that a nebula is made of gas.<ref>{{Cite web |date=2014-08-14 |title=The First Planetary Nebula Spectrum |url=https://www.skyandtelescope.com/astronomy-news/observing-news/first-planetary-nebula-spectrum-08142014/ |access-date=2019-11-29 |website=Sky & Telescope |language=en-US}}</ref> Huggins had a private observatory with an 8-inch telescope, with a lens by [[Alvan Clark]]; but it was equipped for spectroscopy, which enabled breakthrough observations.<ref>{{Cite web |title=William Huggins (1824–1910) |url=http://www.messier.seds.org/xtra/Bios/huggins.html |access-date=2019-11-29 |website=www.messier.seds.org}}</ref> From around 1889, [[Edward Emerson Barnard|Edward Barnard]] pioneered deep photography of the sky, finding many 'holes in the Milky Way'. At first he compared them to [[sunspot]]s, but by 1899 was prepared to write: "One can scarcely conceive a vacancy with holes in it, unless there is nebulous matter covering these apparently vacant places in which holes might occur".<ref>{{Cite journal |last=Barnard |first=E. E. |date=1899 |title=Photographs of Comets and of the Milky Way |url=https://academic.oup.com/mnras/issue/59/6 |journal=Monthly Notices of the Royal Astronomical Society |volume=59 |issue=6 |pages=361–370}}</ref> These holes are now known as [[dark nebula]]e, dusty molecular clouds silhouetted against the background star field of the galaxy; the most prominent are listed in his [[Barnard Catalogue]]. The first direct detection of cold diffuse matter in interstellar space came in 1904, when [[Johannes Franz Hartmann|Johannes Hartmann]] observed the [[binary star]] [[Mintaka]] (Delta Orionis) with the [[Potsdam Great Refractor]].<ref name=":5">{{Cite book |last=Kanipe |first=Jeff |url=https://books.google.com/books?id=VYvQ_8I_kTwC&q=hartmann+interstellar+matter+potsdam&pg=PA154 |title=The Cosmic Connection: How Astronomical Events Impact Life on Earth |date=2011-01-27 |publisher=Prometheus Books |isbn=9781591028826 |language=en}}</ref><ref>{{Citation|author-link=Isaac Asimov|first=Isaac|last=Asimov|title=Asimov's Biographical Encyclopedia of Science and Technology|edition=2nd|title-link=Asimov's Biographical Encyclopedia of Science and Technology}}</ref> Hartmann reported<ref>{{Cite journal |last=Hartmann |first=J. |date=1904 |title=Investigations on the spectrum and orbit of delta Orionis. |url=http://adsabs.harvard.edu/doi/10.1086/141112 |journal=The Astrophysical Journal |language=en |volume=19 |pages=268 |doi=10.1086/141112 |bibcode=1904ApJ....19..268H |issn=0004-637X}}</ref> that absorption from the "K" line of [[calcium]] appeared "extraordinarily weak, but almost perfectly sharp" and also reported the "quite surprising result that the calcium line at 393.4 nanometres does not share in the periodic displacements of the lines caused by the orbital motion of the [[spectroscopic binary]] star". The stationary nature of the line led Hartmann to conclude that the gas responsible for the absorption was not present in the atmosphere of the star, but was instead located within an isolated cloud of matter residing somewhere along the [[line of sight]] to this star. This discovery launched the study of the interstellar medium. Interstellar gas was further confirmed by [[Vesto M. Slipher|Slipher]] in 1909, and then by 1912 interstellar dust was confirmed by Slipher.<ref name="azarchivesonline.org">{{Cite web |title=V. M. Slipher Papers, 1899-1965 |url=http://www.azarchivesonline.org/xtf/view?docId=ead/lowell/VMSlipher.xml}}</ref> Interstellar [[sodium]] was detected by [[Mary Lea Heger]] in 1919 through the observation of stationary absorption from the atom's "D" lines at 589.0 and 589.6 nanometres towards Delta Orionis and [[Beta Scorpii]].<ref>{{Cite journal |last=Heger |first=Mary Lea |title=Stationary Sodium Lines in Spectroscopic Binaries |date=1919 |journal=Publications of the Astronomical Society of the Pacific |volume=31 |issue=184 |pages=304–305 |doi=10.1086/122890 |bibcode=1919PASP...31..304H |s2cid=121462375 |issn=0004-6280|doi-access=free }}</ref> In the series of investigations, [[Viktor Ambartsumian]] introduced the now commonly accepted notion that interstellar matter occurs in the form of clouds.<ref>{{Citation | title= To Victor Ambartsumian on his 80th birthday | author=S. Chandrasekhar | author-link=Subrahmanyan Chandrasekhar | journal= Journal of Astrophysics and Astronomy | volume=18 | issue=1 | pages=408–409 | doi=10.1007/BF01005852|bibcode = 1988Ap.....29..408C | year= 1989 | s2cid=122547053 }}</ref> Subsequent observations of the "H" and "K" lines of calcium by {{harvtxt|Beals|1936}} revealed double and asymmetric profiles in the spectra of [[Epsilon Orionis|Epsilon]] and [[Zeta Orionis]]. These were the first steps in the study of the very complex interstellar sightline towards [[Orion (constellation)|Orion]]. Asymmetric absorption line profiles are the result of the superposition of multiple absorption lines, each corresponding to the same [[atomic transition]] (for example the "K" line of calcium), but occurring in interstellar clouds with different [[Radial velocity|radial velocities]]. Because each cloud has a different velocity (either towards or away from the observer/Earth), the absorption lines occurring within each cloud are either [[Blueshift|blue-shifted]] or [[Red shift|red-shifted]] (respectively) from the lines' rest wavelength through the [[Doppler Effect]]. These observations confirming that matter is not distributed homogeneously were the first evidence of multiple discrete clouds within the ISM. [[File:Hubble sees a cosmic caterpillar.jpg|thumb|This light-year-long knot of interstellar gas and dust resembles a [[caterpillar]].<ref>{{cite web|title=Hubble sees a cosmic caterpillar|url=http://www.spacetelescope.org/images/opo1335a/|work=Image Archive|publisher=ESA/Hubble|access-date=9 September 2013}}</ref> ]] The growing evidence for interstellar material led {{harvtxt|Pickering|1912}} to comment: "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by [[Jacobus Kapteyn|Kapteyn]], is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars." The same year, [[Victor Francis Hess|Victor Hess]]'s discovery of [[cosmic rays]], highly energetic charged particles that rain onto the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year, the Norwegian explorer and physicist [[Kristian Birkeland]] wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or [[nebula]]e, but in 'empty' space" {{harv|Birkeland|1913}}. {{harvtxt|Thorndike|1930}} noted that "it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles emitted by the Sun.<!--transcription: one of these phrases may be duplicative--> If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy." In September 2012, [[NASA|NASA scientists]] reported that [[polycyclic aromatic hydrocarbons|polycyclic aromatic hydrocarbons (PAHs)]], subjected to ''interstellar medium (ISM)'' conditions, are transformed, through [[hydrogenation]], [[Oxygenate|oxygenation]] and [[hydroxylation]], to more complex [[Organic compound|organics]], "a step along the path toward [[amino acids]] and [[nucleotides]], the raw materials of [[proteins]] and [[DNA]], respectively".<ref name="Space-20120920">{{Citation|author=<!--Staff writer(s); no by-line.--> |title=NASA Cooks Up Icy Organics to Mimic Life's Origins|url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html|date=September 20, 2012 |publisher=[[Space.com]] |access-date=September 22, 2012 }}</ref><ref name="AJL-20120901">{{Citation |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui|title=In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs – Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies|date=September 1, 2012 |journal=[[The Astrophysical Journal Letters]] |volume=756 |doi=10.1088/2041-8205/756/1/L24|bibcode = 2012ApJ...756L..24G |issue=1 |pages=L24 |s2cid=5541727 }}</ref> Further, as a result of these transformations, the PAHs lose their [[Spectroscopy|spectroscopic signature]], which could be one of the reasons "for the lack of PAH detection in [[interstellar ice]] [[Cosmic dust#Dust grain formation|grains]], particularly the outer regions of cold, dense clouds or the upper molecular layers of [[protoplanetary disks]]."<ref name="Space-20120920" /><ref name="AJL-20120901" /> In February 2014, NASA announced a greatly upgraded database<ref>{{cite web |title=PAH IR Spectroscopic Database |url=http://www.astrochem.org/pahdb/ |work=The Astrophysics & Astrochemistry Laboratory |publisher=[[Ames Research Center|NASA Ames Research Center]] |access-date=October 20, 2019}}</ref> for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible [[PAH world hypothesis|starting materials]] for the [[Abiogenesis#PAH world hypothesis|formation]] of [[Life#Extraterrestrial|life]]. PAHs seem to have been formed shortly after the [[Big Bang]], are widespread throughout the universe, and are associated with [[Star formation|new stars]] and [[exoplanets]].<ref name="NASA-20140221">{{cite web |last=Hoover |first=Rachel |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |date=February 21, 2014 |work=NASA |access-date=February 22, 2014 |archive-date=May 10, 2020 |archive-url=https://web.archive.org/web/20200510124801/https://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |url-status=dead }}</ref> In April 2019, scientists, working with the [[Hubble Space Telescope]], reported the confirmed detection of the large and complex ionized molecules of [[buckminsterfullerene]] (C<sub>60</sub>) (also known as "buckyballs") in the interstellar medium spaces between the stars.<ref name="SA-20190429">{{cite news |last=Starr |first=Michelle |title=The Hubble Space Telescope Has Just Found Solid Evidence of Interstellar Buckyballs |url=https://www.sciencealert.com/the-hubble-space-telescope-has-found-evidence-of-interstellar-buckyballs |date=29 April 2019 |work=ScienceAlert.com |access-date=29 April 2019 }}</ref><ref name="AJL-20190422">{{cite journal |author=Cordiner, M.A. |display-authors=et al. |title=Confirming Interstellar C60 + Using the Hubble Space Telescope |date=22 April 2019 |journal=[[The Astrophysical Journal Letters]] |volume=875 |pages=L28 |number=2 |doi=10.3847/2041-8213/ab14e5 |arxiv=1904.08821 |bibcode=2019ApJ...875L..28C |s2cid=121292704 |doi-access=free }}</ref> In September 2020, evidence was presented of [[Water|solid-state water]] in the interstellar medium, and particularly, of [[Ice|water ice]] mixed with [[Silicate mineral|silicate grains]] in cosmic dust grains.<ref name="NAT-20200921">{{cite journal |author=Potpov, Alexey |display-authors=et al. |date=21 September 2020 |title=Dust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium |url=https://www.nature.com/articles/s41550-020-01214-x |journal=[[Nature Astronomy]] |volume=5 |pages=78–85 |arxiv=2008.10951 |bibcode=2021NatAs...5...78P |doi=10.1038/s41550-020-01214-x |s2cid=221292937 |access-date=26 September 2020}}</ref>
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