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==Research and discovery== [[File:Henk-van-de-hulst.jpg|thumb|left|Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals.]] The history pertaining to the discovery of molecular clouds is closely related to the development of [[radio astronomy]] and [[astrochemistry]]. During [[World War II]], at a small gathering of scientists, [[Hendrik C. van de Hulst|Henk van de Hulst]] first reported he had calculated the [[Hydrogen line|neutral hydrogen atom]] should transmit a detectable [[Radio wave|radio signal]].<ref name=":3">{{Cite book |last=Verschuur |first=Gerrit L. |title=The invisible universe: the story of radio astronomy |date=2015 |publisher=Springer |isbn=978-3-319-13421-5 |edition=3 |series=Astronomers' universe |location=Cham Heidelberg |pages=71}}</ref> This discovery was an important step towards the research that would eventually lead to the detection of molecular clouds. [[File:JanskyatAntenna hi.tif|thumb|right|Jansky and his rotating directional radio antenna (early 1930s), the world's first radio telescope.]] Once the war ended, and aware of the pioneering radio astronomical observations performed by [[Karl Guthe Jansky|Jansky]] and [[Grote Reber|Reber]] in the US, the Dutch astronomers repurposed the dish-shaped antennas running along the Dutch coastline that were once used by the Germans as a warning radar system and modified into [[radio telescope]]s, initiating the search for the [[Hydrogen spectral series|hydrogen signature]] in the depths of space.<ref name=":3" /><ref name=":4">{{Cite journal |last=Beech |first=Martin |date=2017 |title=The Pillars of Creation |url=https://doi.org/10.1007/978-3-319-48775-5 |journal=SpringerLink |language=en |doi=10.1007/978-3-319-48775-5|isbn=978-3-319-48774-8 |url-access=subscription }}</ref> The neutral hydrogen atom consists of a [[proton]] with an [[electron]] in its orbit. Both the proton and the electron have a [[Spin (physics)|spin property.]] When the spin state flips from a parallel condition to antiparallel, which contains less energy, the atom gets rid of the excess energy by radiating a [[spectral line]] at a frequency of 1420.405 [[Hertz|MHz]].<ref name=":3" /> This frequency is generally known as the [[Hydrogen line|21 cm line]], referring to its [[wavelength]] in the [[Radio spectrum|radio band]]. The 21 cm line is the signature of [[Hydrogen atom|HI]] and makes the gas detectable to astronomers back on earth. The discovery of the 21 cm line was the first step towards the technology that would allow astronomers to detect compounds and molecules in interstellar space.<ref name=":3" /> [[File:Green_Banks_-_Ewen-Purcell_Horn_Antenna_info.jpg|thumb|left|Plaque commemorating the discovery of 21-cm radiation from the Milky Way]] In 1951, two research groups nearly simultaneously discovered radio emission from interstellar neutral hydrogen. [[Harold Irving Ewen|Ewen]] and [[Edward Mills Purcell|Purcell]] reported the detection of the 21-cm line in March, 1951. Using the radio telescope at the Kootwijk Observatory, Muller and [[Jan Oort|Oort]] reported the detection of the hydrogen emission line in May of that same year.<ref name=":4" /> [[File:LeidenObservatory1961c.jpg|thumb|right|Left to right: Jan Oort, Hendrik C. van de Hulst, Pieter Oosterhoff. Jan Oort had a pivotal role in the research that lead to the discovery of molecular clouds.]] Once the 21-cm emission line was detected, radio astronomers began mapping the neutral hydrogen distribution of the [[Milky Way]] Galaxy. Van de Hulst, Muller, and Oort, aided by a team of astronomers from Australia, published the Leiden-Sydney map of neutral hydrogen in the galactic disk in 1958 on the [[Monthly Notices of the Royal Astronomical Society]]. This was the first neutral hydrogen map of the [[galactic disc]] and also the first map showing the spiral arm structure within it.<ref name=":4" /> Following the work on atomic hydrogen detection by van de Hulst, Oort and others, astronomers began to regularly use radio telescopes, this time looking for [[List of interstellar and circumstellar molecules|interstellar molecules]]. In 1963 Alan Barrett and Sander Weinred at [[Massachusetts Institute of Technology|MIT]] found the emission line of [[Hydroxyl radical|OH]] in the [[supernova]] remnant [[Cassiopeia A]]. This was the first radio detection of an interstellar molecule at radio wavelengths.<ref name=":0" /> More interstellar OH detections quickly followed and in 1965, Harold Weaver and his team of radio astronomers at [[University of California, Berkeley|Berkeley]], identified OH emissions lines coming from the direction of the [[Orion Nebula]] and in the constellation of [[Cassiopeia (constellation)|Cassiopeia]].<ref name=":4" /> In 1968, Cheung, Rank, Townes, Thornton and Welch detected [[Ammonia|NHβ]] inversion line radiation in interstellar space. A year later, Lewis Snyder and his colleagues found interstellar [[formaldehyde]]. Also in the same year [[George Robert Carruthers|George Carruthers]] managed to identify [[Hydrogen|molecular hydrogen]]. The numerous detections of molecules in interstellar space would help pave the way to the discovery of molecular clouds in 1970.<ref name=":4" /> [[File:Horn_Antenna-in_Holmdel,_New_Jersey_-_restoration1.jpg|thumb|left|Penzias and Wilson with the [[Holmdel horn antenna]] used to detect microwave emissions from the Big Bang]] Hydrogen is the most abundant species of atom in molecular clouds, and under the right conditions it will form the H<sub>2</sub> molecule. Despite its abundance, the detection of H<sub>2</sub> proved difficult. Due to its symmetrical molecule, H<sub>2</sub> molecules have a weak rotational and vibrational modes, making it virtually invisible to direct observation. The solution to this problem came when [[Arno Allan Penzias|Arno Penzias]], Keith Jefferts, and [[Robert Woodrow Wilson|Robert Wilson]] identified [[Carbon monoxide|CO]] in the [[Star formation|star-forming]] region in the [[Omega Nebula]]. Carbon monoxide is a lot easier to detect than H<sub>2</sub> because of its rotational energy and asymmetrical structure. CO soon became the primary tracer of the clouds where star-formation occurs.<ref name=":4" /> In 1970, Penzias and his team quickly detected CO in other locations close to the [[Galactic Center|galactic center]], including the giant molecular cloud identified as [[Sagittarius B2]], 390 [[Light-year|light years]] from the galactic center, making it the first detection of a molecular cloud in history.<ref name=":4" /> This team later would receive the Nobel prize of physics for their discovery of [[Microwave|microwave emission]] from the [[Big Bang]]. Due to their pivotal role, research about these structures have only increased over time. A paper published in 2022 reports over 10,000 molecular clouds detected since the discovery of Sagittarius B2.<ref>{{Cite journal |last1=Neralwar |first1=K. R. |last2=Colombo |first2=D. |last3=Duarte-Cabral |first3=A. |last4=Urquhart |first4=J. S. |last5=Mattern |first5=M. |last6=Wyrowski |first6=F. |last7=Menten |first7=K. M. |last8=Barnes |first8=P. |last9=Sanchez-Monge |first9=A. |last10=Beuther |first10=H. |last11=Rigby |first11=A. J. |last12=Mazumdar |first12=P. |last13=Eden |first13=D. |last14=Csengeri |first14=T. |last15=Dobbs |first15=C. L. |date=2022 |title=The SEDIGISM survey: Molecular cloud morphology. I. Classification and star formation |journal=Astronomy & Astrophysics |volume=663 |pages=A56 |doi=10.1051/0004-6361/202142428 |arxiv=2203.02504 |bibcode=2022A&A...663A..56N |issn=0004-6361|doi-access=free }}</ref>
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