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111-398: Submillimetre astronomy or submillimeter astronomy (see spelling differences ) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation ) of the electromagnetic spectrum . Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and

222-451: A millimetre . It is still common in submillimetre astronomy to quote wavelengths in 'microns', the old name for micrometre. Using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of star formation from earliest collapse to stellar birth . Submillimetre observations of these dark clouds can be used to determine chemical abundances and cooling mechanisms for

333-455: A stellar nursery (if star formation is occurring within), is a type of interstellar cloud , the density and size of which permit absorption nebulae , the formation of molecules (most commonly molecular hydrogen , H 2 ), and the formation of H II regions . This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas . Molecular hydrogen is difficult to detect by infrared and radio observations, so

444-485: A GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps. Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control

555-691: A consonant followed by an unstressed -re (pronounced /ə(r)/ ). In modern American English, most of these words have the ending -er . The difference is most common for words ending in -bre or -tre : British spellings calibre , centre , fibre , goitre , litre , lustre , manoeuvre , meagre , metre (length) , mitre , nitre , ochre , reconnoitre , sabre , saltpetre , sepulchre , sombre , spectre , theatre (see exceptions ) and titre all have -er in American spelling. In Britain, both -re and -er spellings were common before Johnson's 1755 dictionary

666-459: A crucial role in the initial conditions of star formation and the origin of the stellar IMF. The densest parts of the filaments and clumps are called molecular cores, while the densest molecular cores are called dense molecular cores and have densities in excess of 10 to 10 particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with ammonia . The concentration of dust within molecular cores

777-475: A distinctive set of Canadian English spellings is viewed by many Canadians as one of the unique aspects of Canadian culture (especially when compared to the United States). In Australia, -or endings enjoyed some use throughout the 19th century and in the early 20th century. Like Canada, though, most major Australian newspapers have switched from " -or " endings to " -our " endings. The " -our " spelling

888-495: A emium , and a enigma . In others, it is kept in all varieties: for example, phoenix , and usually subpoena , but Phenix in Virginia . This is especially true of names: Aegean (the sea), Caesar , Oedipus , Phoebe , etc., although "caesarean section" may be spelled as "cesarean section". There is no reduction of Latin -ae plurals (e.g., larv ae ); nor where the digraph <ae>/<oe> does not result from

999-466: A eon , an a emia , an a esthesia , c a ecum , c a esium , c o eliac , diarrh o ea , encyclop a edia , f a eces , f o etal , gyn a ecology , h a emoglobin , h a emophilia , leuk a emia , o esophagus , o estrogen , orthop a edic , pal a eontology , p a ediatric , p a edophile . Oenology is acceptable in American English but is deemed

1110-424: A fast transition, forming "envelopes" of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary. Cosmic dust and ultraviolet radiation emitted by stars are key factors that determine not only gas and column density, but also the molecular composition of a cloud. The dust provides shielding to the molecular gas inside, preventing dissociation by

1221-532: A minor variant of enology , whereas although archeology and ameba exist in American English, the British versions amoeba and archaeology are more common. The chemical haem (named as a shortening of h a emoglobin ) is spelled heme in American English, to avoid confusion with hem . Canadian English mostly follows American English in this respect, although it is split on gynecology (e.g. Society of Obstetricians and Gynaecologists of Canada vs.

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1332-569: A mistaken etymology. The etymologically correct original spelling fetus reflects the Latin original and is the standard spelling in medical journals worldwide; the Oxford English Dictionary notes that "In Latin manuscripts both fētus and foetus are used". The Ancient Greek diphthongs <αι> and <οι> were transliterated into Latin as <ae> and <oe>. The ligatures æ and œ were introduced when

1443-441: A molecular cloud assembles enough mass, the densest regions of the structure will start to collapse under gravity, creating star-forming clusters. This process is highly destructive to the cloud itself. Once stars are formed, they begin to ionize portions of the cloud around it due to their heat. The ionized gas then evaporates and is dispersed in formations called ‘ champagne flows ’. This process begins when approximately 2% of

1554-667: A suffix for agentive ( reader , user , winner ) and comparative ( louder , nicer ) forms. One outcome is the British distinction of meter for a measuring instrument from metre for the unit of length. However, while " poetic metre " is often spelled as -re , pentameter , hexameter , etc. are always -er . Many other words have -er in British English. These include Germanic words, such as anger , mother , timber and water , and such Romance-derived words as danger , quarter and river . The ending -cre , as in acre , lucre , massacre , and mediocre ,

1665-441: A timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Perpendicularly to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height , Z , of approximately 50 to 75 parsecs, much thinner than the warm atomic ( Z from 130 to 400 parsecs) and warm ionized ( Z around 1000 parsecs) gaseous components of

1776-466: A weak rotational and vibrational modes, making it virtually invisible to direct observation. The solution to this problem came when Arno Penzias , Keith Jefferts, and Robert Wilson identified CO in the star-forming region in the Omega Nebula . Carbon monoxide is a lot easier to detect than H 2 because of its rotational energy and asymmetrical structure. CO soon became the primary tracer of

1887-432: Is - /s/ for the noun and - /z/ for the verb). For licence / license or practice / practise , British English also keeps the noun–verb distinction graphically (although phonetically the two words in each pair are homophones with - /s/ pronunciation). On the other hand, American English uses license and practice for both nouns and verbs (with - /s/ pronunciation in both cases too). American English has kept

1998-545: Is another interferometer, located at Mauna Kea, consisting of eight 6-metre diameter radio telescopes. The largest existing submillimetre telescope, the James Clerk Maxwell Telescope , is also located on Mauna Kea. With high-altitude balloons and aircraft, one can get above more of the atmosphere. The BLAST experiment and SOFIA are two examples, respectively, although SOFIA can also handle near infrared observations. Space-based observations at

2109-409: Is dispersed after this time. The lack of large amounts of frozen molecules inside the clouds also suggest a short-lived structure. Some astronomers propose the molecules never froze in very large quantities due to turbulence and the fast transition between atomic and molecular gas. Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating

2220-399: Is dropped for other derivations, for example, central , fibrous , spectral . However, the existence of related words without e before the r is not proof for the existence of an -re British spelling: for example, entry and entrance come from enter , which has not been spelled entre for centuries. The difference relates only to root words; -er rather than -re is universal as

2331-506: Is generally known as the 21 cm line , referring to its wavelength in the radio band . The 21 cm line is the signature of 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. In 1951, two research groups nearly simultaneously discovered radio emission from interstellar neutral hydrogen. Ewen and Purcell reported

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2442-502: Is generally preferred over oe and often over ae , but oe and ae are sometimes found in academic and scientific writing as well as government publications (for example, the fee schedule of the Ontario Health Insurance Plan ) and some words such as palaeontology or aeon . In Australia, it can go either way, depending on the word: for instance, medieval is spelled with the e rather than ae , following

2553-601: Is known as the Pacific Parlour car, not Pacific Parlor . Proper names such as Pearl Harbor or Sydney Harbour are usually spelled according to their native-variety spelling vocabulary. The name of the herb savory is spelled thus everywhere, although the related adjective savo(u)ry , like savo(u)r , has a u in the UK. Honor (the name) and arbor (the tool) have -or in Britain, as mentioned above, as does

2664-414: Is likely to be the main mechanism. Those regions with more gas will exert a greater gravitational force on their neighboring regions, and draw surrounding material. This extra material increases the density, increasing their gravitational attraction. Mathematical models of gravitational instability in the gas layer predict a formation time within the timescale for the estimated cloud formation time. Once

2775-513: Is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae . GMCs are so large that local ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion molecular cloud (OMC) or the Taurus molecular cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of

2886-719: Is only through "windows" between these bands that observations are possible. The ideal submillimetre observing site is dry, cool, has stable weather conditions and is away from urban population centres. Only a handful of sites have been identified. They include Mauna Kea ( Hawaii , United States), the Llano de Chajnantor Observatory on the Atacama Plateau ( Chile ), the South Pole , and Hanle in India (the Himalayan site of

2997-613: Is sometimes used. The ratio between -ise and -ize stood at 3:2 in the British National Corpus up to 2002. The spelling -ise is more commonly used in UK mass media and newspapers, including The Times (which switched conventions in 1992), The Daily Telegraph , The Economist and the BBC . The Government of the United Kingdom additionally uses -ise , stating "do not use Americanisms" justifying that

3108-714: Is standard worldwide and complection is rare. However, the adjective complected (as in "dark-complected"), although sometimes proscribed, is on equal ground in the U.S. with complexioned. It is not used in this way in the UK, although there exists a rare alternative meaning of complicated . In some cases, words with "old-fashioned" spellings are retained widely in the U.S. for historical reasons (cf. connexionalism ). Many words, especially medical words, that are written with ae/æ or oe/œ in British English are written with just an e in American English. The sounds in question are /iː/ or /ɛ/ (or, unstressed, /i/ , /ɪ/ or /ə/ ). Examples (with non-American letter in bold ):

3219-607: Is taught in schools nationwide as part of the Australian curriculum. The most notable countrywide use of the -or ending is for one of the country's major political parties, the Australian Labor Party , which was originally called "the Australian Labour Party" (name adopted in 1908), but was frequently referred to as both "Labour" and "Labor". The "Labor" was adopted from 1912 onward due to

3330-549: Is the usual form of the male given name, as a surname both the spellings Peter and Petre (the latter notably borne by a British lord ) are found. For British accoutre , the American practice varies: the Merriam-Webster Dictionary prefers the -re spelling, but The American Heritage Dictionary of the English Language prefers the -er spelling. More recent French loanwords keep

3441-484: Is used in both British and American English to show that the c is pronounced /k/ rather than /s/ . The spellings euchre and ogre are also the same in both British and American English. Fire and its associated adjective fiery are the same in both British and American English, although the noun was spelled fier in Old and Middle English. Theater is the prevailing American spelling used to refer to both

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3552-591: The -meter suffix (from Ancient Greek - μέτρον métron , via French -mètre ) normally had the -re spelling from earliest use in English but were superseded by -er . Examples include thermometer and barometer . The e preceding the r is kept in American-inflected forms of nouns and verbs, for example, fibers , reconnoitered , centering , which are fibres , reconnoitred , and centring respectively in British English. According to

3663-560: The -ise form as an alternative. Publications by Oxford University Press (OUP)—such as Henry Watson Fowler 's A Dictionary of Modern English Usage , Hart's Rules , and The Oxford Guide to English Usage —also recommend -ize . However, Robert Allan's Pocket Fowler's Modern English Usage considers either spelling to be acceptable anywhere but the U.S. American spelling avoids -ise endings in words like organize , realize and recognize . British spelling mostly uses -ise ( organise , realise , recognise ), though -ize

3774-687: The -ise form is preferred in Australian English at a ratio of about 3:1 according to the Macquarie Dictionary . In Canada, the -ize ending is more common, although the Ontario Public School Spelling Book spelled most words in the -ize form, but allowed for duality with a page insert as late as the 1970s, noting that, although the -ize spelling was in fact the convention used in the OED ,

3885-439: The -ise variation, once more common amongst older Canadians, is employed less and less often in favour of the -ize spelling. (The alternate convention offered as a matter of choice may have been due to the fact that although there were an increasing number of American- and British-based dictionaries with Canadian Editions by the late 1970s, these were largely only supplemental in terms of vocabulary with subsequent definitions. It

3996-450: The -re spelling in American English. These are not exceptions when a French-style pronunciation is used ( /rə/ rather than /ə(r)/ ), as with double entendre , genre and oeuvre . However, the unstressed /ə(r)/ pronunciation of an -er ending is used more (or less) often with some words, including cadre , macabre , maître d' , Notre Dame , piastre , and timbre . The -re endings are mostly standard throughout

4107-512: 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. Within the Milky Way , molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of it. The bulk of

4218-495: The Canadian Medical Association 's Canadian specialty profile of Obstetrics/gynecology ). Pediatrician is preferred roughly 10 to 1 over paediatrician , while foetal and oestrogen are similarly uncommon. Words that can be spelled either way in American English include a esthetics and arch a eology (which usually prevail over esthetics and archeology ), as well as pal a estra , for which

4329-678: The Indian Astronomical Observatory ). Comparisons show that all four sites are excellent for submillimetre astronomy, and of these sites Mauna Kea is the most established and arguably the most accessible. There has been some recent interest in high-altitude Arctic sites, particularly Summit Station in Greenland where the PWV ( precipitable water vapor) measure is always better than at Mauna Kea (however Mauna Kea's equatorial latitude of 19 degrees means it can observe more of

4440-469: The Milky Way per year. Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the collision theory have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures. Gravitational instability

4551-548: The OED , centring is a "word ... of 3 syllables (in careful pronunciation)" (i.e., /ˈsɛntərɪŋ/ ), yet there is no vowel in the spelling corresponding to the second syllable ( /ə/ ). The OED third edition (revised entry of June 2016) allows either two or three syllables. On the Oxford Dictionaries Online website, the three-syllable version is listed only as the American pronunciation of centering . The e

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4662-705: The Organisation for Economic Co-operation and Development ). The European Union 's style guides require the usage of - ise . Proofreaders at the EU's Publications Office ensure consistent spelling in official publications such as the Official Journal of the European Union (where legislation and other official documents are published), but the -ize spelling may be found in other documents. Molecular cloud A molecular cloud , sometimes called

4773-557: The Simplified Spelling Board in the early 20th century, but most were not adopted. In Britain, the influence of those who preferred the Norman (or Anglo-French ) spellings of words proved to be decisive. Later spelling adjustments in the United Kingdom had little effect on today's American spellings and vice versa. For the most part, the spelling systems of most Commonwealth countries and Ireland closely resemble

4884-463: The molecules which comprise them . In addition, submillimetre observations give information on the mechanisms for the formation and evolution of galaxies . The most significant limitations to the detection of astronomical emission at submillimetre wavelengths with ground-based observatories are atmospheric emission, noise and attenuation. Like the infrared , the submillimetre atmosphere is dominated by numerous water vapour absorption bands and it

4995-543: The theatre spelling. (The word "theater" in American English is a place where both stage performances and screenings of films take place, but in British English a "theatre" is where stage performances take place but not film screenings – these take place in a cinema, or "picture theatre" in Australia.) In the United States, the spelling theatre is sometimes used when referring to the art form of theatre, while

5106-519: The u has since been dropped: ambassadour , emperour , errour , governour , horrour , inferiour , mirrour , perturbatour , superiour , tenour , terrour , tremour . Johnson, unlike Webster, was not an advocate of spelling reform, but chose the spelling best derived, as he saw it, from among the variations in his sources. He preferred French over Latin spellings because, as he put it, "the French generally supplied us". English speakers who moved to

5217-435: The u : In American usage, derivatives and inflected forms are built by simply adding the suffix in all cases (for example, favorite , savory etc.) since the u is absent to begin with. American usage, in most cases, keeps the u in the word glamour , which comes from Scots , not Latin or French. Glamor is sometimes used in imitation of the spelling reform of other -our words to -or . Nevertheless,

5328-652: The ‑our/or group do not have a Latin counterpart that ends in ‑or ; for example, armo(u)r , behavio(u)r , harbo(u)r , neighbo(u)r ; also arbo(u)r , meaning "shelter", though senses "tree" and "tool" are always arbor , a false cognate of the other word. The word arbor would be more accurately spelled arber or arbre in the US and the UK, respectively, the latter of which is the French word for "tree". Some 16th- and early 17th-century British scholars indeed insisted that ‑or be used for words from Latin (e.g., color ) and ‑our for French loans; however, in many cases,

5439-441: The 17th and 18th centuries, whereas there are thousands of examples of their -our counterparts. One notable exception is honor . Honor and honour were equally frequent in Britain until the 17th century; honor only exists in the UK now as the spelling of Honor Oak , a district of London, and of the occasional given name Honor . In derivatives and inflected forms of the -our/or words, British usage depends on

5550-615: The 1970s, but had by then been overtaken by connection in regular usage (for example, in more popular newspapers). Connexion (and its derivatives connexional and connexionalism ) is still in use by the Methodist Church of Great Britain to refer to the whole church as opposed to its constituent districts, circuits and local churches, whereas the US-majority United Methodist Church uses Connection . Complexion (which comes from complex )

5661-743: The American National Theatre was referred to by The New York Times as the "American National Theater ", but the organization uses "re" in the spelling of its name. The John F. Kennedy Center for the Performing Arts in Washington, D.C. has the more common American spelling theater in its references to the Eisenhower Theater, part of the Kennedy Center. Some cinemas outside New York also use

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5772-480: The American and British varieties of English. However, English-language spelling reform has rarely been adopted otherwise. As a result, modern English orthography varies only minimally between countries and is far from phonemic in any country. In the early 18th century, English spelling was inconsistent. These differences became noticeable after the publication of influential dictionaries . Today's British English spellings mostly follow Johnson's A Dictionary of

5883-413: The American usage along with numerous other words such as eon or fetus , while other words such as oestrogen or paediatrician are spelled the British way. The Macquarie Dictionary also notes a growing tendency towards replacing ae and oe with e worldwide and with the exception of manoeuvre, all British or American spellings are acceptable variants. Elsewhere, the British usage prevails, but

5994-587: The Anglo-French spelling for defense and offense , which are defence and offence in British English. Likewise, there are the American pretense and British pretence ; but derivatives such as defensive , offensive , and pretension are always thus spelled in both systems. Australian and Canadian usages generally follow British usage. The spelling connexion is now rare in everyday British usage, its use lessening as knowledge of Latin attenuates, and it has almost never been used in

6105-440: The British system. In Canada, the spelling system can be said to follow both British and American forms, and Canadians are somewhat more tolerant of foreign spellings when compared with other English-speaking nationalities. Australian English mostly follows British spelling norms but has strayed slightly, with some American spellings incorporated as standard. New Zealand English is almost identical to British spelling, except in

6216-668: The British usage of -our . This coincided with a renewed interest in Canadian English, and the release of the updated Gage Canadian Dictionary in 1997 and the first Canadian Oxford Dictionary in 1998. Historically, most libraries and educational institutions in Canada have supported the use of the Oxford English Dictionary rather than the American Webster's Dictionary . Today, the use of

6327-471: The Commonwealth. The -er spellings are recognized as minor variants in Canada, partly due to United States influence. They are sometimes used in proper names (such as Toronto's controversially named Centerpoint Mall ). For advice / advise and device / devise , American English and British English both keep the noun–verb distinction both graphically and phonetically (where the pronunciation

6438-647: The Earth and Sun. Herschel's mission focused primarily on the origins of galaxies and galactic formation. Spelling differences A "British standard" began to emerge following the 1755 publication of Samuel Johnson 's A Dictionary of the English Language , and an "American standard" started following the work of Noah Webster and, in particular, his An American Dictionary of the English Language , first published in 1828. Webster's efforts at spelling reform were effective in his native country, resulting in certain well-known patterns of spelling differences between

6549-521: The English Language (1755), while many American English spellings follow Webster's An American Dictionary of the English Language ("ADEL", "Webster's Dictionary", 1828). Webster was a proponent of English spelling reform for reasons both philological and nationalistic. In A Companion to the American Revolution (2008), John Algeo notes: "it is often assumed that characteristically American spellings were invented by Noah Webster. He

6660-488: The Greek-style ligature as, for example, in maelstrom or toe ; the same is true for the British form aeroplane (compare other aero- words such as aerosol ) . The now chiefly North American airplane is not a respelling but a recoining, modelled after airship and aircraft . The word airplane dates from 1907, at which time the prefix aero- was trisyllabic, often written aëro- . In Canada, e

6771-457: The ISM . The exceptions to the ionized-gas distribution are H II regions , which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars ; and as such they have approximately the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of

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6882-708: 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. 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 interstellar molecules . In 1963 Alan Barrett and Sander Weinred at MIT found

6993-593: The Sun coinciding with the Gould Belt . The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than

7104-518: The US: the more common connection has become the standard worldwide. According to the Oxford English Dictionary , the older spelling is more etymologically conservative, since the original Latin word had -xio- . The American usage comes from Webster , who abandoned -xion and preferred -ction . Connexion was still the house style of The Times of London until the 1980s and was still used by Post Office Telecommunications for its telephone services in

7215-497: The United States took these preferences with them. In the early 20th century, H. L. Mencken notes that " honor appears in the 1776 Declaration of Independence , but it seems to have been put there rather by accident than by design". In Jefferson 's original draft it is spelled "honour". In Britain, examples of behavior , color , flavor , harbor , and neighbor rarely appear in Old Bailey court records from

7326-423: The United States use Centre in their names. Examples include the villages of Newton Centre and Rockville Centre , the city of Centreville , Centre County and Centre College . Sometimes, these places were named before spelling changes but more often the spelling serves as an affectation. Proper names are usually spelled according to their native-variety spelling vocabulary; so, for instance, although Peter

7437-617: The adjective glamorous often drops the first "u". Saviour is a somewhat common variant of savior in the US. The British spelling is very common for honour (and favour ) in the formal language of wedding invitations in the US. The name of the Space Shuttle Endeavour has a u in it because the spacecraft was named after British Captain James Cook 's ship, HMS Endeavour . The (former) special car on Amtrak 's Coast Starlight train

7548-518: The building itself, as noted above, generally is spelled theater . For example, the University of Wisconsin–Madison has a "Department of Theatre and Drama", which offers courses that lead to the "Bachelor of Arts in Theatre ", and whose professed aim is "to prepare our graduate students for successful 21st Century careers in the theatre both as practitioners and scholars". Some placenames in

7659-456: The choice to spell such words in the -ise form was a matter of personal preference ; however, a pupil having made the decision, one way or the other, thereafter ought to write uniformly not only for a given word, but to apply that same uniformity consistently for all words where the option is found. Just as with -yze spellings, however, in Canada the ize form remains the preferred or more common spelling, though both can still be found, yet

7770-426: The close historic, economic, and cultural relationship with the United States, -or endings are also sometimes used. Throughout the late 19th and early to mid-20th century, most Canadian newspapers chose to use the American usage of -or endings, originally to save time and money in the era of manual movable type . However, in the 1990s, the majority of Canadian newspapers officially updated their spelling policies to

7881-477: The clouds where star-formation occurs. In 1970, Penzias and his team quickly detected CO in other locations close to the galactic center , including the giant molecular cloud identified as Sagittarius B2 , 390 light years from the galactic center, making it the first detection of a molecular cloud in history. This team later would receive the Nobel prize of physics for their discovery of microwave emission from

7992-461: The detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width. A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation. Recent studies have suggested that filamentary structures in molecular clouds play

8103-666: The detection of the 21-cm line in March, 1951. Using the radio telescope at the Kootwijk Observatory, Muller and Oort reported the detection of the hydrogen emission line in May of that same year. 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

8214-403: The dramatic arts and buildings where stage performances and screenings of films take place (i.e., " movie theaters "); for example, a national newspaper such as The New York Times would use theater in its entertainment section. However, the spelling theatre appears in the names of many New York City theatres on Broadway (cf. Broadway theatre) and elsewhere in the United States. In 2003,

8325-404: The dust and gas to collapse. 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, Henk van de Hulst first reported he had calculated the neutral hydrogen atom should transmit a detectable radio signal . This discovery was an important step towards

8436-665: The emission line of OH in the supernova remnant Cassiopeia A . This was the first radio detection of an interstellar molecule at radio wavelengths. More interstellar OH detections quickly followed and in 1965, Harold Weaver and his team of radio astronomers at Berkeley , identified OH emissions lines coming from the direction of the Orion Nebula and in the constellation of Cassiopeia . In 1968, Cheung, Rank, Townes, Thornton and Welch detected NH₃ inversion line radiation in interstellar space. A year later, Lewis Snyder and his colleagues found interstellar formaldehyde . Also in

8547-715: The ending became ‑our to match the later Old French spelling. The ‑our ending was used not only in new English borrowings, but was also applied to the earlier borrowings that had used ‑or . However, ‑or was still sometimes found. The first three folios of Shakespeare 's plays used both spellings before they were standardised to ‑our in the Fourth Folio of 1685. After the Renaissance , new borrowings from Latin were taken up with their original ‑or ending, and many words once ending in ‑our (for example, chancellour and governour ) reverted to ‑or . A few words of

8658-406: The etymology was not clear, and therefore some scholars advocated ‑or only and others ‑our only. Webster's 1828 dictionary had only -or and is given much of the credit for the adoption of this form in the United States. By contrast, Johnson's 1755 (pre-U.S. independence and establishment) dictionary used -our for all words still so spelled in Britain (like colour ), but also for words where

8769-444: The gas is highly irregular, with most of it concentrated in discrete clouds and cloud complexes. Molecular clouds typically have interstellar medium densities of 10 to 30 cm , and constitute approximately 50% of the total interstellar gas in a galaxy . Most of the gas is found in a molecular state . The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in

8880-685: The influence of the American labor movement and King O'Malley . On top of that, some place names in South Australia such as Victor Harbor , Franklin Harbor or Outer Harbor are usually spelled with the -or spellings. Aside from that, -our is now almost universal in Australia but the -or endings remain a minority variant. New Zealand English , while sharing some words and syntax with Australian English , follows British usage. In British English, some words from French, Latin or Greek end with

8991-570: The larger substructure of the cloud, having the average size of 1 pc . Clumps are the precursors of star clusters , though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are gravitationally bound and go through a collapse during star formation . In astronomical terms, molecular clouds are short-lived structures that are either destroyed or go through major structural and chemical changes approximately 10 million years into their existence. Their short life span can be inferred from

9102-492: The launch of the near-infrared James Webb Space Telescope ) and studied radiation in the far infrared and submillimetre wavebands. Rather than an Earth orbit, Herschel entered into a Lissajous orbit around L 2 , the second Lagrangian point of the Earth-Sun system. L 2 is located approximately 1.5 million km from Earth and the placement of Herschel there lessened the interference by infrared and visible radiation from

9213-401: The mass of the Sun is called a giant molecular cloud ( GMC ). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser than

9324-544: The mass of the cloud has been converted into stars. Stellar winds are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability. Star formation involves the collapse of the densest part of the molecular cloud, fragmenting the collapsed region in smaller clumps. These clumps aggregate more interstellar material, increasing in density by gravitational contraction. This process continues until

9435-486: The molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years ) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on

9546-477: The molecule most often used to determine the presence of H 2 is carbon monoxide (CO). The ratio between CO luminosity and H 2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where much dust and many gas cores reside, called clumps. These clumps are the beginning of star formation if gravitational forces are sufficient to cause

9657-414: The nature of the suffix used. The u is kept before English suffixes that are freely attachable to English words (for example in humourless , neighbourhood , and savoury ) and suffixes of Greek or Latin origin that have been adopted into English (for example in behaviourism , favourite , and honourable ). However, before Latin suffixes that are not freely attachable to English words,

9768-602: The processes taking place in the upper layers of the Earth's atmosphere . The Submillimeter Wave Astronomy Satellite (SWAS) was launched into low Earth orbit on December 5, 1998 as one of NASA 's Small Explorer Program (SMEX) missions. The mission of the spacecraft is to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines: water (H 2 O), isotopic water (H 2 O), isotopic carbon monoxide (CO), molecular oxygen (O 2 ), and neutral carbon (C I). The SWAS satellite

9879-401: The range in age of young stars associated with them, of 10 to 20 million years, matching molecular clouds’ internal timescales. Direct observation of T Tauri stars inside dark clouds and OB stars in star-forming regions match this predicted age span. The fact OB stars older than 10 million years don’t have a significant amount of cloud material about them, seems to suggest most of the cloud

9990-409: The rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars is approximately 3 M ☉ per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 M ☉ of gas being assembled in molecular clouds in

10101-824: The relationship between molecular clouds and star formation. Embedded in the Taurus molecular cloud there are T Tauri stars . These are a class of variable stars in an early stage of stellar development and still gathering gas and dust from the cloud around them. Observation of star forming regions have helped astronomers develop theories about stellar evolution . Many O and B type stars have been observed in or very near molecular clouds. Since these star types belong to population I (some are less than 1 million years old), they cannot have moved far from their birth place. Many of these young stars are found embedded in cloud clusters, suggesting stars are formed inside it. A vast assemblage of molecular gas that has more than 10 thousand times

10212-513: The research that would eventually lead to the detection of molecular clouds. Once the war ended, and aware of the pioneering radio astronomical observations performed by Jansky and 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 telescopes , initiating

10323-460: The same year George Carruthers managed to identify molecular hydrogen . The numerous detections of molecules in interstellar space would help pave the way to the discovery of molecular clouds in 1970. Hydrogen is the most abundant species of atom in molecular clouds, and under the right conditions it will form the H 2 molecule. Despite its abundance, the detection of H 2 proved difficult. Due to its symmetrical molecule, H 2 molecules have

10434-425: The search for the hydrogen signature in the depths of space. The neutral hydrogen atom consists of a proton with an electron in its orbit. Both the proton and the electron have a 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 MHz . This frequency

10545-461: The second most common compound. Molecular clouds also usually contain other elements and compounds. Astronomers have observed the presence of long chain compounds such as methanol , ethanol and benzene rings and their several hydrides . Large molecules known as polycyclic aromatic hydrocarbons have also been detected. The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form

10656-475: The simplified form palestra is described by Merriam-Webster as "chiefly Brit[ish]." This is a reverse of the typical rule, where British spelling uses the ae / oe and American spelling simply uses e . Words that can be spelled either way in British English include cham a eleon , encyclop a edia , hom o eopathy , medi a eval (a minor variant in both AmE and BrE ), f o etid and f o etus . The spellings f o etus and f o etal are Britishisms based on

10767-410: The sounds became monophthongs , and later applied to words not of Greek origin, in both Latin (for example, cœli ) and French (for example, œuvre ). In English, which has adopted words from all three languages, it is now usual to replace Æ/æ with Ae/ae and Œ/œ with Oe/oe . In many words, the digraph has been reduced to a lone e in all varieties of English: for example, o economics , pr

10878-619: The southern skies than Greenland). The Llano de Chajnantor Observatory site hosts the Atacama Pathfinder Experiment (APEX), the largest submillimetre telescope operating in the southern hemisphere, and the world's largest ground based astronomy project, the Atacama Large Millimeter Array (ALMA), an interferometer for submillimetre wavelength observations made of 54 12-metre and 12 7-metre radio telescopes. The Submillimeter Array (SMA)

10989-695: The spelling "is often seen as such". The -ize form is known as Oxford spelling and is used in publications of the Oxford University Press, most notably the Oxford English Dictionary , and of other academic publishers such as Nature , the Biochemical Journal and The Times Literary Supplement . It can be identified using the IETF language tag en-GB-oxendict (or, historically, by en-GB-oed ). In Ireland, India, Australia, and New Zealand -ise spellings strongly prevail:

11100-608: The spellings with just e are increasingly used. Manoeuvre is the only spelling in Australia, and the most common one in Canada, where maneuver and manoeuver are also sometimes found. The -ize spelling is often incorrectly seen in Britain as an Americanism. It has been in use since the 15th century, predating the -ise spelling by over a century. The verb-forming suffix -ize comes directly from Ancient Greek -ίζειν ( -ízein ) or Late Latin -izāre , while -ise comes via French -iser . The Oxford English Dictionary ( OED ) recommends -ize and lists

11211-735: The submillimetre wavelengths remove the ground-based limitations of atmospheric absorption. The first submillimeter telescope in space was the Soviet BST-1M, located in the scientific equipment compartment of the Salyut-6 orbital station . It was equipped with a mirror with a diameter of 1.5 m and was intended for astrophysical research in the ultraviolet (0.2 - 0.36 microns), infrared (60 - 130 microns) and submillimeter (300 - 1000 microns) spectral regions, which are of interest to those who are interested in which makes it possible to study molecular clouds in space, as well as obtain information about

11322-494: The temperature reaches a point where the fusion of hydrogen can occur. The burning of hydrogen then generates enough heat to push against gravity, creating hydrostatic equilibrium . At this stage, a protostar is formed and it will continue to aggregate gas and dust from the cloud around it. One of the most studied star formation regions is the Taurus molecular cloud due to its close proximity to earth (140 pc or 430 ly away), making it an excellent object to collect data about

11433-419: The ultraviolet radiation. The dissociation caused by UV photons is the main mechanism for transforming molecular material back to the atomic state inside the cloud. Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance. Most of the gas constituting a molecular cloud is molecular hydrogen , with carbon monoxide being

11544-412: The vowel is unreduced in pronunciation (e.g., devour , contour , flour , hour , paramour , tour , troubadour , and velour ), the spelling is uniform everywhere. Most words of this kind came from Latin, where the ending was spelled ‑or . They were first adopted into English from early Old French , and the ending was spelled ‑our , ‑or or ‑ur . After the Norman conquest of England ,

11655-624: The word fiord (instead of fjord ) . There is an increasing use of macrons in words that originated in Māori and an unambiguous preference for -ise endings (see below). Most words ending in an unstressed ‑our in British English (e.g., behaviour , colour , favour , flavour , harbour , honour , humour , labour , neighbour , rumour , splendour ) end in ‑or in American English ( behavior , color , favor , flavor , harbor , honor , humor , labor , neighbor , rumor , splendor ). Wherever

11766-428: The word pallor . As a general noun, rigour / ˈ r ɪ ɡ ər / has a u in the UK; the medical term rigor (sometimes / ˈ r aɪ ɡ ər / ) does not, such as in rigor mortis , which is Latin. Derivations of rigour / rigor such as rigorous , however, are typically spelled without a u , even in the UK. Words with the ending -irior , -erior or similar are spelled thus everywhere. The word armour

11877-666: Was not until the mid-1990s that Canadian-based dictionaries became increasingly common.) Worldwide, -ize endings prevail in scientific writing and are commonly used by many international organizations, such as United Nations Organizations (such as the World Health Organization and the International Civil Aviation Organization ) and the International Organization for Standardization (but not by

11988-428: Was once somewhat common in American usage but has disappeared except in some brand names such as Under Armour . The agent suffix -or ( separator , elevator , translator , animator , etc.) is spelled thus both in American and British English. Commonwealth countries normally follow British usage. Canadian English most commonly uses the -our ending and -our- in derivatives and inflected forms. However, owing to

12099-929: Was published. Following this, -re became the most common usage in Britain. In the United States, following the publication of Webster's Dictionary in the early 19th century, American English became more standardized, exclusively using the -er spelling. In addition, spelling of some words have been changed from -re to -er in both varieties. These include September , October , November , December , amber, blister , cadaver , chamber , chapter , charter , cider , coffer , coriander , cover , cucumber , cylinder , diaper , disaster , enter , fever , filter , gender , leper , letter , lobster , master , member , meter (measuring instrument) , minister , monster , murder , number , offer , order , oyster , powder , proper , render , semester , sequester , sinister , sober , surrender , tender , and tiger . Words using

12210-631: Was repurposed in June, 2005 to provide support for the NASA Deep Impact mission. SWAS provided water production data on the comet until the end of August 2005. The European Space Agency launched a space-based mission known as the Herschel Space Observatory (formerly called Far Infrared and Sub-millimetre Telescope or FIRST) in 2009. Herschel deployed the largest mirror ever launched into space (until December 2021, with

12321-437: Was very influential in popularizing certain spellings in the United States, but he did not originate them. Rather [...] he chose already existing options such as center, color and check for the simplicity, analogy or etymology". William Shakespeare 's first folios , for example, used spellings such as center and color as much as centre and colour . Webster did attempt to introduce some reformed spellings, as did

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