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Triangulum Galaxy
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==Observation history== The Triangulum Galaxy was probably discovered by the Italian astronomer [[Giovanni Battista Hodierna]] before 1654. In his work [[De systemate orbis cometici, deque admirandis coeli characteribus|''De systemate orbis cometici; deque admirandis coeli caracteribus'']] ("About the systematics of the cometary orbit, and about the admirable objects of the sky"), he listed it as a cloud-like nebulosity or obscuration and gave the cryptic description, "near the Triangle ''hinc inde''". This is in reference to the constellation Triangulum as a pair of triangles. The magnitude of the object matches M33, so it is most likely a reference to the Triangulum Galaxy.<ref name=jha16_1/> The galaxy was independently discovered by [[Charles Messier]] on the night of August 25β26, 1764. It was published in his ''Catalog of Nebulae and Star Clusters'' (1771) as object number 33; hence the name M33.<ref>{{Cite web|last=|title=Triangulum Galaxy Snapped by VST|url=https://www.eso.org/public/news/eso1424/|access-date=2021-05-16|website=www.eso.org|language=en}}</ref> When [[William Herschel]] compiled his extensive catalog of nebulae, he was careful not to include most of the objects identified by Messier.<ref name=jones91/> However, M33 was an exception, and he cataloged this object on September 11, 1784, as H V-17.<ref name=mullaney07/> Herschel also cataloged the Triangulum Galaxy's brightest and largest H [[H II region|II region]] (diffuse [[emission nebula]] containing [[ion]]ized [[hydrogen]]) as H III.150 separately from the galaxy itself; the nebula eventually obtained NGC [[NGC 604|number 604]]. As seen from [[Earth]], NGC 604 is located northeast of the galaxy's central core. It is one of the largest H II regions known, with a diameter of nearly 1500 [[light-years]] and a [[electromagnetic spectrum|spectrum]] similar to that of the [[Orion Nebula]]. Herschel also noted three other smaller H II regions (NGC 588, 592, and 595). It was among the first "[[spiral nebulae]]" identified as such by [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] in 1850. In 1922β23, [[John Charles Duncan]] and [[Max Wolf]] discovered variable stars in the nebulae. [[Edwin Hubble]] showed in 1926 that 35 of these stars were [[Classical Cepheid variable|classical Cepheids]], thereby allowing him to estimate their distances. The results were consistent with the concept of spiral nebulae being independent galactic systems of gas and dust, rather than just nebulae in the Milky Way.<ref name=bergh2000/> <gallery widths="200" heights="200"> File:Nursery of New Stars - GPN-2000-000972.jpg|[[NGC 604]] in the Triangulum Galaxy File:Messier33 - HST - Heic1901a.jpg|Composite of about 54 different pointings with Hubble's Advanced Camera for Surveys<ref>{{cite web |title=Hubble takes gigantic image of the Triangulum Galaxy |url=https://www.spacetelescope.org/news/heic1901/ |publisher=www.spacetelescope.org |access-date=8 January 2019 |language=en}}</ref> </gallery>
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