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La '''Galassia del Triangolo''', conosciuta anche con il nome di '''M33''' (oggetto numero 33 del catalogo di [[Charles Messier]]) e di '''[[New General Catalogue|NGC]] 598''', è una [[galassia a spirale]] di tipo [[Sequenza di Hubble|Sc]] distante circa 3 milioni di [[anni luce|anno luce]] dalla [[Terra]] e situata nella [[costellazione]] del [[Triangulum|Triangolo]]. Nel mondo anglosassone questa galassia è anche informalmente chiamata ''Pinwheel Galaxy'' (in italiano letteralmente ''Galassia Girandola'') in alcune pubblicazioni amatoriali di astronomia<ref name="omeara98">{{cita libro|autore= S. J. O'Meara | titolo=The Messier Objects | anno=1998 | editore=Cambridge University | città=Cambridge | lingua=en | id=ISBN 0-521-55332-6}}</ref>, ma anche in alcuni comunicati ufficiali di siti professionali<ref name="Spitzer">{{cita web | titolo=NASA Spitzer Telescope Reveals Pinwheel Galaxy's Hidden Wonders | url=http://www.spaceref.com/news/viewpr.html?pid=14321 | lingua=en | accesso=24-11-2009}}</ref>. Tuttavia sia il database professionale ''[[SIMBAD Astronomical Database]]'', che contiene i nomi e le designazioni formali di vari oggetti astronomici, che diversi altri siti per l'astronomia amatoriale indicano con il nome di "Pinwheel Galaxy" la galassia [[M101]]<ref name="simbad_m101">SIMBAD Astronomical Database: risultato per {{cita web | titolo=M101 | url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=M+101 | lingua=en | accesso=24-11-2009}}</ref><ref name="seds_m101">{{cita web | titolo=Messier Object 101 | url=http://www.seds.org/messier/m/m101.html | lingua=en | accesso=24-11-2009}}</ref>.

==Osservazione ==

[[File:Andromeda constellation map.png|left|thumb|170px|Triangulum (M33) and Andromeda (M31)]]

and in some public outreach websites.<ref name="spaceref.com">{{cite web

| title=NASA Spitzer Telescope Reveals Pinwheel Galaxy's Hidden Wonders

| url=http://www.spaceref.com/news/viewpr.html?pid=14321

| accessdate=2007-04-07}}</ref>

However, the [[SIMBAD Astronomical Database]], a professional astronomy database that contains formal designations for astronomical objects, indicates that the name "Pinwheel Galaxy" is used to refer to [[Messier 101]],<ref name="simbadm101">{{cite web

| title=SIMBAD Astronomical Database

| url=http://simbad.u-strasbg.fr/simbad/

| work=Results for Messier 101.

| accessdate=2007-04-07}}</ref>

and several other amateur astronomy resources and other public outreach websites also identify Messier 101 by that name.<ref name="seds">{{cite web

| title=Messier Object 101

| url=http://www.seds.org/messier/m/m101.html

| accessdate=2007-04-07}}</ref><ref name="hubble/esa">{{cite web

| title=Best of AOP: M101: Pinwheel Galaxy

| url=http://simbad.u-strasbg.fr/simbad/

| accessdate=2007-04-07}}</ref>

With a diameter of about 50,000 light-years, it is the third largest galaxy in the [[Local Group]], a [[group of galaxies]] which also contains the [[Milky Way Galaxy]] and the [[Andromeda Galaxy]], and it may be a gravitationally bound companion of the Andromeda Galaxy. Estimates indicate that Triangulum may be home to between 30 and 40 billion stars,<ref>http://home.att.net/~numericana/answer/sagan.htm</ref> compared to the 1000 billion stars for Andromeda and c. 200-400 billion for the Milky Way. The [[Pisces Dwarf]] (LGS 3), one of the small Local Group member galaxies, is possibly a [[satellite]] of Triangulum.

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==General information==

The Triangulum Galaxy can be seen with the naked eye, if the level of light pollution is sufficiently low<ref name="bort">http://www.skyandtelescope.com/resources/darksky/3304011.html</ref>. Being a diffuse object, its visibility is strongly affected by even small amounts of light pollution, ranging from easily visible in direct vision in truly dark skies to a difficult averted vision object in rural/suburban skies<ref name="bort"/>. The fainter and more distant galaxies [[Messier 81]] and [[Centaurus A]] have also been seen with the naked eye by very experienced observers<ref>[http://www.maa.clell.de/Messier/E/Xtra/Supp/m81naked.txt]; [http://astronomy-mall.com/Adventures.In.Deep.Space/aintno.htm]</ref>. However, some [[amateur astronomy|amateur astronomers]] may confuse the object with the nearby [[NGC 752]], an [[open cluster]] that is brighter than the Triangulum Galaxy. {{Citation needed|date=October 2007}} No known pre-telescopic observer notes it, which is not surprising: given its indistinctness, it is not likely to be noticed as an object unless one already knows of its existence.

The Triangulum Galaxy was probably discovered by [[Giovanni Batista Hodierna]] before 1654, who may have grouped it together with [[open cluster]], [[NGC 752]]. It was independently discovered by [[Charles Messier]] in 1764, who catalogued it as [[Messier object|M33]] on August 25. M33 was also catalogued independently by [[William Herschel]] on September 11, 1784 number H V.17. It was among the first "[[spiral nebulae]]" identified as such by [[William Parsons, 3rd Earl of Rosse|Lord Rosse]].

[[Image:Triangulum.nebula.full.jpg|left|thumb|170px|[[NGC 604]] in the Triangulum Galaxy]]

Herschel also cataloged The Triangulum Galaxy's brightest and largest [[H II region]] (diffuse [[emission nebula]] containing [[ion]]ized [[hydrogen]]) as H III.150 separately from the galaxy itself, which eventually obtained [[NGC 604|NGC number 604]]. As seen from Earth NGC 604 is located northeast of the galaxy's central core, and is one of the largest H II regions known with a diameter of nearly 1500 [[light-year]]s and a [[electromagnetic spectrum|spectrum]] similar to the [[Orion Nebula]]. Herschel also noted 3 other smaller H II regions (NGC 588, 592 and 595).

In 2005, using observations of two [[Astrophysical maser|water masers]] on opposite sides of Triangulum via the [[VLBA]], researchers were, for the first time, able to estimate the angular rotation and proper motion of Triangulum. A velocity of 190 ± 60 [[kilometre|km]]/[[second|s]] relative to the Milky Way is computed which means Triangulum is moving towards [[Andromeda Galaxy|Andromeda]].<ref>{{cite journal

| author=Brunthaler, Andreas; Reid, Mark J.; Falcke, Heino; Greenhill, Lincoln J.; Henkel, Christian

| year=2005

| title=The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)

| journal=Science

| volume=307

| issue=5714

| pages=1440–1443

| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005Sci...307.1440B

| doi=10.1126/science.1108342

| pmid=15746420

}}</ref>

In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy using data from the [[Chandra X-ray Observatory]]. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days.<ref>Morcone, Jennifer, [http://chandra.harvard.edu/press/07_releases/press_101707.html Heaviest Stellar Black Hole Discovered in Nearby Galaxy], [[Chandra X-ray Observatory]] press release, {{date|2007-10-17|mdy}}</ref>

The galaxy has an H&nbsp;II nucleus.<ref name="hoetal1997">{{citation

| last1 = Ho

| first1 = Luis C.

| last2 = Filippenko

| first2 = Alexei V.

| last3 = Sargent

| first3 = Wallace L. W.

| title = A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies

| year = 1997

| date = October 1997

| periodical = Astrophysical Journal Supplement

| volume = 112

| pages = 315–390

| url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ApJS..112..315H

| doi = 10.1086/313041

| author = Ho, Luis C.

| journal = The Astrophysical Journal Supplement Series

}}</ref>

<references/>

| accessdate=2007<!-04-07}}</ref> <ref name="hubble/esa">{{cite web

| title=Best of AOP: M101: Pinwheel Galaxy

| url=http://simbad.u-strasbg.fr/simbad/

| accessdate=2007-04-07}}</ref> -->