Editing Triangulum Galaxy

{{Short description|Spiral galaxy in the constellation Triangulum}}
{{Sky|01|33|50.9||30|39|36|2810}}
{{Infobox galaxy
| name = Triangulum Galaxy
| image = [[File:VST snaps a very detailed view of the Triangulum Galaxy.jpg|250px]]
| caption = Galaxy Messier 33 in Triangulum (the Triangulum Galaxy)
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| pronounce = {{IPAc-en|t|r|aɪ|ˈ|æ|ŋ|ɡ|j|ʊ|l|ə|m}}
| constellation name = [[Triangulum]]
| ra = {{RA|01|33|50.02}}<ref name="SIMBAD-M33"/>
| dec = {{DEC|+30|39|36.7}}<ref name="SIMBAD-M33"/>
| z = {{nowrap|-0.000607 ± 0.000010}}<ref name="SIMBAD-M33"/>
| h_radial_v = {{nowrap|-179 ± 3 [[Metre per second|km/s]]}}<ref name=ned/>
| gal_v = {{nowrap|-44 ± 6 km/s}}<ref name=ned/>
| dist_ly = {{convert|970|kpc|Mly|abbr=on|lk=on }}<ref name=vivian/>
| type = SA(s)cd<ref name=ned/>
| mass = {{Val|5e10}}<ref name=mnras342_1/>
| stars = 40 billion ({{Val|4|e=10}})<ref name=michon/>
| appmag_v = 5.72<ref name="SIMBAD-M33"/>
| size_v = {{nowrap|70.8 × 41.7 [[arcminutes]]}}<ref name="SIMBAD-M33" />
| size = {{cvt|18.74|kpc|ly|sigfig=4|lk=on}}<br />(diameter; 25.0 mag/arcsec<sup>2</sup> B-band isophote)<ref name="RC3">{{Cite book |last1=De Vaucouleurs |first1=Gerard |title=Third Reference Catalogue of Bright Galaxies |last2=De Vaucouleurs |first2=Antoinette |last3=Corwin |first3=Herold G. |last4=Buta |first4=Ronald J. |last5=Paturel |first5=Georges |last6=Fouque |first6=Pascal |year=1991 |bibcode=1991rc3..book.....D}}</ref><ref name=michon/>
| notes =
| names = {{odlist | NGC= 598 | MCG= +05-04-069 | UGC= 1117 | PGC= 5818 | CGCG= 502-110 | IRAS= 01310+3024}}<ref name=ned/>
}}

The '''Triangulum Galaxy''' is a [[spiral galaxy]] 2.73&nbsp;million [[light-year]]s (ly) from Earth in the [[constellation]] [[Triangulum]]. It is catalogued as '''Messier 33''' or '''NGC 598'''. With the D<sub>25</sub> isophotal diameter of {{convert|18.74|kpc|ly|sigfig=3|abbr=off|lk=on}}, the Triangulum Galaxy is the third-largest member of the [[Local Group]] of galaxies, behind the [[Andromeda Galaxy]] and the [[Milky Way]].

The galaxy is the second-smallest spiral galaxy in the Local Group after the [[Large Magellanic Cloud]], which is a [[Magellanic spiral|Magellanic-type]] spiral galaxy.<ref name="rydenpeterson">{{Cite book |last1=Ryden |first1=Barbara |author-link1=Barbara Ryden |title=Foundations of astrophysics |last2=Peterson |first2=Bradley M. |date=2010 |publisher=[[Addison-Wesley]] |isbn=978-0-321-59558-4 |location=San Francisco}}</ref> It is believed to be a satellite of the Andromeda Galaxy or on its rebound into the latter due to their interactions, velocities,<ref name="science307_5714" /> and proximity to one another in the night sky. It also has an [[H II region|H II]] [[active galactic nucleus|nucleus]].<ref name="hoetal1997">{{Cite journal |last1=Ho |first1=Luis C. |last2=Filippenko |first2=Alexei V. |last3=Sargent |first3=Wallace L. W. |date=October 1997 |title=A Search for ''Dwarf'' Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies |journal=[[The Astrophysical Journal Supplement Series]] |language=en |volume=112 |issue=2 |pages=315–390 |arxiv=astro-ph/9704107 |bibcode=1997ApJS..112..315H |doi=10.1086/313041 |issn=0067-0049 |s2cid=17086638}}</ref>

==Etymology==
The galaxy gets its name from the constellation [[Triangulum]], where it can be spotted.  

It is sometimes informally referred to as the "[[Pinwheel Galaxy (disambiguation)|Pinwheel Galaxy]]" by some astronomy references,<ref name=omeara1998/> in some computerized telescope software, and in some public outreach websites.<ref name="spaceref.com"/> However, the [[SIMBAD Astronomical Database]], a professional database, collates formal designations for astronomical objects and indicates that [[Pinwheel Galaxy]] refers to Messier 101,<ref name=simbadm101/> which several amateur astronomy resources including public outreach websites identify by that name, and that is within the bounds of [[Ursa Major]].<ref name="hubble/esa"/>

==Visibility==
Under exceptionally good viewing conditions with no [[light pollution]], the Triangulum Galaxy can be seen by some people with the fully dark-adapted [[naked eye]];<ref name=bort/> to those viewers, it is the farthest permanent entity visible without magnification, being about half again as distant as Messier 31, the Andromeda Galaxy.<ref name=naeye08/><ref name=skiff97/> It is a diffuse, or extended, object rather than a starlike point, even without magnification, because of its physical extent.

Its observability without optical aid ranges from being relatively easily seen by people using direct vision in deep rural locations under a dark, clear, transparent sky, to requiring use of [[averted vision]] by observers in locations beyond the suburbs in shallow rural areas under good viewing conditions.<ref name=bort/> It is one of the reference objects of the [[Bortle Dark-Sky Scale]].

[[Andrew Crumey|Crumey]] has shown that although the total apparent V-magnitude of M33 is 5.72, it has an effective visual magnitude of approximately 6.6, meaning that a precondition for visibility is that the observer can see stars at least as faint as that latter figure.<ref>{{Cite journal |last=Crumey |first=Andrew |author-link=Andrew Crumey |date=11 August 2014 |title=Human contrast threshold and astronomical visibility |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=442 |issue=3 |pages=2600–2619 |arxiv=1405.4209 |bibcode=2014MNRAS.442.2600C |doi=10.1093/mnras/stu992 |issn=1365-2966 |doi-access=free}}</ref> This is fainter than many people are able to see, even at a very dark site.<ref>{{Cite journal |last=Weaver |first=Harold F. |date=October 1947 |title=The Visibility of Stars Without Optical Aid |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=59 |issue=350 |pages=232 |bibcode=1947PASP...59..232W |doi=10.1086/125956 |issn=0004-6280 |s2cid=51963530 }}</ref>

==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 25–26, 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&nbsp;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 1922–23, [[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>

==Properties==
The Triangulum Galaxy is the third largest member of the [[Local Group]] of galaxies. It has a diameter measured through the D<sub>25</sub> standard - the [[isophote]] where the surface brightness of the galaxy reaches 25 mag/arcsec<sup>2</sup>, to be about {{convert|18.74|kpc|ly|sigfig=3|abbr=off}},<ref name="RC3" /> making it roughly 70% the size of the Milky Way. It may be a gravitationally bound [[#Relationship with the Andromeda Galaxy|companion of the Andromeda Galaxy]]. Triangulum may be home to 40&nbsp;billion stars, compared to 400&nbsp;billion for the Milky Way and 1 trillion for Andromeda.<ref name=michon/>

The disk of Triangulum has an estimated mass of {{nowrap|(3–6) × 10<sup>9</sup>}} [[solar mass]]es, while the gas component is about {{nowrap|3.2 × 10<sup>9</sup>}} solar masses. Thus, the combined mass of all [[baryon]]ic matter in the galaxy may be 10<sup>10</sup> solar masses. The contribution of the [[dark matter|dark]] matter component out to a radius of {{Convert|55|e3ly|kpc|abbr=on}} is equivalent to about {{nowrap|5 × 10<sup>10</sup>}} solar masses.<ref name=mnras342_1/>

=== Location – distance – motion ===
[[File:Andromeda constellation map (1).png|thumb|upright=1.45|Triangulum (M33; lower left of center) and Andromeda Galaxy ([[Andromeda Galaxy|M31]]; above center)]]
Estimates of the distance from the Milky Way to the Triangulum Galaxy range from {{Convert|2380|to|3070|e3ly|kpc|abbr=on|lk=on}} (or 2.38 to 3.07&nbsp;[[Mly]]), with most estimates since the year 2000 lying in the middle portion of this range,<ref name=aass06/><ref name=apj696/> making it slightly more distant than the Andromeda Galaxy ([[Andromeda Galaxy#Distance estimate|at 2,540,000 light-years]]). At least three techniques have been used to measure distances to M 33. Using the [[Cepheid variable]] method, an estimate of {{Convert|2770|+/-|130|e3ly|kpc|abbr=on}} was achieved in 2004.<ref name=aj127/><ref name=ap49/> In the same year, the [[tip of the red-giant branch]] (TRGB) method was used to derive a distance estimate of {{Convert|2590|+/-|80|e3ly|kpc|abbr=on}}.<ref name=mnras350/> The Triangulum Galaxy is around 750,000 light years from the Andromeda Galaxy.<ref>{{cite web |url=http://www.messier.seds.org/m/m033.html |title=Messier 33 |access-date=22 July 2024 |website=SEDS Messier Catalog}}</ref>

In 2006, a group of astronomers announced the discovery of an [[eclipsing binary|eclipsing binary star]] in the Triangulum Galaxy. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure the [[absolute magnitude|absolute]] magnitude of the stars. When the [[Apparent magnitude|visual]] and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of {{Convert|3070|+/-|240|e3ly|kpc|abbr=on}}.<ref name=aass06/>  The average of 102 distance estimates published since 1987 gives a [[distance modulus]] of 24.69, or .883 [[Megaparsec|Mpc]] (2,878,000 light-years).<ref>{{Cite web | url=http://ned.ipac.caltech.edu/cgi-bin/objsearch?objname=messier+33&extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES | title=Your NED Search Results}}</ref>

The Triangulum Galaxy is a source of [[Astrophysical maser|H<sub>2</sub>O maser]] emission.<ref name=apj708/> In 2005, using observations of two water masers on opposite sides of Triangulum via the [[Very Long Baseline Array|VLBA]], researchers were for the first time able to estimate the angular rotation and [[proper motion]] of Triangulum. A velocity of {{nowrap|190 ± 60&nbsp;km/s}} relative to the Milky Way was computed, which means Triangulum is moving towards [[Andromeda Galaxy]] and suggesting it may be a satellite of the larger galaxy (depending on their relative distances and margins of error).<ref name=science307_5714/> 

In 2004, evidence was announced of a clumpy stream of hydrogen gas linking the Andromeda Galaxy with Triangulum, suggesting that the two may have tidally interacted in the past. This discovery was confirmed in 2011.<ref name=nra020120613/>  A distance of less than 300 kiloparsecs between the two supports this hypothesis.<ref name="pawlowski">{{Cite journal |last1=Pawlowski |first1=Marcel S. |last2=Kroupa |first2=Pavel |last3=Jerjen |first3=Helmut |date=November 2013 |title=Dwarf galaxy planes: the discovery of symmetric structures in the Local Group |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=435 |issue=3 |pages=1928–1957 |arxiv=1307.6210 |bibcode=2013MNRAS.435.1928P |doi=10.1093/mnras/stt1384 |issn=1365-2966 |s2cid=53991672 |doi-access=free}}</ref>

The [[Pisces Dwarf]] (LGS 3), one of the small Local Group member galaxies, is located {{Convert|2022|e3ly|kpc|abbr=on}} from the Sun. It is 20° from the [[Andromeda Galaxy]] and 11° from Triangulum. As LGS 3 lies at a distance of {{Convert|913|e3ly|kpc|abbr=on}} from both galaxies, it could be a [[satellite galaxy]] of either Andromeda or Triangulum. LGS 3 has a core radius of {{Convert|483|ly|pc|abbr=on}} and {{nowrap|2.6 × 10<sup>7</sup>}} solar masses.<ref name=apj562_2/>

Pisces VII/Triangulum (Tri) III may be another satellite of Triangulum.<ref>{{Cite web |last=Lazaro |first=Enrico de |date=18 November 2021 |title=Amateur Astronomer Discovers New Dwarf Galaxy |url=https://www.sci.news/astronomy/pisces-vii-dwarf-galaxy-10283.html |website=Sci.News |language=en-US}}</ref>

===Structure===
{{multiple image
| align             = right
| direction         = horizontal
| image1            = Spitzer m33.jpg
| width1            = 
| alt1              = 
| caption1          = [[Infrared]] image of M33 taken with the [[Spitzer Space Telescope]]
| alt               = In the center is an irregular spiral ellipse of red dust and a haze of blue light from the stars. The black surroundings are dotted with individual stars.
| image2            = M33.jpg
| width2            = 
| alt2              = 
| caption2          = Ultraviolet image of M33 by [[GALEX]] observatory
| total_width       = 400
}}

In the French astronomer [[Gérard de Vaucouleurs]]' revised Hubble Sandage (VRHS) system of [[galaxy morphological classification]], the Triangulum Galaxy is classified as type SA(s)cd. The ''S'' prefix indicates that it is a disk-shaped galaxy with prominent arms of gas and dust that spiral out from the nucleus—what is commonly known as a [[spiral galaxy]]. The ''A'' is assigned when the galactic nucleus lacks a bar-shaped structure, in contrast to ''SB'' class [[barred spiral galaxy|barred spiral galaxies]]. American astronomer [[Allan Sandage]]'s "(s)" notation is used when the spiral arms emerge directly from the nucleus or central bar, rather than from an inner ring as with an (r)-type galaxy. Finally, the ''cd'' suffix represents a stage along the spiral sequence that describes the openness of the arms. A rating of ''cd'' indicates relatively loosely wound arms.<ref name=buta_corwin_odewahn07/>

This galaxy has an inclination of 54° to the line of sight from Earth, allowing the structure to be examined without significant obstruction by gas and dust.<ref name=apj602/><ref name=aaa493/> The disk of the Triangulum Galaxy appears warped out to a radius of about 8 kpc. There may be a halo surrounding the galaxy, but there is no bulge at the nucleus.<ref name=aaa506/> This is an isolated galaxy and there are no indications of recent mergers or interactions with other galaxies,<ref name=aaa493/> and it lacks the dwarf spheroidals or [[tidal tail]]s associated with the Milky Way.<ref name=aa58/>

Triangulum is classified as unbarred, but an analysis of the galaxy's shape shows what may be a weak bar-like structure about the galactic nucleus. The radial extent of this structure is about 0.8&nbsp;kpc.<ref name=hernandez_lopez_et_al_09/> 

The nucleus of this galaxy is an [[H&nbsp;II region]],<ref name=apj708/> and it contains an [[ultraluminous X-ray source]] with an emission of {{nowrap|1.2 × 10<sup>39</sup> [[erg]] s<sup>−1</sup>}}, which is the most luminous source of X-rays in the [[Local Group]] of galaxies. This source is modulated by 20% over a 106-day cycle.<ref name=aaa425/> However, the nucleus does not appear to contain a [[supermassive black hole]], as a best-fit value of zero mass and an upper limit of {{solar mass|1,500}} is placed on the mass of a central black hole based on models and the [[Hubble Space Telescope]] (HST) data.<ref name=science122_5/> This is significantly lower than the mass expected from the velocity dispersion of the nucleus and far below any mass predicted from the disk kinematics.<ref name=science122_5/> This may suggest that supermassive black holes are associated only with galaxy bulges instead of with their disks.<ref name=science122_5/> Assuming that the upper limit of the central black hole is correct, it would be rather an [[intermediate-mass black hole]].

The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features.<ref name=apj602/><ref name=aaa493/> The main arms are designated IN (north) and IS (south).<ref name=aaa205_1_2/>

===Star formation===
[[File:M33.tif|thumb|[[NGC 604]], a star-forming region in the Triangulum Galaxy, as imaged by the [[Hubble Space Telescope]]]]

In the central 4′ region of this galaxy, atomic gas is being efficiently converted to molecular gas, resulting in a strong [[Spectral line|spectral emission]] of [[carbon monoxide|CO]]. This effect occurs as giant [[molecular cloud]]s condense out of the surrounding [[interstellar medium]]. A similar process is taking place outside the central 4′, but at a less efficient pace. About 10% of the gas content in this galaxy is in the molecular form.<ref name=apj602/><ref name=aaa493/>

[[Star formation]] is taking place at a rate that is strongly correlated with local gas density, and the rate per unit area is higher than in the neighboring [[Andromeda Galaxy]]. (The rate of star formation is about 3.4 solar masses&nbsp;Gyr<sup>−1</sup>&nbsp;pc<sup>−2</sup> in the Triangulum Galaxy, compared to 0.74 in Andromeda.<ref name="aaa495"/>) The total integrated rate of star formation in the Triangulum Galaxy is about {{nowrap|0.45 ± 0.1 [[solar mass]]es per year}}. It is uncertain whether this net rate is currently decreasing or remaining constant.<ref name=apj602/><ref name=aaa493/>

Based on analysis of the chemical composition of this galaxy, it appears to be divided into two distinct components with differing histories. The inner disk within a radius of {{Convert|30|e3ly|kpc|abbr=on|sigfig=1}} has a typical composition gradient that decreases linearly from the core. Beyond this radius, out to about {{Convert|82|e3ly|kpc|abbr=on}}, the gradient is much flatter. This suggests a different star formation history between the inner disk and the outer disk and halo, and may be explained by a scenario of "inside-out" galaxy formation.<ref name=aaa506/> This occurs when gas is accumulated at large radii later in a galaxy's life space, while the gas at the core becomes exhausted. The result is a decrease in the average age of stars with increasing radius from the galaxy core.<ref name=apjl695/>

===Discrete features===
Using infrared observations from the [[Spitzer Space Telescope]], a total of 515 discrete candidate sources of 24&nbsp;μm emission within the Triangulum Galaxy have been catalogued as of 2007. The brightest sources lie within the central region of the galaxy and along the spiral arms.

Many of the emission sources are associated with [[H II region]]s of star formation.<ref name=aaa476/> The four brightest HII regions are designated [[NGC 588]], [[NGC 592]], [[NGC 595]], and [[NGC 604]]. These regions are associated with [[molecular cloud]]s containing {{nowrap|(1.2–4) × 10<sup>5</sup>}} solar masses. The brightest of these regions, NGC 604, may have undergone a discrete outburst of star formation about three million years ago.<ref name=apj128_1/> This nebula is the second most luminous HII region within the Local Group of galaxies, at {{nowrap|(4.5 ± 1.5) × 10<sup>7</sup>}} times the [[Solar luminosity|luminosity of the Sun]].<ref name=aaa495/> Other prominent HII regions in Triangulum include IC&nbsp;132, IC&nbsp;133, and IK&nbsp;53.<ref name=aaa205_1_2/>

The northern main spiral arm contains four large [[HII region]]s, while the southern arm has greater concentrations of young, hot stars.<ref name=aaa205_1_2/> The estimated rate of [[supernova]] explosions in the Triangulum Galaxy is 0.06 [[Type Ia supernova|Type Ia]] and 0.62 [[Type Ib and Ic supernovae|Type Ib]]/[[Type II supernova|Type II]] per century. This is equivalent to a supernova explosion every 147 years, on average.<ref name=apjss92_2/> As of 2008, a total of 100 [[supernova remnant]]s have been identified in the Triangulum Galaxy,<ref name=apjss174_2/> the majority of which lie in the southern half of the spiral galaxy. Similar asymmetries exist for H I and H II regions, plus highly luminous concentrations of massive, [[O-type main sequence star|O type stars]]. The center of the distribution of these features is offset about two arc minutes to the southwest.<ref name=aaa205_1_2/> M33 being a local galaxy, the [[Central Bureau for Astronomical Telegrams]] (CBAT) tracks [[nova]]e in it along with M31 and [[Messier 81|M81]].<ref name="supernovae.net">
{{cite web
 |author      = David Bishop
 |title       = Extragalactic Novae
 |url         = http://www.supernovae.net/novae.html
 |publisher   = supernovae.net ([[International Supernovae Network]])
 |access-date  = 2010-09-11
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20100408150756/http://www.supernovae.net/novae.html
 |archive-date = 2010-04-08
}}</ref>

About 54 [[globular cluster]]s have been identified in this galaxy, but the actual number may be 122 or more.<ref name=aa58/> The confirmed clusters may be several billion years younger than globular clusters in the Milky Way, and cluster formation appears to have increased during the past 100&nbsp;million years. This increase is correlated with an inflow of gas into the center of the galaxy. The [[ultraviolet]] emission of massive stars in this galaxy matches the level of similar stars in the [[Large Magellanic Cloud]].<ref name=grebel99/>

In 2007, a [[black hole]] about 15.7 times the [[Solar mass|mass of the Sun]] was detected in this 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. It is the largest [[stellar mass black hole]] known.<ref name=ar53_3/><ref name=morcone07/>

Unlike the Milky Way and Andromeda galaxies, the Triangulum Galaxy does not appear to have a [[supermassive black hole]] at its center.<ref name=science293_5532/>  This may be because the mass of a galaxy's central supermassive black hole correlates with the size of the galaxy's [[Galactic bulge|central bulge]], and unlike the Milky Way and Andromeda, the Triangulum Galaxy is a pure [[disk galaxy]] with no bulge.<ref name="science122_5">{{Cite journal |last1=Gebhardt |first1=Karl |last2=Lauer |first2=Tod R. |last3=Krmendy |first3=John |last4=Pinkney |first4=Jason |last5=Bower |first5=Gary A. |last6=Green |first6=Richard |last7=Gull |first7=Theodore |last8=Hutchings |first8=J.B. |last9=Kaiser |first9=M.E. |last10=Nelson |first10=Charles H. |date=November 2001 |title=M33: A Galaxy with No Supermassive Black Hole |journal=[[Astronomical Journal]] |volume=122 |issue=5 |pages=2469–2476 |arxiv=astro-ph/0107135 |bibcode=2001AJ....122.2469G |doi=10.1086/323481 |s2cid=118864132 }}</ref>

===Relationship with the Andromeda Galaxy===
[[File:Collision paths of our Milky Way galaxy and the Andromeda galaxy.jpg|thumb|upright=1.35|Triangulum on the collision paths of the Milky Way and Andromeda galaxies]]

As mentioned above, M33 is linked to M31 by several streams of [[Hydrogen line|neutral hydrogen]]<ref name=triangulumvsandromeda>{{cite web | work=New Scientist | title=Milky Way's twin caught dismembering neighbour  | url=https://www.newscientist.com/article/dn17728-milky-ways-twin-caught-dismembering-neighbour.html | access-date=2012-07-31}}</ref> and stars,<ref name=triangulumvsandromeda/> which suggests that a past interaction between these two galaxies took place from 2 to 8&nbsp;billion years ago,<ref name="Davidge & McCoonnachie 2012">
{{cite journal
| last1 = Davidge
| first1 = T. J.
| last2 = McConnachie
| display-authors = 4
| first2 = A. W.
| last3 = Fardal
| first3 = M. A.
| last4 = Fliri
| first4 = J.
| last5 = Valls-Gabaud
| first5 = D.
| last6 = Chapman
| first6 = S. C.
| last7 = Lewis
| first7 = G. F.
| last8 = Rich
| first8 = R. M.
| date = 2012
| title = The Recent Stellar Archeology of M31 – The Nearest Red Disk Galaxy
| journal = [[The Astrophysical Journal]]
| volume = 751
| issue = 1
| bibcode = 2012ApJ...751...74D
| doi = 10.1088/0004-637X/751/1/74
|arxiv = 1203.6081 | pages=74 | s2cid = 59933737
}}</ref><ref name=Bekki2008>{{cite journal | author=Bekki K.| title=Formation of a giant HI bridge between M31 and M33 from their tidal interaction | journal=Monthly Notices of the Royal Astronomical Society Letters | volume=390 | issue=1 |date=October 2008 | pages=L24–L28 | bibcode=2008MNRAS.390L..24B |arxiv = 0807.1161 |doi = 10.1111/j.1745-3933.2008.00528.x | doi-access=free | s2cid=119090934 }}</ref> and a more violent encounter will occur 2.5&nbsp;billion years in the future.<ref name=triangulumvsandromeda/>

The fate of M33 was uncertain in 2009 beyond seeming to be linked to its larger neighbor M31. Suggested scenarios include being torn apart and absorbed by the greater companion, fueling the latter with [[hydrogen]] to form new stars; eventually exhausting all of its gas, and thus the ability to form new stars;<ref name=Putman2009>{{cite journal | display-authors=1 | last1=Putman | first1=M. E. | last2=Peek | first2=J. E. G. | last3=Muratov | first3=A. | last4=Gnedin | first4=O. Y. | last5=Hsu | first5=W. | last6=Douglas | first6=K. A. | last7=Heiles | first7=C. | last8=Stanimirovic | first8=S. | last9=Korpela | first9=E. J. | first10=S. J. | title=The Disruption and Fueling of M33 | journal=The Astrophysical Journal | volume=703 | issue=2 |date=October 2009 | pages=1486–1501 | bibcode=2009ApJ...703.1486P|arxiv = 0812.3093 |doi = 10.1088/0004-637X/703/2/1486 | last10=Gibson | s2cid=119310259 }}</ref> or participating in the [[Andromeda–Milky Way collision|collision between the Milky Way and M31]], likely ending up orbiting the merger product and fusing with it much later. Two other possibilities are a collision with the Milky Way before the Andromeda Galaxy arrives or an ejection out of the Local Group.<ref name=Roeland2012>{{cite journal | display-authors=1 | first1=Roeland P. | last1=van der Marel | first2=Gurtina | last2=Besla | first3=T. J. | first4=Sangmo Tony | last4=Sohn | first5=Jay | last5=Anderson | title=The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun | journal=The Astrophysical Journal | volume=753 | issue=1 |date=July 2012 | bibcode=2012ApJ...753....9V|arxiv = 1205.6865 |doi = 10.1088/0004-637X/753/1/9 | last3=Cox | pages=9  | s2cid=53071454 }}</ref> Astrometric data from [[Gaia (spacecraft)|Gaia]] appears in 2019 to rule out the possibility that M33 and M31 are in orbit. If correct, M33 is on its first infall proper into the Andromeda Galaxy (M31).<ref>{{cite journal | display-authors=1 |first1=Roeland P. |last1=van der Marel |first2=Mark A. |last2=Fardal |first3=Sangmo Tony |last3=Sohn |first4=Ekta |last4=Patel |first5=Gurtina |last5=Besla |first6=Andrés del |last6=Pino |first7=Johannes |last7=Sahlmann |first8=Laura L. |last8=Watkins |title=First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33 |journal=The Astrophysical Journal | volume=872 | issue=1 |date=7 February 2019 |pages = 24|doi=10.3847/1538-4357/ab001b |arxiv=1805.04079 |bibcode=2019ApJ...872...24V |s2cid=119011033 |doi-access=free }}</ref>

==See also==
* [[List of galaxies]]
* [[Messier object]]
* [[List of Messier objects]]
* ''[[New General Catalogue]]''
* [[NGC 55]]
* [[Pisces Dwarf]]
*[[Andromeda–Milky Way collision|Andromeda-Milky Way collision]]

==References==
{{Reflist|colwidth=25em|refs=

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<ref name=wilson_mitchell>{{cite web | author=Wilson, Barbara | author2=Mitchell, Larry | title=The Revised AINTNO 100 | publisher=Astronomy-Mall | url=http://astronomy-mall.com/Adventures.In.Deep.Space/aintno.htm | access-date=2010-02-11 }}</ref>
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<ref name=aaa205_1_2>{{cite journal | last=Buczilowski | first=U. R. | title=A multifrequency radio continuum survey of M33. II – Thermal and non-thermal emission | journal=Astronomy and Astrophysics | volume=205 | issue=1–2 |date=October 1988 | pages=29–40 | bibcode=1988A&A...205...29B }}</ref>

<ref name=apjss92_2>{{cite journal | author=Tammann, G. A. | author2=Loeffler, W. | author3=Schroeder, A. | title=The Galactic supernova rate | journal=The Astrophysical Journal Supplement Series | volume=92 | issue=2 | pages=487–493 |date=June 1994 | bibcode=1994ApJS...92..487T | doi=10.1086/192002 | doi-access=free }}</ref>

<ref name=apjss174_2>{{cite journal | display-authors=4 | author=Plucinsky, Paul P. |author2=Williams, Benjamin | title=Chandra ACIS Survey of M33 (ChASeM33): A First Look | journal=The Astrophysical Journal Supplement Series | issue=2 | pages=366–378 |date=February 2008 | doi=10.1086/522942 | bibcode=2008ApJS..174..366P | volume=174 |arxiv = 0709.4211 |last3=Long  |last4=Gaetz  |last5=Sasaki  |last6=Pietsch  |last7=Tüllmann  |last8=Smith  |last9=Blair  |last10=Helfand  |last11=Hughes  |last12=Winkler  |last13=De Avillez  |last14=Bianchi  |last15=Breitschwerdt  |last16=Edgar  |last17=Ghavamian  |last18=Grindlay  |last19=Haberl  |last20=Kirshner  |last21=Kuntz  |last22=Mazeh  |last23=Pannuti  |last24=Shporer  |last25=Thilker  | s2cid=18857065 }}</ref>

<ref name=naeye08>The following source lists it as the most distant object:
:{{cite web | first=Robert | last=Naeye | title=A Stellar Explosion You Could See on Earth! | date=March 21, 2008 | publisher=NASA's Goddard Space Flight Center | url=http://www.nasa.gov/mission_pages/swift/bursts/brightest_grb.html | access-date=2010-04-13 | archive-date=2012-03-03 | archive-url=https://web.archive.org/web/20120303175418/http://www.nasa.gov/mission_pages/swift/bursts/brightest_grb.html | url-status=dead }}
However, the more distant galaxy [[Messier 81]] has also been sighted with the naked eye:
:{{cite web | author=Christensen, Lars Lindberg | author2=Zezas, Andreas | author3=Noll, Keith | author4=Villard, Ray | date=May 28, 2007 | title=Hubble photographs grand spiral galaxy Messier 81 | publisher=ESA | url=http://www.spacetelescope.org/news/heic0710/ | access-date=2010-06-15 }}</ref>

<ref name=bergh2000>{{cite book | first=Sidney | last=Van den Bergh | title=The galaxies of the Local Group | volume=35 | series=Cambridge astrophysics series | publisher=Cambridge University Press | date=2000 | isbn=978-0-521-65181-3 | page=72 }}</ref>

<ref name=nra020120613>{{cite news | first1=Dave | last1=Finley | title=Neighbor galaxies may have brushed closely, astronomers find | publisher=National Radio Astronomy Observatory | date=June 11, 2012 | url=http://www.nrao.edu/pr/2012/m31m33/ | access-date=2012-06-13 }}</ref>
}}

==Further reading==
* {{cite book |last=Hodge |first=Paul |author-link=Paul W. Hodge |title=The Spiral Galaxy M33 |series=Astrophysics and Space Science Library |volume=379 |date=2012 |publisher=Springer Science+Business Media |isbn=978-94-007-2024-4 |oclc=757338008 |doi=10.1007/978-94-007-2025-1|url=https://cds.cern.ch/record/1399129 }}

==External links==
{{Commons|Messier 33}}
* {{WikiSky|name=The Triangulum Galaxy}}
* [http://www.messier.seds.org/m/m033.html Messier 33, SEDS Messier pages]
* [https://web.archive.org/web/20070930014801/http://www.spacetelescope.org/bin/images.pl?searchtype=freesearch&string=m33 M33 at ESA/Hubble]
* [https://web.archive.org/web/20071110000208/http://starrymirror.com/m331103.htm  Triangulum Galaxy High In Northern Skies]
* [http://www.darkatmospheres.com/astro/gallery/galaxies/enlarge.php?fileBase=galaxies_4 Dark Atmospheres Photography – M33 (dust lane enhancement)]
* [http://www.starpointing.com/dslr/m33.html Pointing to the Universe – M33]
* {{cite web|last=Balcells|first=Marc|title=M33 – Triangulum Galaxy|url=http://www.deepskyvideos.com/videos/messier/M33_triangulum_galaxy.html|work=Deep Sky Videos|publisher=[[Brady Haran]]|author2=Szymanek, Nik|author2-link=Nik Szymanek|author3=Merrifield, Michael}}
* [http://ned.ipac.caltech.edu/cgi-bin/objsearch?objname=messier+33&extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES  NASA/IPAC Extragalactic Database entry for Messier 33]
* [http://www.constellation-guide.com/triangulum-galaxy-messier-33/ Triangulum Galaxy (M33) on Constellation Guide]
* [https://www.spacetelescope.org/images/heic1901a/zoomable/ Triangulum Galaxy – Zoomable UltraHighRez (Hubble; 11 January 2019)]

{{Triangulum Galaxy}}
{{Messier objects}}
{{Stars of Triangulum}}
{{NGC10}}
{{Authority control}}
{{Portal bar|Astronomy|Stars|Outer space}}

{{DEFAULTSORT:Triangulum Galaxy}}
[[Category:Triangulum Galaxy| ]]
[[Category:Astronomical objects discovered in 1654|?]]
[[Category:Local Group]]
[[Category:Messier objects]]
[[Category:NGC objects]]
[[Category:Principal Galaxies Catalogue objects|005818]]
[[Category:Triangulum]]
[[Category:Triangulum Subgroup]]
[[Category:UGC objects|01117]]
[[Category:Unbarred spiral galaxies]]
[[Category:IRAS catalogue objects|01310+3024]]
[[Category:MCG objects|+05-04-069]]