Triangulum Galaxy

Coordinates: Sky map 01h 33m 50.9s, 30° 39′ 36″

The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way. It is one of the most distant permanent objects that can be viewed with the naked eye.[6]

Triangulum Galaxy
VST snaps a very detailed view of the Triangulum Galaxy.jpg
Galaxy Messier 33 in Triangulum
Credit: VLT Survey Telescope, Atacama Desert, Chile
Observation data (J2000 epoch)
Right ascension01h 33m 50.02s[1]
Declination+30° 39′ 36.7″[1]
Redshift-0.000607 ± 0.000010[1]
Helio radial velocity-179 ± 3 km/s[2]
Galactocentric velocity-44 ± 6 km/s[2]
Distance (comoving)970 kpc (3.2 Mly)[3]
Apparent magnitude (V)5.72[1]
Mass5 × 1010[4] M
Number of stars40 billion (4×1010)[5]
Size~60,000 ly (diameter)[5]
Apparent size (V)70.8 × 41.7 moa[1]
Other designations
NGC 0598, MCG+05-04-069, UGC 1117, PGC 5818[2]

The galaxy is the smallest spiral galaxy in the Local Group (although the smaller Large and Small Magellanic Clouds may have been spirals before their encounters with the Milky Way), and is believed to be a satellite of the Andromeda Galaxy or on its rebound into the latter due to their interactions, velocities,[7] and proximity to one another in the night sky. It also has an H II nucleus.[8]


The galaxy gets its name from the constellation Triangulum, where it can be spotted. It is sometimes informally referred to as the "Pinwheel Galaxy" by some astronomy references,[9] in some computerized telescope software, and in some public outreach websites.[10] However, the SIMBAD Astronomical Database, a professional database, collates formal designations for astronomical objects and indicates that Pinwheel Galaxy refers to Messier 101,[11] which several amateur astronomy resources including public outreach websites identify by that name, and that is within the bounds of Ursa Major.[12][13]


Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen with the 20/20 vision naked eye;[14] to those viewers, it will sometimes be the farthest permanent entity visible without magnification.[15][16] Its light diffuses (spreads) across a little more than a pinprick of the unmagnified sky, the cause of which is its broadness – this astronomers term a diffuse, rather than compact, object.

Observers range from finding the galaxy easily visible by direct vision in a truly dark (and impliedly dry, cloud-free) sky to needing to use averted vision in rural or suburban skies with good viewing conditions.[14] It has been chosen as one of the critical sky marks of the Bortle Dark-Sky Scale,[17] supported by its relative invariability, reasonable northern declination and brightness described.

Triangulum Galaxy (Messier 33), taken with amateur equipment.

Observation historyEdit

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 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 of Triangulum as a pair of triangles. The magnitude of the object matches M33, so it is most likely a reference to the Triangulum Galaxy.[18]

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.[19] When William Herschel compiled his extensive catalogue of nebulae, he was careful not to include most of the objects identified by Messier.[20] However, M33 was an exception and he catalogued this object on September 11, 1784, as H V-17.[21]

Herschel also catalogued the Triangulum Galaxy's brightest and largest H II region (diffuse emission nebula containing ionized hydrogen) as H III.150 separately from the galaxy itself; the nebula eventually obtained NGC 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 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 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 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.[22]


Another image of the Triangulum Galaxy made by an amateur telescope

With a diameter of about 60,000 light-years, the Triangulum Galaxy is the third largest member of the Local Group of galaxies, roughly 60% the size of the Milky Way. It may be a gravitationally bound companion of the Andromeda Galaxy. Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion stars for Andromeda Galaxy.[5]

The disk of Triangulum has an estimated mass of (3–6) × 109 solar masses, while the gas component is about 3.2 × 109 solar masses. Thus the combined mass of all baryonic matter in the galaxy may be 1010 solar masses. The contribution of the dark matter component out to a radius of 55×10^3 ly (17 kpc) is equivalent to about 5 × 1010 solar masses.[4]


Triangulum (M33; lower left of center) and Andromeda Galaxy (M31; above center)

Estimates of the distance from the Milky Way to the Triangulum Galaxy range from 2,380×10^3 to 3,070×10^3 ly (730 to 940 kpc) (or 2.38 to 3.07 Mly), with most estimates since the year 2000 lying in the middle portion of this range,[24][25] making it slightly more distant than the Andromeda Galaxy (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 2,770×10^3 ± 130×10^3 ly (849 ± 40 kpc) was achieved in 2004.[26][27] In the same year, the tip of the red-giant branch (TRGB) method was used to derive a distance estimate of 2,590×10^3 ± 80×10^3 ly (794 ± 25 kpc).[28] The Triangulum Galaxy is around 750000 light years from the Andromeda Galaxy.[29]

In 2006, a group of astronomers announced the discovery of an 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 of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 3,070×10^3 ± 240×10^3 ly (941 ± 74 kpc).[24] The average of 102 distance estimates published since 1987 gives a distance modulus of 24.69, or .883 Mpc (2,878,000 light-years).[30]

The Triangulum Galaxy is a source of H2O maser emission.[31] In 2005, using observations of two 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 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).[7] 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.[32] A distance of less than 300 kiloparsecs between the two supports this hypothesis.[33]

128 minutes of amateur astrophotography directed at the Triangulum Galaxy (RC10, ASI2400, EQ-6R)

The Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is located 2,022×10^3 ly (620 kpc) from the Sun. It is 20° from the Andromeda Galaxy and 11° from Triangulum. As LGS 3 lies at a distance of 913×10^3 ly (280 kpc) from both galaxies, it could be a satellite galaxy of either Andromeda or Triangulum. LGS 3 has a core radius of 483 ly (148 pc) and 2.6 × 107 solar masses.[34]


Infrared image of M33 taken with the Spitzer Space Telescope
Ultraviolet image of M33 by GALEX observatory

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 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.[35]

This galaxy has an inclination of 54° to the line of sight from the Earth, allowing the structure to be examined without significant obstruction by gas and dust.[36][37] 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.[38] This is an isolated galaxy and there are no indications of recent mergers or interactions with other galaxies,[37] and it lacks the dwarf spheroidals or tidal tails associated with the Milky Way.[39]

Triangulum is classified as unbarred, but an analysis of the galaxy shape shows what may be a weak bar-like structure about the galactic nucleus. The radial extent of this structure is about 0.8 kpc.[40] The nucleus of this galaxy is an H II region,[31] and it contains an ultraluminous X-ray source with an emission of 1.2 × 1039 erg s−1, 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.[41] However, the nucleus does not appear to contain a supermassive black hole, as an upper limit of 3,000 solar masses is placed on the mass of a central black hole based upon the velocity of stars in the core region.[42]

The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features.[36][37] The main arms are designated IN (north) and IS (south).[43]

Star formationEdit

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 emission of CO. This effect occurs as giant molecular clouds 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.[36][37]

Star formation is taking place at a rate that is strongly correlated with the 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 Gyr−1 pc−2 in the Triangulum Galaxy, compared to 0.74 in Andromeda.[44]) The total integrated rate of star formation in the Triangulum Galaxy is about 0.45 ± 0.1 solar masses per year. It is uncertain whether this net rate is currently decreasing or remaining constant.[36][37]

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 30×10^3 ly (9 kpc) has a typical composition gradient that decreases linearly from the core. Beyond this radius, out to about 82×10^3 ly (25 kpc), 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.[38] 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.[45]

Discrete featuresEdit

Using infrared observations from the Spitzer Space Telescope, a total of 515 discrete candidate sources of 24 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 regions of star formation.[46] The four brightest HII regions are designated NGC 588, NGC 592, NGC 595, and NGC 604. These regions are associated with molecular clouds containing (1.2–4) × 105 solar masses. The brightest of these regions, NGC 604, may have undergone a discrete outburst of star formation about three million years ago.[47] This nebula is the second most luminous HII region within the Local Group of galaxies, at (4.5 ± 1.5) × 107 times the luminosity of the Sun.[44] Other prominent HII regions in Triangulum include IC 132, IC 133, and IK 53.[43]

The northern main spiral arm contains four large HII regions, while the southern arm has greater concentrations of young, hot stars.[43] The estimated rate of supernova explosions in the Triangulum Galaxy is 0.06 Type Ia and 0.62 Type Ib/Type II per century. This is equivalent to a supernova explosion every 147 years, on average.[48] As of 2008, a total of 100 supernova remnants have been identified in the Triangulum Galaxy,[49] the majority of which lies 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 stars. The center of the distribution of these features is offset about two arc minutes to the southwest.[43] M33 being a local galaxy, the Central Bureau for Astronomical Telegrams (CBAT) tracks novae in it along with M31 and M81.[50]

About 54 globular clusters have been identified in this galaxy, but the actual number may be 122 or more.[39] 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 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.[51]

In 2007, a black hole about 15.7 times the 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.[52][53]

Unlike the Milky Way and Andromeda galaxies, the Triangulum Galaxy does not appear to have a supermassive black hole at its center. This may be because the mass of a galaxy's central supermassive black hole correlates with the size of the galaxy's central bulge, and unlike the Milky Way and Andromeda, the Triangulum Galaxy is a pure disk galaxy with no bulge.[54]

Relationship with the Andromeda GalaxyEdit

Triangulum on the collision paths of the Milky Way and Andromeda galaxies.

As mentioned above, M33 is linked to M31 by several streams of neutral hydrogen[55] and stars,[55] which suggests that a past interaction between these two galaxies took place from 2 to 8 billion years ago,[56][57] and a more violent encounter will occur 2.5 billion years in the future.[55]

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;[58] or participating in the 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.[59] Astrometric data from Gaia appears 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).[60]

Planetary nebulaeEdit

Planetary nebulae are not only important contributors to the chemical enrichment of galaxies, but provide valuable information on single and binary stellar evolution. In addition, these objects seem to always produce very bright planetary nebulae with consistent luminosities, regardless of the galaxy's mass, age or metallicity. This feature is very useful as a standard candle for distance measurements.

Large systematic research on this topic has been done by Rebeca Galera-Rosillo and co-authors in 2018.[61] This work benefited from use of the INT and WHT telescope located at La Palma island. As a result of this study three new planetary nebulae were discovered.

Newly discovered PNe (2018), Rebeca named the PNe after her closest family members.
GCM 1 (Ovejisaurio), 01:34:48.86 +31:05:14.8
GCM 2, (Cuchilla Andante) 01:33:45.20 +30:21:22.0
GCM 3, (Sewi) 01:33:52.30 +30:21:12.0

See alsoEdit


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