The Triangulum Galaxy is a

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s (ly) from Earth in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Triangulum Triangulum is a small constellation in the northern sky. Its name is Latin for "triangle", derived from its three brightest stars, which form a long and narrow triangle. Known to the ancient Babylonians and Greeks, Triangulum was one of the 48 ...

. It is catalogued as Messier 33 or NGC (''New General Catalogue)'' 598. With the D₂₅ isophotal diameter of , the Triangulum Galaxy is the third-largest member of the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

of galaxies, behind the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

and the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. The galaxy is the smallest spiral galaxy in the Local Group (although the smaller

Large Large means of great size. Large may also refer to: Mathematics * Arbitrarily large, a phrase in mathematics * Large cardinal, a property of certain transfinite numbers * Large category, a category with a proper class of objects and morphisms (or ...

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, and proximity to one another in the night sky. It also has an

H II An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundred ...

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

Etymology

The galaxy gets its name from the constellation

, where it can be spotted. It is sometimes informally referred to as the "

Pinwheel Galaxy The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy away from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who ...

" by some astronomy references, in some computerized telescope software, and in some public outreach websites. However, the

SIMBAD Astronomical Database SIMBAD (the Set of Identifications, Measurements and Bibliography for Astronomical Data) is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de Strasbourg (CDS), France. SIMBA ...

, a professional database, collates formal designations for astronomical objects and indicates that

refers to Messier 101, which several amateur astronomy resources including public outreach websites identify by that name, and that is within the bounds of

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

Visibility

Under exceptionally good viewing conditions with no

light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...

, the Triangulum Galaxy can be seen with the 20/20 vision

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

; to those viewers, it will sometimes be the farthest permanent entity visible without magnification. 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. It has been chosen as one of the critical sky marks of the

Bortle Dark-Sky Scale The Bortle scale (also known as the Bottle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by lig ...

, supported by its relative invariability, reasonable northern declination, and brightness described.

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 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. The galaxy was independently discovered by

Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects''. Messier's purpose f ...

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. William Herschel compiled his extensive catalog of nebulae, he was careful not to include most of the objects identified by Messier. However, M33 was an exception, and he cataloged this object on September 11, 1784, as H V-17. Herschel also cataloged the Triangulum Galaxy's brightest and largest H II region (diffuse

emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emissio ...

containing

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

ized

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

) 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-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s and a

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

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 William Parsons, 3rd Earl of Rosse (17 June 1800 – 31 October 1867), was an Irish astronomer, naturalist, and engineer. He was president of the Royal Society (UK), the most important association of naturalists in the world in the nineteenth ...

in 1850. In 1922–23,

John Charles Duncan John Charles Duncan (February 8, 1882 in Knightstown, Indiana – September 10, 1967 in Chula Vista, California) was an American astronomer. Life John Charles Duncan was the son of Daniel Davidson and Naomi, née Jessup, Duncan. He graduated fr ...

and

Max Wolf Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K� ...

discovered variable stars in the nebulae.

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects ...

showed in 1926 that 35 of these stars were

classical Cepheids Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cepheid variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a ...

, 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. File:Nursery of New Stars - GPN-2000-000972.jpg,

NGC 604 NGC 604 is an H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is among the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-y ...

in the Triangulum Galaxy File:Messier33 - HST - Heic1901a.jpg, Composite of about 54 different pointings with Hubble's Advanced Camera for Surveys.

Properties

The Triangulum Galaxy is the third largest member of the

of galaxies. It has a diameter measured through the D₂₅ standard - the

isophote In geometry, an isophote is a curve on an illuminated surface that connects points of equal brightness. One supposes that the illumination is done by parallel light and the brightness is measured by the following scalar product: :b(P)= \vec n(P) ...

where the surface brightness of the galaxy reaches 25 mag/arcsec², to be about , making it 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. The disk of Triangulum has an estimated mass of

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es, while the gas component is about solar masses. Thus, the combined mass of all

baryon In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified ...

ic matter in the galaxy may be 10¹⁰ solar masses. The contribution of the

dark Darkness, the direct opposite of lightness, is defined as a lack of illumination, an absence of visible light, or a surface that absorbs light, such as black or brown. Human vision is unable to distinguish colors in conditions of very low lu ...

matter component out to a radius of is equivalent to about solar masses.

Location – distance – motion

Estimates of the distance from the Milky Way to the Triangulum Galaxy range from (or 2.38 to 3.07

Mly A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

), with most estimates since the year 2000 lying in the middle portion of this range, 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 A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

method, an estimate of was achieved in 2004. In the same year, the

tip of the red-giant branch Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjun ...

(TRGB) method was used to derive a distance estimate of . The Triangulum Galaxy is around 750,000 light years from the Andromeda Galaxy. In 2006, a group of astronomers announced the discovery of an

eclipsing binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

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 Absolute may refer to: Companies * Absolute Entertainment, a video game publisher * Absolute Radio, (formerly Virgin Radio), independent national radio station in the UK * Absolute Software Corporation, specializes in security and data risk manage ...

magnitude of the stars. When the

visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the ...

and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of . The average of 102 distance estimates published since 1987 gives a

distance modulus The distance modulus is a way of expressing distances that is often used in astronomy. It describes distances on a logarithmic scale based on the astronomical magnitude system. Definition The distance modulus \mu=m-M is the difference between th ...

of 24.69, or .883 Mpc (2,878,000 light-years). The Triangulum Galaxy is a source of H₂O maser emission. In 2005, using observations of two water masers on opposite sides of Triangulum via the

VLBA The Very Long Baseline Array (VLBA) is a system of ten radio telescopes which are operated remotely from their Array Operations Center located in Socorro, New Mexico, as a part of the National Radio Astronomy Observatory (NRAO). These ten radi ...

, researchers were, for the first time, able to estimate the angular rotation and

proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...

of Triangulum. A velocity of relative to the Milky Way was computed, which means Triangulum is moving towards

and suggesting it may be a satellite of the larger galaxy (depending on their relative distances and margins of error). 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. A distance of less than 300 kiloparsecs between the two supports this hypothesis. The

Pisces Dwarf The Pisces Dwarf, also known as Pisces I, is an irregular dwarf galaxy that is part of the Local Group. The galaxy, taking its name from the constellation Pisces where it appears, is suspected of being a satellite galaxy of the Triangulum Gal ...

(LGS 3), one of the small Local Group member galaxies, is located from the Sun. It is 20° from the

and 11° from Triangulum. As LGS 3 lies at a distance of from both galaxies, it could be a

satellite galaxy A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents are bound ...

of either Andromeda or Triangulum. LGS 3 has a core radius of and solar masses. Pisces VII/Triangulum (Tri) III may be another satellite of Triangulum.

Structure

In the French astronomer

Gérard de Vaucouleurs Gérard Henri de Vaucouleurs (25 April 1918 – 7 October 1995) was a French astronomer. Life and career Born in Paris, he had an early interest in amateur astronomy and received his undergraduate degree in 1939 at the Sorbonne in that city. ...

' revised Hubble Sandage (VRHS) system of

galaxy morphological classification Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most f ...

, 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 Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''barred spiral galaxies. American astronomer

Allan Sandage Allan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He determined the first reasonably accurate values for the Hubble con ...

'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. 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. 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. This is an isolated galaxy and there are no indications of recent mergers or interactions with other galaxies, and it lacks the dwarf spheroidals or

tidal tail A tidal tail is a thin, elongated region of stars and interstellar gas that extends into space from a galaxy. Tidal tails occur as a result of galactic tide forces between interacting galaxies. Examples of galaxies with tidal tails include the ...

s associated with the Milky Way. 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 kpc. The nucleus of this galaxy is an

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundred ...

, and it contains an

ultraluminous X-ray source An ultraluminous X-ray source (ULX) is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process (over 1039 erg/s, or 1032 watts), assuming that it radiat ...

with an emission of , which is the most luminous source of X-rays in the

of galaxies. This source is modulated by 20% over a 106-day cycle. However, the nucleus does not appear to contain a

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical obj ...

, as a best-fit value of zero mass and an upper limit of is placed on the mass of a central black hole based on models and the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

(HST) data. 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. This may suggest that supermassive black holes are associated only with galaxy bulges instead of with their disks. Assuming that the upper limit of the central black hole is correct, it would be rather an

intermediate-mass black hole An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes. Several IMBH candidate obje ...

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

Star formation

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 cloud A molecular cloud, sometimes called 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 ...

s condense out of the surrounding

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

. 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.

Star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

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

. (The rate of star formation is about 3.4 Gyr⁻¹ pc⁻² in the Triangulum Galaxy, compared to 0.74 in Andromeda.) The total integrated rate of star formation in the Triangulum Galaxy is about . It is uncertain whether this net rate is currently decreasing or remaining constant. 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 has a typical composition gradient that decreases linearly from the core. Beyond this radius, out to about , 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. 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.

Discrete features

Using infrared observations from the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...

, 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

s of star formation. The four brightest HII regions are designated

NGC 588 NGC 588 is a diffuse nebula located in the outskirts of the galaxy Messier 33, within the Triangulum constellation. It was discovered October 2, 1861 by the German-Danish astronomer Heinrich d'Arrest Heinrich Louis d'Arrest (13 August 1822 � ...

NGC 592 NGC 592 is an H II region type emission nebula located in the Triangulum galaxy (M33) and thus in the constellation of Triangulum. The nebula contains an open cluster of stars and is approximately 2.86 million light-years away from Earth. Obse ...

NGC 595 NGC 595 is an H II region in the Triangulum Galaxy. It was discovered by Heinrich Ludwig d'Arrest on October 1, 1864. References External links * * NGC 0595 NGC 0595 0595 Triangulum Galaxy 18641001 Star-forming regions ...

, and

. These regions are associated with

s containing solar masses. The brightest of these regions, NGC 604, may have undergone a discrete outburst of star formation about three million years ago. This nebula is the second most luminous HII region within the Local Group of galaxies, at times the

luminosity of the Sun The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal ...

. Other prominent HII regions in Triangulum include IC 132, IC 133, and IK 53. The northern main spiral arm contains four large

HII region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundre ...

s, while the southern arm has greater concentrations of young, hot stars. The estimated rate of

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

explosions in the Triangulum Galaxy is 0.06

Type Ia A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...

and 0.62 Type Ib/ Type II per century. This is equivalent to a supernova explosion every 147 years, on average. As of 2008, a total of 100

supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...

s have been identified in the Triangulum Galaxy, 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. M33 being a local galaxy, the

Central Bureau for Astronomical Telegrams The Central Bureau for Astronomical Telegrams (CBAT) is the official international clearing house for information relating to transient astronomical events. The CBAT collects and distributes information on comets, natural satellites, novae, supern ...

(CBAT) tracks

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

e in it along with M31 and M81. About 54

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

s have been identified in this galaxy, but the actual number may be 122 or more. 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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

emission of massive stars in this galaxy matches the level of similar stars in the

Large Magellanic Cloud The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...

. In 2007, a

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

about 15.7 times the

mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

was detected in this galaxy using data from the

Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 1 ...

. 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. Unlike the Milky Way and Andromeda galaxies, the Triangulum Galaxy does not appear to have a

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 A disc galaxy (or disk galaxy) is a galaxy characterized by a galactic disc, a flattened circular volume of stars. These galaxies may or may not include a central non-disc-like region (a galactic bulge). Disc galaxy types include: * Spiral gala ...

with no bulge.

Relationship with the Andromeda Galaxy

Collision paths of our Milky Way galaxy and the Andromeda galaxy

As mentioned above, M33 is linked to M31 by several streams of

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

and stars, which suggests that a past interaction between these two galaxies took place from 2 to 8 billion years ago, and a more violent encounter will occur 2.5 billion years in the future. 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

to form new stars; eventually exhausting all of its gas, and thus the ability to form new stars; 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. Astrometric data from

Gaia In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthenog ...

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).

Planetary nebulae

Planetary nebulae A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated t ...

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. This work benefited from the use of the INT and WHT telescopes located at

La Palma La Palma (, ), also known as ''La isla bonita'' () and officially San Miguel de La Palma, is the most north-westerly island of the Canary Islands, Spain. La Palma has an area of making it the fifth largest of the eight main Canary Islands. The ...

island. As a result of this study, three new planetary nebulae were discovered.

References

External links

*
Messier 33, SEDS Messier pages

M33 at ESA/Hubble

Dark Atmospheres Photography – M33 (dust lane enhancement)

*
NASA/IPAC Extragalactic Database entry for Messier 33

Triangulum Galaxy (M33) on Constellation Guide

Triangulum Galaxy – Zoomable UltraHighRez (Hubble; 11 January 2019)
{{DEFAULTSORT:Triangulum Galaxy ? Local Group Messier objects NGC objects 05818 Triangulum (constellation) Triangulum Subgroup 01117 Unbarred spiral galaxies