Triangulum Galaxy in the constellation
 

The Triangulum Galaxy is a spiral galaxy approximately 3 million light years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, which includes the Milky Way Galaxy, the Andromeda Galaxy and about 30 other smaller galaxies. It is one of the most distant permanent objects that can be viewed with the naked eye.

The Triangulum Galaxy is sometimes informally referred to as the "Pinwheel Galaxy" by some amateur astronomy references and in some public outreach websites. 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, and several other amateur astronomy resources and other public outreach websites also identify Messier 101 by that name.

Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen with the naked eye. It is one of the most distant permanent objects that can be viewed without the aid of a telescope. Being a diffuse object, its visibility is strongly affected by small amounts of light pollution. It ranges from easily visible by direct vision in dark skies to a difficult averted vision object in rural or suburban skies. For this reason, Triangulum is one of the critical sky marks of the Bortle Dark-Sky Scale.

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.

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

NGC 604 in the Triangulum Galaxy

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, which 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 3 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 classic 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.

With a diameter of about 50,000 light years, the Triangulum galaxy is the third largest member of the Local Group of galaxies. It may be a gravitationally bound companion of the Andromeda Galaxy. (See below.) Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion (1000 billion) stars for Andromeda. 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 kly (17 kpc) is equivalent to about 5 × 1010 solar masses.

Estimates of the distance to the Triangulum galaxy range from 2,380 to 3,070 kly (730 to 940 kpc), with most estimates since the year 2000 lying in the middle portion of this range. At least three techniques have been used to measure distances to M 33. Using the Cepheid variable method, an estimate of 2,770 ± 130 kly (850 ± 40 kpc) was achieved in 2004. In the same year, the Tip of the Red Giant Branch (TRGB) method was used to derive a distance estimate of 2,590 ± 80 kilolight-years (790 ± 25 kpc).

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 ± 240 kly (940 ± 74 kpc).

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, 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. 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.

The Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is located 2,022 kly (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 kly (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.

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.

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. 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 tails associated with the Milky Way.

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. The nucleus of this galaxy is an H II region, 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. 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.

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).

 

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.

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.) 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.

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 kly (9 kpc) has a typical composition gradient that decreases linearly from the core. Beyond this radius, out to about 82 kly (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. 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.

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 HII regions of star formation. 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. 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. Other prominent HII regions in Triangulum include IC 132, IC 133 and IK 53.

The northern main spiral arm contains four large HII regions, while the southern arm has greater concentrations of young, hot stars. 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. As of 2008, a total of 100 supernova remnants have been identified in the Triangulum Galaxy, and a majority of the remnants 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 stars. The center of the distribution of these features is offset about two arc minutes to the southwest. Being a local galaxy, the Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M33 along with M31 and M81.

About 54 globular clusters 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 emission of massive stars in this galaxy matches the level of similar stars in the Large Magellanic Cloud.

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.

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

The fate of the Triangulum Galaxy is unclear, but seems to be linked to its larger neighbor the Andromeda Galaxy. Suggested future scenarios for M33 range from being torn apart and absorbed by Andromeda, fueling the latter with hydrogen to form new stars; to exhausting all of its gas, and thus the ability to form new stars; or participating in the collision between the Milky Way and M31, most likely ending orbiting the merger remnant of the latter two galaxies and fusing with it much later. Two other possibilities are a collision with the Milky Way before Andromeda arrives or an ejection out of the Local Group.