Messier Index/M33

The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy approximately 3 million light-years away in the constellation Triangulum. The galaxy is also 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. It is the third largest galaxy in the Local Group, a group of galaxies that also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy. The Pisces Dwarf (LGS 3), one of the small Local Group member galaxies, is possibly a satellite of Triangulum.

General information
The Triangulum Galaxy can be seen with the naked eye under exceptionally good conditions. While the fainter and more distant galaxy Messier 81 has also been seen with the naked eye by very experienced observers. However, some amateur astronomers may confuse the object with the nearby NGC 752, an open cluster that is brighter than the Triangulum Galaxy. No known pre-telescopic observer notes it, which is not surprising: given its indistinctness, it is not likely to be noticed as an object unless one already knows of its existence.

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

Herschel also cataloged 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, and is one of the largest H II regions known with a diameter of nearly 1500 light-years and a spectrum similar to the Orion Nebula. Herschel also noted 3 other smaller H II regions (NGC 588, 592 and 595).

In 2005, using observations of two 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 is computed which means Triangulum is moving towards Andromeda.

In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy using data from the Chandra X-ray Observatory. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days.

The galaxy has an H II nucleus.

Distance
At least three techniques have been used to measure distances to M 33. Using the Cepheid variable method, an estimate of 2.77 ± 0.13 Mly (850 ± 40 kpc) was achieved in 2004.

Also 2004, the Tip of the Red Giant Branch (TRGB) method was used to derive a distance estimate of 2.59 ± 0.08 Mly (794 ± 23 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, the 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 3.1 ± 0.2 Mly (940 ± 70 kpc).

Averaged together, all these distance measurements give a combined distance estimate of 2.81 ± 0.09 Mly (861 ± 28 kpc).

References in fiction
In the episode "Where No One Has Gone Before" of the first season of w:Star Trek: The Next Generation, the Enterprise is hurled into the far side of M33 by an alien visitor known only as "The Traveler".

In the TV series Andromeda, Triangulum Galaxy is one of the three galaxies forming the Systems Commonwealth, along with The Milky Way and Andromeda.

In the 2007 computer game Crysis, the alien antagonists of the game are said to have originated from the galaxy M33.

In the comic-book series Yoko Tsuno, the Vineans originated from M33, with the first mention of it being in the sixth album Les Trois soleils de Vinéa (The Three Suns of Vinea).