Galaxy M33

 

 

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 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 obsuration 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).