Triangulum Galaxy — The Triangulum Galaxy, Messier object M33, is a spiral galaxy of type Sc located in the constellation Triangulum.
Triangulum is small relative to its larger neighbors such as the Milky Way and Andromeda Galaxy, but is about average compared to most spiral galaxies in the universe.
Triangulum is a member of the Local Group of galaxies and may be a gravitationally bound companion of the Andromeda Galaxy. LGS 3, one of the small Local Group member galaxies, is itself possibly a satellite of Triangulum.
Triangulum was probably discovered by 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.
Triangulum was also catalogued independantly by William Herschel on September 11, 1784, who assigned it the number H V.17. Triangulum was among the first “spiral Nebulae” identified as such by William Parsons, the Third Earl of Rosse.
Herschel also cataloged Triangulum’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.
NGC 604 is located in the northeastern corner of the galaxy as seen from Earth, 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.
This galaxy can be seen with the naked eye under exceptionally good conditions, and for most people is the most distant object visible.