M33

M33, also known as the Triangulum Galaxy, is a spiral galaxy in the constellation Triangulum. It is designated as Messier 33 (M33) and NGC 598, and it is the third-largest member of the Local Group after the Milky Way and Andromeda, located about 2.73 million light-years from the Milky Way.

M33 is a late-type, nearly face-on spiral galaxy (classification SA(s)cd) with loosely wound arms, a small central bulge, and no obvious bar. It contains numerous H II regions and OB associations, indicating ongoing star formation across its disk. One of its most notable star-forming complexes is NGC 604, which is among the largest H II regions in the Local Group.

The galaxy possesses an extended neutral hydrogen (HI) disk that stretches beyond its optical edge and shows warps and asymmetries that may result from gravitational interactions. M33 exhibits a metallicity gradient typical of spiral disks, with lower metal content toward the outer regions.

Observations across radio, optical, infrared, and ultraviolet wavelengths have been used to study its structure, dynamics, and stellar populations. The system is believed to have experienced past interactions with its larger neighbor Andromeda (M31), events that could have influenced its outer disk morphology.

Discovered by Charles Messier in 1764, M33 serves as a nearby, well-studied example of a late-type spiral and provides insight into star formation and disk dynamics in galaxies of modest mass.