The third-largest member of our Local Group of galaxies, after Andromeda and the Milky Way. M33 lies about 2.7 million light-years away in the constellation Triangulum and is one of the most distant objects visible to the naked eye under dark skies. Unlike the more tightly wound Andromeda, the Triangulum Galaxy is a loosely structured spiral with prominent, well-defined arms dotted with enormous HII regions — clouds of ionised hydrogen where new stars are actively forming.

The brightest of these, NGC 604, is one of the largest known star-forming regions in the Local Group, dwarfing the Orion Nebula by a factor of 40. Imaging M33 at sufficient depth reveals individual star clusters, dust lanes, and the galaxy's extended outer arms.