What is the third-largest galaxy in the Local Group, after Andromeda and the Milky Way?

The Triangulum Galaxy is the third-largest galaxy in the Local Group, after Andromeda and the Milky Way.

Also called Messier 33 or M33, the Triangulum Galaxy is a spiral galaxy about 2.9 million light-years away in the constellation Triangulum. Its disk measures roughly 61,000 light-years across by one standard diameter measure, making it smaller than both the Milky Way and Andromeda but larger than most other Local Group galaxies.

M33 may contain around 40 billion stars and is especially rich in star-forming regions. Its most prominent nebula, NGC 604, is one of the largest known star-forming regions in the Local Group. Unlike the barred Milky Way and Andromeda, Triangulum is generally classified as an unbarred spiral galaxy.

Astronomers continue to study whether M33 is gravitationally bound to Andromeda. The two galaxies are separated by about 750,000 light-years and may have passed near each other several billion years ago. The Large Magellanic Cloud is smaller, despite being a substantial satellite of the Milky Way.