Triangulum Galaxy

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

Structure
Triangulum has a diameter of about 50,000 light years, and is home to around forty billion stars. It has an estimated mass of ten billion solar masses, though it has fifty billion solar masses if dark matter is included.

It is a non-barred spiral with two major inner arms and a large number of spurs connecting the arms. Star formation is currently very rapid, possibly due to its interactions with the Andromeda Galaxy. Whether it could be classed as a satellite of Andromeda is uncertain.

History
Triangulum formed over 13 billion years ago.

Several billion years ago, it interacted with the Andromeda Galaxy, and several streams of gas and stars still connect the two. It will return for another pass in around 2.5 billion years. It may be torn apart and fuel further star formation in Andromeda or may escape again and participate in the eventual Milky Way/Andromeda merger.