The Triangulum Galaxy is a spiral galaxyapproximately 3 million light-years (ly) from Earth in the constellation Triangulum.
It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Milky Way and the Andromeda Galaxy. It is one of the most distant permanent objects that can be viewed with the naked eye.
The galaxy is the smallest spiral galaxy in the Local Group and it is believed to be a satellite of the Andromeda Galaxy due to their interactions, velocities and proximity to one another in the night sky. It also has an H-II nucleus.
With a diameter of about 60,000 light-years, the Triangulum galaxy is the third largest member of the Local Group of galaxies, roughly 40% the size of the Milky Way. It may be a gravitationally bound companion of the Andromeda Galaxy. (See below.) Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion stars for Andromeda Galaxy. The disk of Triangulum has an estimated mass of (3–6) × 109 solar masses, while the gas component is about 3.2 × 109 solar masses. Thus the combined mass of all baryonic matter in the galaxy may be 1010 solar masses. The contribution of the dark matter component out to a radius of 55×103 ly (17 kpc) is equivalent to about 5 × 1010 solar masses.