M31, aka NGC 224, and more commonly known as the “Great Galaxy in Andromeda”, is probably the farthest object (2.5 million light years) visible to the naked eye (albeit, only the bright core region). Spanning about 3 degrees across the sky, M31 is as long as 6 full moons, end to end! Variable stars in M31 allowed Hubble to deduce this so-called spiral nebula was much more distant than previously thought — not a denizen of our Milky Way, but representing an entirely separate “island universe” of billions of stars.
Technical details as follows;
Takahashi FSQ 106N, SBIG STL11K, Paramount ME, RGB 3 hrs/channel, processed in PixInsight and Photoshop.
Data acquired from Pennsylvania home observatory in the fall 2015.