2.5 million light-years

The closest spiral galaxy to us, the Andromeda Galaxy (also called Messier 31) would appear eight times the size of the full Moon in the sky if our eyes were sensitive enough. Spanning 150,000 light years, Messier 31 has a shape very much like the Milky Way with older, yellow stars in the centre and younger, blue stars in the spiral arms.

Credit: Image made in visible light by Robert Gendler.