The mystery is: Is this one galaxy or two? This question came to light in 1950 when astronomer Art Hoag discovered this odd extragalactic object. On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars that are likely much older. Between the two is a gap that appears almost completely dark. How the object formed remains unknown. Possibilities include a galaxy collision billions of years ago and the gravitational affect of a central bar that has since vanished. Hoag's Object spans about 100,000 light years and lies about 600 million light years away toward the constellation of the Snake (Serpens). Coincidentally, visible in the gap (at about one o'clock) is yet another ring galaxy that likely lies far in the distance. Fascinating!
Image Credit: R. Lucas (STScI/AURA), Hubble Heritage Team, NASA