Of all the galaxies observed by astronomers, barely any are as strange or geometrically unique as Hoag's Object. This bizarre galactic circle, categorized as a "ring galaxy," is located some 600 million light-years from Earth in the constellation Serpens and spans about 120,000 light-years across. Even more befuddling is that Hoag's Object is not one, but two galaxies — a cosmic arrangement that has baffled researchers since its discovery by American astronomer Arthur Hoag in 1950.
At first, astronomers believed the unusual arrangement of Hoag's Object was a trick of the eye caused by gravitational lensing. This phenomenon, first proposed by Einstein's Theory of General Relativity, occurs when the gravitational mass of one object can bend light in such a way as to magnify the appearance of a more distant object. Astronomers have used such cosmic lenses before to peer into the distant hearts of other galaxies that would otherwise be impossible to detect with modern instruments. This idea was later disproved after observations of Hoag's Object in 1974 showed it weighed far too little (about the mass of 700 billion suns) to cause gravitational lensing of any magnitude.
Instead, Hoag's Objects features what appear to be two distinct galaxies, with young bright blue stars encircling a center core of older reddish-stars. Between them lies a perceptible moat of darkness.
So how did these incredibly rare cosmic anomalies, accounting for only .01% of all the galaxies discovered, ever come to be? The most popular theory presently is that Hoag's Object was once a common disc-shaped galaxy that suffered a direct hit from a smaller, neighboring galaxy. The resulting collision, which would have happened billions of years ago, wrapped the original galaxy's gravitational pull and formed the beautiful symmetry we see today.
Another theory posits that the galaxy has simply sucked in enough intergalactic mass over time to form the beautiful ring we see today.
Formation theories aside, astronomers noted something else incredible about Hoag's Object when they trained the Hubble Telescope's sensitive optics on it back in 2002. Hiding behind this cosmic wonder at the one o'clock position in the above photo is yet another rare ring galaxy –– making this an image of a galaxy within a galaxy within a galaxy within a galaxy!