This strange galaxy has puzzled astronomers for six decades. Its outer ring of bright blue stars is separated from an inner cluster of older stars by a massive void tens of thousands of light-years across.

First discovered by Art Hoag in 1950, the galaxy is located some 600 million light-years away and measures 100,000 light-years across. Now known as Hoag's Object, we've since found other similarly bifurcated ring galaxies, but that doesn't mean we're any closer to understanding how they form.

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There's some thought they could be the result of galactic collisions, in which a smaller, young galaxy (the source of the blue stars) collides with an older, bigger galaxy (where the central stars come from). This collision might have caused the older stars to become very tightly concentrated while throwing the younger stars into a wide ring around the galaxy, with both then settling into orbit around the galactic center. It's also possible a vast bar once connected the inner and outer portions of the galaxy, but it's since faded away for reasons unknown.

This recently released image was taken by the Hubble Telescope in 2001, and it gives us the most detailed (and most breathtaking) view yet of Hoag's Object. You can click the image up top for a much closer look at this remarkable galaxy. There's also an added galactic bonus - a third galaxy located even further away can be spotted in the void between the inner and outer portions, at about the one o'clock position.

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[NASA's Astronomy Picture of the Day; thanks to Roklimber for the tip!]