Chown's Cosmos: Hoag's Object
A spiral galaxy has Marcus Chown's head spinning
Galaxies come in only a limited number of types. There are blob-like “ellipticals”, whirlpool “spirals”, amorphous “irregulars”. Then there is “Hoag’s Object”.
When astronomer Arthur Hoag discovered it in 1950 he at first thought it was a star which, in its death throes, had shrugged off its outer envelope to create a ring of glowing gas. Unhappy with that explanation, he later speculated that the ring might be an illusion, the light from a distant galaxy distorted into a ring by the “gravitational lens” of a dark foreground galaxy. However, the truth, realised by others, is that Hoag’s Object is actually a “ring galaxy” seen face-on – not a single star but an agglomeration of hundreds of billions of stars.
Since 1950, astronomers have pulled their hair out trying to understand how such a peculiar object could have formed. One idea is that a small galaxy passed through a bigger one and the shock wave, like an expanding ripple on a pond, triggered a rash of star formation that created the ring. Another idea is that a galaxy pulled in gas from its surroundings and that shockwaves in the in-falling material created the stellar ring. The truth is nobody knows how Hoag’s Object came to be.
Hoag’s Object has one other surprise up its sleeve. Look inside the ring at about one o’clock. There you will see another, more distant ring galaxy. Hoag’s Object is not unique. It has a twin in the very same field of view. Now, what are the chances of that?
You can view a larger image of Hoag's Object on the Hubble Space Telescope website