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Hoag's Object Is a Galaxy Within a Galaxy Within a Galaxy (and Nobody Knows Why)

With a perfectly symmetrical ring circling a red sphere of stars, Hoag's object is one of the prettiest mysteries in the universe. (Image credit: NASA/ESA, Processing: Benoit Blanco)

Look closely at the serpent constellation slithering through the northern sky, and you might see a galaxy within a galaxy within a galaxy.

This cosmic turducken is known as Hoag's object, and it has befuddled stargazers since astronomer Arthur Hoag discovered it in 1950. 

The object in question is a rare, ring-shaped galaxy measuring some 100,000 light-years across (slightly larger than the Milky Way) and located 600 million light-years from Earth. In a recent image of the oddball object taken by the Hubble Space Telescope and processed by geophysicist Benoit Blanco, a bright ring of billions of blue stars forms a perfect circle around a much smaller and denser sphere of reddish stars. In the dark gap between the two stellar circles, another ring galaxy — much, much farther away from us — peeks out to say hello.

Related: The 15 Weirdest Galaxies in Our Universe

What's going on here, and what tore Hoag's object in two? Astronomers still aren't sure; ring galaxies account for less than 0.1% of all known galaxies, and so they aren't the easiest objects to study. Hoag himself suggested that the galaxy's peculiar ring formation was merely an optical illusion caused by gravitational lensing (an effect that occurs when extremely high-mass objects bend and magnify light). Later studies with better telescopes disproved this idea.

Another popular hypothesis suggests that Hoag's object was once a more common, disk-shaped galaxy but an ancient collision with a neighboring galaxy ripped a hole through the disk's belly and permanently warped its gravitational pull. If such a collision occurred in the last 3 billion years, then astronomers looking through radio telescopes should have been able to see some of the fallout from the accident. No such evidence has been found. 

If there was a cosmic crash at the core of Hoag's object, it must have happened so long ago that all the evidence has been swept away. With only a handful of other known ring galaxies available to study (none of which shows the perfectly symmetrical characteristics found in this one), Hoag's object remains a mystery wrapped in a riddle inside an enigma — you know, like a turducken. 

Originally published on Live Science.

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  • rod
    The report stated "The object in question is a rare, ring-shaped galaxy measuring some 100,000 light-years across (slightly larger than the Milky Way) and located 600 million light-years from Earth. In a recent image of the oddball object taken by the Hubble Space Telescope and processed by geophysicist Benoit Blanco, a bright ring of billions of blue stars forms a perfect circle around a much smaller and denser sphere of reddish stars. In the dark gap between the two stellar circles, another ring galaxy — much, much farther away from us — peeks out to say hello."

    Good information but I like the z numbers for both objects shown. A ring of blue stars circling a denser area of red stars is a problem using H-R star diagram dating, thus the implied great age differences between the stars shown in the ring and denser sphere of red stars. Galaxy z numbers for redshifts can be compared using the flat universe model for inflation (the common and accepted model used) and the open universe model. You get different distances and age results.
    Reply
  • Sgt Jmack
    Is it possible that it is an optical illusion, and that the smaller galaxy/galaxies are farther away or behind the ring galaxy and we are looking 'through' the center of the rings and seeing the other galaxy? I mean, we are only looking at it from one specific angle.
    Reply
  • Mikael L
    How about this one?:
    A supermassive black hole exploded, in the center of the original galaxy!
    Reply
  • rod
    Mikael L said:
    How about this one?:
    A supermassive black hole exploded, in the center of the original galaxy!

    Someone pointing this solution out, perhaps viewed the event too :) Have fun observing---Rod
    Reply