Hoag’s object is a bizarre and unusual ring galaxy, with an old population of stars in the central nucleus well-separated from a younger, bluer population of stars in an outer ring. 70 years after its discovery, we still can’t fully explain what’s happening. (NASA AND THE HUBBLE HERITAGE TEAM (STSCI/AURA); ACKNOWLEDGMENT: RAY A. LUCAS (STSCI/AURA))

Astronomy’s Most Perfect Ring Galaxy, Hoag’s Object, Is Still A Mystery After 70 Years

We still don’t know how it came to be this way.

Every once in a while, we find an object in the Universe that completely mystifies us. For generations, astronomers have been observing distant galaxies present all throughout the Universe, cataloguing them and noting their various characteristics. Overwhelmingly, galaxies fell into three different categories:

• spiral galaxies, where stars are concentrated in vast, sweeping arms,
• elliptical galaxies, where stars swarm around a central region,
• and irregular galaxies, which are neither spiral nor elliptical, and which often correspond to two or more galaxies in the process of interacting.

Spirals and ellipticals are ubiquitous, with spirals more common among isolated or sparsely populated regions of space, while ellipticals often dominate the centers of large galaxy clusters. But in 1950, astronomer Arther Hoag discovered a galaxy unlike any other: Hoag’s object, dominated by a vast, ring-like halo. 70 years later, we’re still struggling to piece together this galactic mystery.