Starts With A Bang

Mostly Mute Monday: Overlapping, but not colliding galaxies (Synopsis)

Image credit: Sloan Digital Sky Survey / William Keel, of Arp 198.

“I know there is strength in the differences between us. I know there is comfort where we overlap.” -Ani DiFranco

The observable Universe is a big, big place. Some 46 billion light years in all directions, with hundreds of billions of galaxies inside, it’s no wonder that we see stars and galaxies in all directions we’ve ever looked. What’s even more of a surprise, though, is that sometimes galaxies that aren’t at all close together look like they are, and overlap.

Image credit: Sloan Digital Sky Survey / William Keel, of NGC 3861.

These optical illusions are caused by the uncommon occurrence of two galaxies coincident along the same line-of-sight, even though they may be separated by tremendous cosmic distances. Thanks to the combined efforts of Galaxy Zoo, the Sloan Digital Sky Survey and professor William Keel, more than a thousand such overlapping galaxies have been found so far.

Image credit: SDSS / Galaxy Zoo / William Keel, from

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