The 30-degree arc is likely an expanding shock front from a star that exploded about 100,000 years ago.
A strange, geometrically perfect formation, located right in front of the Big Dipper's Chalice. But even though it covers a third of the northern sky, you will never see this circle visually through your telescope.
Ultraviolet and narrowband photography captured a thin and extremely faint arc of hydrogen gas.
The arc, unveiled at a recent virtual meeting of the American Astronomical Society, is likely a primeval shockwave expanding from a supernova about 100,000 years ago, and holds the record for the largest in the sky.
Andrea Bracco (University of Paris) and his colleagues accidentally stumbled upon the Ursa's large arc while viewing NASA's ultraviolet images from the GALEX spacecraft.
Based on the interaction of the gas with its surroundings, the researchers estimate that the arc is about 600 light years away.
Interestingly, there are two well-known "holes" in interstellar space, called the Lockman Hole and the Extended Groth Strip, that appear to lie within the bubble.
These "windows" gave astronomers an extremely clear view of intergalactic space.
Given that the arc has such an ideal shape, it is likely that some other explosive event already cleared the area when this supernova impact front passed through it.
However, the explosion may have played a role in keeping these windows open.