Sometimes looking for answers just creates a lot more questions. Fifty years ago, astronomers observed a mysterious structure in the night sky. The object, called Loop I, is a "bubble" that covers about one-third of the sky. Researchers don’t yet know where or how far away the loop is, but they do know that it appears in several wavelengths.

Using ESA’s Planck satellite, researchers studied Loop I in microwaves to form a picture of its external and internal structure. In doing so, the team extended its apparent size to the southern hemisphere as well as put some limits on where it is situated.

New work, published in Astronomy & Astrophysics, expands our understanding of Loop I and other akin structures, but the question of its origin remains shrouded in mystery. The microwaves detected by Planck are created by electrons being accelerated by the object's magnetic field, and the different colors in the map indicate the direction they are oscillating.

Loop I is more strongly visible in the sky’s northern hemisphere, with several filaments being detected about the galactic plane (shown in yellow and green above). The team believe these filaments are associated with Loop I, strongly suggesting that these "bubble walls" were formed by shockwaves.

Many scientists believe that Loop I is a relic of a supernova remnant whose gas was stretched and dispersed by a nearby episode of intense star formation in the Scorpius-Centaurus OB Association. This region has been active for more than 10 million years, so its stars could have easily carved out the nebula left over by one or more supernovae and pushed the gas outwards. The accelerated material then interacted with the galaxy's magnetic field, creating Loop I.

Proponents of this explanation suggest that the bubble could be as close as 400 light-years, but the Planck collaboration suggests that it should be at least several times that distance.

There’s also a more dramatic explanation for the structure: The bubble might have been caused by an extremely powerful outburst from Sagittarius A*, the supermassive black hole at the center of the Milky Way. If this was the case, Loop I's location could extend to over 25,000 light-years from Earth.

Although the mystery of Loop I remains unsolved, this work has expanded our understanding of the subtle interplay between gas and the magnetic field of the Milky Way.