Fifty years ago, astronomers discovered a mystery. They called it Loop I. Today, we still have not fully resolved the mystery of how this giant celestial structure formed but we do now have the best image of it, thanks to ESA’s Planck satellite.
Loop I is a nearly circular formation that covers one third of the sky. In reality, it is probably a spherical ‘bubble’ that stretches to more than 100º across, making it wider than 200 full Moons. Its absolute size, however, is extremely uncertain because astronomers do not know how close it is to us: estimates to the center of the bubble vary from 400 light-years to 25 000 light-years.
What they do know is that the structure shows up in many different wavelengths, from radio waves to gamma rays. Planck sees Loop I in microwaves. This image’s colors reflect the polarisation – the direction in which the microwaves are oscillating.
Our eyes are not sensitive to this information in the visible light, where we perceive only the intensity and color. Planck, however, can detect all three of these characteristics in the microwaves it targets.
The microwaves detected by Planck are emitted by electrons that are being accelerated by the Galaxy’s magnetic field.
Loop I is most visible in the sky’s northern hemisphere. Astronomers refer to this portion as the north polar spur. It can be seen in this image as the yellow arc. This fades to purple and can be traced into the southern hemisphere, completing the circle. The blue band spanning the image horizontally is the Galactic Plane.
Info and image via ESAhttp://www.esa.int/spaceinimages/Images/2016/06/A_mysterious_ring_of_microwaves
Credit: ESA and the Planck Collaboration