Because of our Solar System's location in the Milky Way's galactic disk, astronomers have a harder time determining the true extent of the Milky Way than they do galaxies millions or even billions of light-years away. And whereas distant galaxies can be well-constrained using optical telescopes, astronomers must rely on instruments that capture light at other wavelengths (radio, infrared, ultraviolet, and X-ray) to better understand the Milky Way's properties.

This includes the Milky Way's outer arms, whose true distances have remained somewhat unclear until recently. Using NASA's Chandra X-ray Observatory and ESA's XMM-Newton satellite, a team of astronomers recently made precise distance measurements to dust clouds in the Milky Way’s spiral arms. Their findings suggest that they may be wider than previously thought, causing astronomers to rethink prevailing theories about our home galaxy's structure.

The results are described in a new paper published in the journal Astronomy & Astrophysics. The team's distance measurements relied on a technique that utilizes light echoes, where they observed rings created by gamma-ray bursts (GRBs) bouncing off of dust clouds in the spiral arms. These GRBs were released by the collapse of massive stars (supernovae) or the merger of neutron stars (kilonova bursts). The diameters of the rings in X-rays provide distance measurements, with larger rings being generated by dust clouds closer to us.