Milky Way's Outer Spiral Arms Extend Further Than Thought, Say X-Ray Echoes

Astronomers used X-ray echoes from rare gamma-ray bursts observed by NASA’s Chandra and ESA’s XMM-Newton to geometrically measure dust rings in the Milky Way’s spiral arms, finding the outer arms extend farther than previously known (one distant dust cloud about 3,500 light-years across). This geometry-based method, not dependent on the galaxy’s rotation, could prompt revisions to estimates of the Milky Way’s mass and structure, though its data are limited by the rarity of bright gamma-ray bursts.