Tour: NASA's Chandra Examines Milky Way at Arms' Length

A new result shows that the outer spiral arms in the Milky Way galaxy may reach wider than previously thought. This finding may lead astronomers to adjust their understanding of our home galaxy’s structure.

A team of astronomers made this discovery by making precise measurements of distances to dust clouds in the Milky Way’s spiral arms using data from both NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. The researchers determined the distances by studying rings around gamma-ray bursts, or GRBs, which are some of the brightest bursts of light in the universe that come from the collapse of massive stars or the merger of neutron stars. GRBs are located at enormous distances, well beyond the confines of the Milky Way galaxy.

The distance measurement technique in this study capitalized on the phenomenon of light echoes, where the light from the GRB bounced off intervening dust clouds in the spiral arms along the line of sight to Earth. The diameters of the rings in X-rays give the distances to Earth, with larger rings being generated by dust clouds closer to us.

The advantage of this method is that this is a very direct way — relying only on geometry — to precisely measure distances to the Milky Way’s spiral arms. Most other methods rely on assumptions about how the Milky Way rotates, which become increasingly uncertain in the outer regions of our Galaxy.

Despite a century of awareness of the Milky Way’s spiral arms, astronomers are still working toward precise characterization of its arms due to Earth’s position within one, along with the dust and gas that block the view to other arms.

Although this technique is a major improvement, it may be difficult to use it for further measurements because bright GRBs that are visible through the plane of the galaxy are rare. In the meantime, astronomers will take a closer look at what could be the new geometry of our home galaxy, thanks to this new study.