Free Astronomy Magazine September-October 2026

39 SEPTEMBER-OCTOBER 2026 Way galaxy’s arms – Outer Scutum-Centaurus Arm and Outer Arm – lie up to 10% further away than we thought. While ESA’s Gaia has revolu- tionised our understanding of the Milky Way galaxy, the distance measurements avail- able so far from the tele- scope are less precise for the outer arms. Using X-rays to probe the distances to dust clouds, as XMM-Newton and Chandra did here, is highly accurate out to longer dis- tances, allowing the research team to revise the map of the outer Milky Way galaxy. “This finding is a great ex- ample of how ESA’s long- er-standing missions – such as XMM-Newton, which launched in 1999 – still have a hugely important role to play in exploring the Uni- verse,” says Erik Kuulkers, ESA XMM-Newton project scientist. “Now in its third decade, XMM-Newton con- tinues to return a steady stream of groundbreaking science on everything from the brightest-ever GRB, to stars being shredded by black holes, to X-ray snapshots of Mars. It’s even more exciting when missions team up, as they did here. Together, they can reveal huge amounts about the skies around us.” What we know of our home galaxy will continue to grow in coming years. Alongside ever more detailed data from Gaia’s fourth and fifth data releases (planned for Decem- ber 2026 and after the end of 2030, respectively), ESA’s next generation X-ray observatory NewAthena is poised to transform X-ray astron- omy, and enable scientists to explore far fainter X-ray echoes in the out- skirts of our galaxy. T his artist’s impressions depict the structure of our galaxy, the Milky Way, based on data from ESA’s Gaia and illustrates how scientists have revised the position of its outer arms thanks to observations from ESA’s XMM-Newton and NASA’s Chandra. The two X-ray telescopes have spotted the aftermath of three bright cosmic explosions echoing through the outer spiral arms of our galaxy. By measuring the distance to these echoes, a research team found that the outer arms are up to 10% further away than we thought. [ESA/Gaia/ DPAC, Stefan Payne-Wardenaar, ESA/XMM-Newton and NASA/Chandra] four spiral arms (we now know the answer to be four). Now, another of ESA’s missions has found a new way to map the extremities of our gal- axy. “We usually model the Milky Way’s outer arms indirectly based on what we know of how our galaxy rotates, but doing it this way leaves room for error,” says Beatrice Vaia of Istituto Nazionale di Astrofisica (INAF), Italy, who led the research as part of her PhD. “Instead, we did something new: we looked at the aftermath of three cosmic explo- sions that took place in far more dis- tant galaxies. These explosions flung out X-rays that echoed through sev- eral of the Milky Way’s outer arms – and we measured the distances to these echoes directly.” X-ray light was thrown out by three bright ex- plosions known as gamma-ray bursts (GRBs). The X-rays bounced around and were scattered by dust grains within the Milky Way galaxy’s spiral arms, forming bright rings that were then picked up by XMM-Newton and Chandra. By studying how these ring-shaped echoes slowly expanded over time, Beatrice and colleagues were able to pinpoint the distance of the scattering dust grains. As these lie in clouds within the arms of our galaxy, the team could directly measure the distance of the arms. Besides confirming the known dis- tance to the Perseus arm, the scien- tists found that two of the Milky !