Free Astronomy Magazine September-October 2025

SEPTEMBER-OCTOBER 2025 N ASA’s James Webb Space Telescope captured the central region of the Bullet Cluster with its NIRCam (Near-Infrared Camera). The scene contains two massive galaxy clusters that sit on either side of the large, light blue spiral galaxy at the center. Webb’s extremely precise images revealed many more distant gal- axies and faint objects, allowing a research team to refine the amount of mass in the two galaxy clusters. [NASA, ESA, CSA, STScI − James Jee (Yonsei University, UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (Caltech/IPAC)] very massive collections of galaxies, known as galaxy clusters, that are themselves bound by gravity. These galaxy clusters act as gravita- tional lenses, magnifying the light of background galaxies. “Gravita- tional lensing allows us to infer the distribution of dark matter,” said James Jee, a co-author, professor at Yonsei University, and research asso- ciate at UC Davis in California. To visualize gravitational lensing and dark matter, think of a pond filled with clear water and pebbles. “You cannot see the water unless there is wind, which causes ripples,” Jee explained. “Those ripples dis- tort the shapes of the pebbles below, causing the water to act like a lens.” The same thing happens in space, but the water is dark matter and the pebbles are background galaxies. In all, the team measured thousands of galaxies in Webb’s im- ages to accurately “weigh” both the visible and invisible mass in these galaxy clusters. They also carefully mapped and measured the collective light emit- ted by stars that are no longer bound to individual galaxies — known as intracluster stars. The revised map of the Bullet Clus- ter is shown in a new image on the previous two pages: Layered on top of an image from Webb’s NIRCam (Near-Infrared Camera) is data from NASA’s Chandra X-ray Observatory that shows hot gas in pink, includ- ing the bullet shape at right. Refined measurements of the dark matter, calculated by the team us- ing Webb’s observations, are repre- sented in blue. Their findings are