Free Astronomy Magazine September-October 2025

SEPTEMBER-OCTOBER 2025 region, researchers have more com- pletely mapped the colliding galaxy clusters’ contents. “With Webb’s observations, we carefully measured the mass of the Bullet Cluster with the largest lens- ing dataset to date, from the galaxy clusters’ cores all the way out to their outskirts,” said Sangjun Cha, the lead author of the paper pub- lished in The Astrophysical Journal Letters and a PhD student at Yonsei University in Seoul, South Korea. (Previous studies of the Bullet Clus- ter with other telescopes relied on significantly less lensing data, which netted out with less precise esti- mates of the system’s mass.) T his is the central region of the Bullet Cluster, which is made up of two mas- sive galaxy clusters. The vast number of galaxies and foreground stars in the image were captured by NASA’s James Webb Space Telescope in near-infrared light. Glowing, hot X-rays captured by NASA’s Chandra X-ray Observatory ap- pear in pink. The blue represents the dark matter, which was precisely mapped by researchers with Webb’s detailed imaging. Normally, gas, dust, stars, and dark matter are combined into galaxies, even when they are gravitationally bound within larger groups known as galaxy clusters. The Bullet Cluster is unusual in that the intracluster gas and dark matter are separated, offering further evidence in support of dark matter. [NASA, ESA, CSA, STScI, CXC − James Jee (Yonsei Uni- versity, UC Davis), Sangjun Cha (Yonsei University), Kyle Finner (Caltech/IPAC)] “Webb’s images dramatically im- prove what we can measure in this scene — including pinpointing the position of invisible particles known as dark matter,” said Kyle Finner, a co-author and an assistant scientist at IPAC at Caltech in Pasadena, Cali- fornia. All galaxies are made up of stars, gas, dust, and dark matter, which are bound together by gravity. The Bullet Cluster is made up of two